Guides
How the things behind the tools actually work — written plainly, with the details most explainers skip.
How QR codes actually workWhat the squares in a QR code mean, how they survive damage, and the four reasons a QR code fails to scan. Written by someone who built an encoder.8 min read · about QR Code GeneratorHow long should a password be?Length beats complexity, and by a wider margin than most advice admits. What the maths actually says, and what to do instead of changing passwords monthly.7 min read · about Password GeneratorHow to reduce image file size without wrecking itWhy your photos are 8 MB, which lever actually shrinks them, and how to pick between JPEG, PNG and WebP. With the sizes each platform expects.7 min read · about Image ResizerInstagram fonts, explainedThose fancy bio fonts aren't fonts at all — they're Unicode look-alike letters. How the trick works, why it breaks search, and when to avoid it.6 min read · about Fancy Font GeneratorWhat emoji really meanEmoji meanings shift by platform, generation and country. What the common ones actually signal, and the ones most likely to be badly misread.9 min read · about Emoji ListHow to make a barcodeWhich barcode type you need, why retail codes require a GS1 prefix, and how to print one that actually scans first time. With a free generator.8 min read · about Barcode GeneratorPassport photo sizes, country by countryPassport and visa photo dimensions for the UK, US, EU, Canada, Australia and more, plus the head-height rule that causes most rejections.9 min read · about Passport Photo MakerWhat Base64 is, and what it is notBase64 turns binary into text so it survives email and URLs. How it works, why it grows files by a third, and why it protects absolutely nothing.8 min read · about Base64 EncoderHow to calculate percentagesPercentage of a number, percentage change, reverse percentages and why stacked discounts never add up. The four cases that cover everything.9 min read · about Percentage CalculatorSocial media image sizes and aspect ratiosCorrect dimensions for Instagram, YouTube, TikTok, X, LinkedIn and Facebook, and why the aspect ratio matters far more than the pixel count.8 min read · about Aspect Ratio CalculatorWorking across time zonesWhy UTC offsets are not fixed, how daylight saving quietly breaks recurring meetings, and a practical way to schedule across a distributed team.8 min read · about Time Zone ConverterHashing, encryption and encodingThree things constantly mistaken for each other. What each is for, which are reversible, and why MD5 is fine for downloads but never for passwords.9 min read · about Hash GeneratorThe Morse code alphabetThe full Morse alphabet, why timing matters more than memorising dots, and the learning method that actually works for hearing it at speed.8 min read · about Morse Code TranslatorHow random is a random number generator?Pseudorandom versus cryptographic randomness, why modulo bias quietly skews results, and when the difference actually matters to you.8 min read · about Random NumberHex colour codes, explainedWhat #FF5733 actually means, how RGB and HSL differ, and why HSL is the one to reach for when you are building a palette by hand.8 min read · about Color ConverterA regex cheat sheet that explains itselfThe regular expression syntax you actually use, why greedy matching bites, and the two patterns you should never write yourself.9 min read · about Regex TesterWhat JSON is, and where it trips people upJSON's six types, the syntax rules that cause most parse errors, and the large-number problem that silently corrupts data.8 min read · about JSON FormatterWhat your photos reveal about youPhotos carry GPS coordinates, timestamps and device serial numbers. What EXIF stores, which sites strip it, and how to check before you share.9 min read · about EXIF Viewer & RemoverUnix time, explainedWhy computers count seconds from 1970, how to spot seconds versus milliseconds, and what actually happens in January 2038.8 min read · about Timestamp ConverterIs BMI accurate?BMI was designed for populations, not people. Where it works, the five groups it misclassifies, and what to measure instead.8 min read · about BMI CalculatorHow loan interest actually worksAmortisation explained without the jargon: why early payments are mostly interest, what APR includes, and how overpaying saves years.9 min read · about Loan CalculatorHow to record your screenScreen recording in the browser, what the codecs mean, how to keep file sizes sane, and the privacy checks worth doing before you hit record.8 min read · about Screen RecorderHow many words is a page?Word counts per page for essays, novels and speeches, what changes the number, and why reading time is the more useful measure online.7 min read · about Word CounterDoes the Pomodoro technique actually work?What the evidence says about timed work blocks, why 25 minutes is arbitrary, and how to adapt the method for work that needs deep focus.8 min read · about Pomodoro TimerHow file compression actually worksWhy a text file shrinks by 80% and a JPEG barely moves, what ZIP does, and the difference between lossless and lossy compression.8 min read · about Create a ZIPHow to type fasterTouch typing beats every shortcut: why accuracy-first training works, what speeds are realistic, and a four-week plan that sticks.8 min read · about Typing Speed TestStick drift, explainedWhy analogue sticks start moving by themselves, which fixes are real, and how to test any controller for drift in the browser.8 min read · about Gamepad TesterReaction time, memory span, aim: what's normal?The honest numbers behind the popular brain benchmarks — average reaction time, digit span, sequence memory — and what actually moves them.9 min read · about Reaction Time TestSleep cycles, and waking up less awfulWhat 90-minute sleep cycles really are, why waking mid-cycle hurts, and how to time an alarm — with the science's error bars included.8 min read · about Sleep CalculatorHow to write an invoice that gets paidWhat legally belongs on an invoice, the wording that speeds payment, numbering and VAT rules, and what to do when it goes unpaid.9 min read · about Invoice GeneratorWhich appliances actually use the most electricityHeat is the whole story: the appliance league table, the standby myth cut to size, and how to cost anything from its label.8 min read · about Electricity CostHow to play Minesweeper properlyThe rules in one minute, the number patterns that solve boards, chording for speed, and when a guess is genuinely forced.8 min read · about Minesweeper2048 strategy that actually reaches 2048The corner method explained with its reasons: monotonic snakes, the forbidden direction, recovering from mistakes, and what comes after 2048.8 min read · about 2048The keyboard shortcuts actually worth learningNot another list of 200 — the twenty shortcuts that repay learning, why they compound, and how to install them as habits.8 min read · about Keyboard TesterHow to run Secret Santa without the dramaBudgets, exclusions, the draw itself, and the etiquette questions that sink office gift exchanges — solved in one organiser's checklist.8 min read · about Secret Santa GeneratorTimes tables: the tricks and the methodThe patterns that shrink 144 facts to a learnable handful, the three genuinely hard ones, and a practice routine that avoids tears.8 min read · about Times TablesWhy adults can't hear the mosquito toneThe teenage-only ringtone, the biology of losing treble, what shops did with it, and testing your own ceiling honestly.8 min read · about Hearing Range TestWhat is a good typing speed?Average typing speed is 40 WPM, but the number moves a lot with age and practice. What counts as good, fast and pro level — and how WPM is measured.7 min read · about Typing Speed TestHow long does it take to build a habit?The 21-day rule is a misquote. The real research says 18 to 254 days, median about 66 — and what actually determines where you land on that range.7 min read · about Habit TrackerSpaced repetition, explained properlyWhy you forget 70% of what you learn within days, and how spacing and active recall reverse it. The forgetting curve, real intervals, and flashcards.8 min read · about FlashcardsTabata: what the 4-minute workout really isThe original 1996 Tabata protocol, what 170% VO2max actually means, why most 'Tabata' classes aren't, and how to do intervals honestly at home.7 min read · about Interval TimerHow to track your work hours honestlyUntracked freelancers systematically underbill — memory rounds small tasks to zero. How to track hours without hating it, and turn the log into invoices.7 min read · about Time TrackerTo-do list methods that actually workWhy most to-do lists fail by lunchtime, and the methods that survive: the Ivy Lee method, 1-3-5 rule, MITs and the two-minute rule — with honest limits.7 min read · about To-Do ListHow to calculate a tip without a calculatorThe 10% trick that makes any tip mental arithmetic, standard tip amounts by situation, and how tipping customs differ around the world.6 min read · about Tip CalculatorCompound interest, explained with real numbersHow compounding actually works, the Rule of 72, why a 10-year head start beats double the deposits, and how the same force works against borrowers.7 min read · about Compound InterestHow GPA is actually calculatedGPA is a credit-weighted average of grade points. The 4.0 scale, how weighted GPAs pass 4.0, worked examples, and what actually moves your number.6 min read · about GPA CalculatorMilitary time, explained in one ruleThe subtract-12 rule for reading 24-hour time, a full conversion chart, why midnight is 0000, how to say it aloud, and why aviation runs on Zulu.5 min read · about Digital ClockDog years: the real conversionThe multiply-by-7 rule is wrong at both ends of a dog's life. How dog aging actually works, the science-backed formula, and why size changes it.6 min read · about Dog Age CalculatorBinary, explained from zeroHow binary counting actually works, how to convert both directions by hand, why 8 bits make a byte, and how text becomes 1s and 0s via ASCII and UTF-8.7 min read · about Text to BinaryThe NATO phonetic alphabet, letter by letterAlfa to Zulu with pronunciations, why these 26 words were chosen, the numbers that changed, and how to spell anything clearly over a bad phone line.6 min read · about NATO Phonetic AlphabetMetric vs imperial: how we got two systemsWhere feet, pounds and gallons came from, how the metre was invented, why the US never switched — and the $327 million crash caused by mixing them up.7 min read · about Unit ConverterIs a coin flip really 50/50?Coin flips are close to fair but measurably biased — about 51% to the side that starts up. The physics, famous streaks, and how to flip fairly.6 min read · about Coin FlipWhat do I need on the final?The one formula that answers it: (target − current × weight) ÷ final's weight. Worked examples, when to stop worrying, and how to plan a term.5 min read · about Final Grade CalculatorHow to add fractions (and why the rule works)Why you can't add denominators, finding common denominators without fear, the butterfly shortcut, mixed numbers, and the pizza picture that explains it.6 min read · about Fraction CalculatorTic-tac-toe: the never-lose strategyTic-tac-toe is solved — perfect play never loses. The best first move, why the double threat wins games, the full defensive rules, and 255,168 games.6 min read · about Tic-Tac-ToeLorem ipsum: what it means and where it came fromLorem ipsum is scrambled Cicero from 45 BC — what the Latin fragments mean, how printers adopted it, why designers still use it, and when not to.6 min read · about Lorem Ipsum GeneratorHow VAT actually worksValue-added tax explained: who really pays it, how to add or back it out of a price, why the maths trips people, and how VAT differs from sales tax.6 min read · about VAT & Sales TaxHow to cite a website in APA and MLAThe exact format for citing a website in APA 7 and MLA 9 — with examples, the no-author and no-date fixes, and the mistakes graders actually flag.7 min read · about Citation GeneratorHow much protein do you actually need?The real numbers behind daily protein: what the 0.8 g/kg minimum means, why athletes aim for 1.6–2.2, and how much your body can use per meal.7 min read · about Protein CalculatorHow to count macros, in plain EnglishWhat macros are, the 4/4/9 calorie rule, how to turn percentages into grams, and a worked 2,000-calorie example — without the influencer fog.7 min read · about Macro CalculatorOne-rep max, explained (no maxing required)What a 1RM is, how the Epley and Brzycki formulas estimate it from any hard set, why estimates beat testing, and how to train with percentages.6 min read · about One Rep MaxWhy Easter is on a different date every yearEaster is set by a formula involving the Moon, an equinox that isn't the equinox, and a 19-year cycle. Here's the computus, explained plainly.7 min read · about Easter CalculatorLeap years, explained properlyThe year is 365.2422 days long and calendars hate fractions. Why leap days exist, why 1900 skipped one, and what happens if you're born on one.6 min read · about Leap Year CheckerHow to win at solitaire (more often)About 80% of Klondike deals are winnable — most losses are self-inflicted. The priority order, the king rule, and the habits that raise your win rate.6 min read · about SolitaireHow to transpose chords to another keyThe semitone-counting method for changing a song's key, why sharps go up and flats come down, and the capo shortcut guitarists should know.6 min read · about Chord TransposerFlexbox vs Grid: which one, whenThe one-dimension rule that settles flexbox vs grid, the cases where each wins, and why real pages use both — with playgrounds to test in.6 min read · about Flexbox PlaygroundWhat DPI you actually need for printingWhy 300 DPI is the print standard, when 150 is fine, and how to check if your photo has enough pixels — with the arithmetic made visible.6 min read · about DPI CalculatorHow background removal works — and why hair is hardWhat a background remover actually does — segmentation, masks, alpha — why it runs on your device now, where it fails (hair, glass), and how to get clean cutouts.7 min read · about Background RemoverHow OCR works, and why scans read better than photosWhat optical character recognition does step by step, why 300-DPI scans read near-perfectly and phone photos don't, and what a searchable PDF is.7 min read · about Image to Text (OCR)HEIC vs JPG: what iPhones shoot, and why it won't openWhy Apple switched to HEIC in 2017, what HEIF and HEVC are, why Windows and websites still choke on them, and how to convert without losing quality.6 min read · about HEIC to JPGWebM vs MP4: containers, codecs, and which one to sendThe difference between a container and a codec, why screen recorders produce WebM, why MP4 opens everywhere, and how browsers can now convert on-device.7 min read · about Video to MP4How video compression works: bitrate, keyframes and codecsWhy a minute of video can be 10 MB or 1 GB: bitrate, keyframes and prediction, what resolution and codec change, and how to shrink a file without wrecking it.8 min read · about Video CompressorHow audio noise reduction works, and why voices go wateryWhat spectral gating does to a recording, which noises it removes, which it can't, and how to avoid the underwater artefact of over-processing.7 min read · about Audio Noise ReducerHow PDF files work: why some have text and some are picturesWhat a PDF contains — page objects, fonts, a text layer or just images — why scans can't be searched, what PDF/A is for, and which tool fits which kind of PDF.7 min read · about Extract Text from PDFHow to redact a PDF so the text is actually goneWhy a black rectangle over text leaves it in the file, the redaction failures that made headlines, and the two methods that really remove content.6 min read · about PDF RedactionHow to write a résumé that both software and recruiters can readWhat applicant-tracking systems do with your CV, the layouts that break them, the one-column format that works, and how to write bullets recruiters stop on.7 min read · about Résumé BuilderWhat kanban is, and why the work-in-progress limit is the whole ideaWhere kanban came from, how a three-column board works, why limiting work in progress makes things finish faster, and how to run a personal board honestly.6 min read · about Kanban BoardFlesch reading ease and grade level, explained honestlyHow the Flesch formulas turn sentence and word length into a score, what 60, 70 and 30 mean, which grade to aim for, and why a low score isn't clear writing.6 min read · about Readability CheckerWhat structured data is, and what it does for a pageWhat schema.org markup tells search engines, how JSON-LD works, which types earn rich results, and the rules that get markup penalised.7 min read · about Schema Markup GeneratorHow long should a title tag be? Pixels, not charactersWhy Google truncates titles by width (about 580 px), what the 60-character rule really means, when Google rewrites titles, and how to write one that lasts.6 min read · about SERP Snippet PreviewHow A/B testing works — and the three ways it liesWhat a p-value means for a split test, why sample size must be fixed in advance, how peeking manufactures false wins, and how to read the confidence interval.7 min read · about A/B Test CalculatorROAS explained: the number every ad platform shows, and the one it doesn'tWhat return on ad spend measures, how to compute break-even ROAS from your margin, how CPM, CPC and CPA connect, and the attribution caveats behind dashboards.7 min read · about Ad Cost CalculatorHow to calculate your break-even point (and what it doesn't tell you)The break-even formula step by step: fixed costs, variable costs, contribution margin, a worked example, the chart, target profit and the limits.6 min read · about Break-Even CalculatorHow mortgage amortization works, and why overpaying early mattersHow a repayment mortgage's payment is calculated, why early payments are mostly interest, what the schedule shows, and how overpayments cut years off.7 min read · about Mortgage CalculatorHow to split group expenses fairly, and settle up in the fewest paymentsWhy dividing the total by six is wrong, how to compute each person's true balance, and the simple method that settles a group in the fewest transfers.6 min read · about Expense SplitterHow authenticator app codes work: TOTP, explainedWhat the six-digit codes in an authenticator app are — a shared secret, the clock, an HMAC — why they change every 30 seconds, and how to back them up.6 min read · about TOTP Code GeneratorWhat an .ics file is, and how 'add to calendar' links workHow iCalendar files carry events between Google, Outlook and Apple, what the VEVENT lines mean, why time zones go wrong, and the links an event page needs.6 min read · about Add to Calendar Link GeneratorHow to scan documents with your phone, and what 'scanning' actually doesWhat a scanning app does to a photo — perspective correction, thresholding, A4 sizing — how to shoot pages so it works, and how to get a searchable PDF.6 min read · about Document ScannerUTM parameters explained: the five tags and how to use them consistentlyWhat utm_source, medium, campaign, content and term mean, how analytics reads them, the naming rules that keep reports clean, and the mistakes that break tracking.6 min read · about UTM BuilderApp icon sizes explained: every size, and why there are so manyWhich icon sizes iOS, Android and web apps need and why — points, densities, adaptive masks, the App Store 1024 rule — plus favicons, all from one file.6 min read · about App Icon GeneratorHow image upscaling works: interpolation, Lanczos and 'AI enhance'What happens when an image is enlarged — nearest neighbour, bicubic and Lanczos resampling — what AI upscalers invent, and which method to use for what.6 min read · about Image UpscalerSRT vs VTT: the two subtitle formats, and how to fix timingHow SRT and WebVTT files are structured, where each is used, how to convert between them, and the shift trick for subtitles that run early or late.6 min read · about Subtitle ConverterWhy GIFs are so big, and when a muted video is the better choiceWhat the 1989 GIF format stores — 256 colours, whole frames, no real compression — why a clip outweighs the video it came from, and how to make sendable GIFs.5 min read · about Video to GIFHow face detection works, and what really hides a faceHow software finds faces, why it misses profiles and small ones, the difference between detection and recognition, and which blur actually anonymises.6 min read · about Face BlurExtractive vs abstractive summaries: how each works and when to trust itHow extractive summarisers pick sentences, how generative models write new ones, the hallucination trade-off, and how to use each kind honestly.6 min read · about Text SummarizerHow to choose a chart type — and the two ways charts lieWhich chart fits which question — comparison, trend, share, distribution — why pies fail past six slices, why bars must start at zero, and honest defaults.6 min read · about Chart MakerCSV vs Excel: what each file really contains, and what breaks in betweenWhat a CSV is, what an .xlsx stores that CSV cannot, how Excel corrupts CSVs on open (leading zeros, dates, long numbers) and how to convert cleanly.6 min read · about CSV to ExcelHow JPEG compression works — and what the quality slider really doesWhat happens inside a JPEG — colour subsampling, 8×8 blocks, the cosine transform and quantisation — why quality 80 looks like 100, and where artefacts come from.7 min read · about Image CompressorAVIF vs WebP vs JPEG: sizes, support and when to use eachHow the three web photo formats compare on size, quality, transparency, animation and browser support, and a practical rule for choosing between them.6 min read · about Image to AVIFWhat an SVG is, why it scales forever, and when not to use itHow SVG describes shapes instead of pixels, why logos and icons belong in it, what makes files bloat, and how to convert to and from PNG.6 min read · about SVG to PNGHow to make a favicon that works everywhere — sizes, formats and the tagsWhich favicon files a modern site needs (ICO, PNG sizes, SVG, apple-touch-icon), the HTML link tags, and why the tab icon refuses to update.6 min read · about Favicon GeneratorAspect ratios explained: what 16:9, 4:3, 3:2 and 9:16 mean, and how to crop between themWhat an aspect ratio is, where 16:9, 4:3, 3:2 and 9:16 come from, how to find a missing dimension, and how to crop between ratios without distortion.6 min read · about Aspect Ratio CalculatorHow dithering works — fewer colours that look like moreWhy reducing colours causes banding, how ordered and error-diffusion dithering trade noise for smoothness, and where dithering is still used today.5 min read · about Dither ImageHow to read an image histogram — exposure, clipping and contrast at a glanceWhat the histogram's axes show, how to spot underexposure, blown highlights and low contrast, why there is no correct shape, and how edits move it.5 min read · about Image HistogramHow to watermark photos — and what a watermark can and can't protectVisible and invisible watermarks, where and how large to place them so they cannot be cropped away, opacity that keeps the image, and the honest limits.5 min read · about Add WatermarkSample rate and bit depth explained: what 44.1 kHz and 16-bit actually meanHow digital audio captures sound, why 44.1 kHz covers everything you can hear, what bit depth does to noise, and which settings to use.6 min read · about Sample Rate ConverterAudio normalization explained: peak, RMS and loudness, and why streaming turns you downPeak versus loudness normalization, what LUFS measures, why Spotify and YouTube turn your track down anyway, and how to set levels for podcasts and music.6 min read · about Audio NormalizerHow MP3 compression works, and which bitrate you can actually hearHow MP3 uses psychoacoustics to drop sounds you cannot hear, what 128 vs 320 kbps changes, why re-encoding compounds loss, and where AAC and Opus do better.6 min read · about Audio to MP3How to compress a PDF — what's inside a large file and what shrinks itWhy PDFs bloat (images, fonts, scans, revisions), what compression does to each, how much quality to expect at each setting, and when a big PDF is correct.5 min read · about Compress PDFElectronic vs digital signatures: what each is, and what a drawn signature on a PDF provesAn e-signature shows intent; a digital signature is a cryptographic seal. When each is valid, and what a signature drawn onto a PDF really proves.6 min read · about Sign PDFHow booklet imposition works — why page 1 prints next to the last pageWhat imposition is, how a folded booklet's page order is calculated, why page counts must be multiples of four, and how to print one at home.5 min read · about PDF Booklet MakerHow to write a meta description: length, structure, and when Google ignores itWhat a meta description is for, the pixel and character limits, a structure that reads as an answer, why Google often rewrites it, and examples.5 min read · about Meta Tag GeneratorWhat Markdown is, the syntax that matters, and where it worksThe plain-text formatting syntax behind READMEs, chat apps and note tools: headings, lists, links, code and tables, the flavours, and converting it.6 min read · about Markdown PreviewUTF-8 explained: how text is stored, and why it turns into é and ’What Unicode and UTF-8 are, how characters become bytes, why mojibake like é appears, what a BOM is, and how to fix encoding problems in files and CSVs.6 min read · about HTML Entity EncoderWhat a URL slug is, and the rules for a clean oneThe readable part of a URL: how slugs are made from titles, the rules on hyphens, case and length, what they do for SEO, and why not to change one.5 min read · about URL Slug GeneratorHow diff tools work — the algorithm behind 'what changed'How a diff finds the smallest set of changes between two texts, why line, word and character diffs differ, what a unified diff means, and where diffs fail.5 min read · about Text CompareWhat YAML is, the syntax that bites, and when JSON is the better choiceHow YAML's indentation-based syntax works, the traps (the Norway problem, tabs, colons), where it is used, and converting to and from JSON.6 min read · about YAML to JSONWhat XML is, how it differs from HTML and JSON, and where it still runs the worldThe rules of well-formed XML, elements versus attributes, namespaces and schemas, why it lost to JSON for APIs, and where it remains standard.6 min read · about XML FormatterWhat a UUID is, the versions that matter, and whether collisions can happenThe 128-bit identifier format, what versions 1, 4 and 7 encode, how random a v4 really is, why collisions are a non-issue, and when a UUID is the wrong key.5 min read · about UUID GeneratorWhat a JWT is, what's inside one, and what it does and doesn't proveThe three parts of a JSON Web Token, why the payload is readable by anyone, what the signature guarantees, the mistakes that make tokens unsafe, and safe decoding.6 min read · about JWT DecoderCron expressions explained: the five fields, the special characters, and the trapsHow to read and write cron schedules — minute, hour, day, month, weekday — with the special characters, common examples, and the traps.5 min read · about Cron Expression ExplainerHow checksums verify downloads and backups — and which one to useWhat a checksum is, why one changed bit changes the whole hash, CRC32 versus MD5 versus SHA-256, how to verify a download, and why it is not a signature.5 min read · about MD5 & CRC32Subnet masks explained: what /24 means and how to size a networkHow an IP address splits into network and host parts, what the mask and CIDR prefix mean, how many hosts a /24 or /26 holds, and private ranges.6 min read · about Subnet CalculatorWhat a user-agent string is, why it lies, and how to read oneWhat the user-agent header tells a website, why every browser claims to be Mozilla, how to read one, and why it is being frozen and replaced by client hints.5 min read · about What Is My User AgentHTTP status codes explained — the ones you'll actually meet, and what to do about themThe five classes of HTTP status, what 200, 301, 302, 404, 410, 429, 500, 502 and 503 mean, which ones matter for SEO, and how to check a URL.6 min read · about HTTP Status CodesURL encoding explained: why spaces become %20 (or +), and what has to be escapedWhich characters are allowed in a URL, how percent-encoding turns bytes into %XX, why a space is %20 in a path but + in a form, and the double-encoding trap.5 min read · about URL Encoderpx vs rem vs em: what each unit is relative to, and which to use whereWhat a CSS pixel actually is, how rem and em scale with the user's font setting, when em compounds, which unit to use where, plus viewport units.5 min read · about px to rem ConverterMean, median and mode: which average to use, and when the mean liesThe three averages, how to calculate each, why house prices and salaries are reported as medians, what outliers do to a mean, and how to pick the right one.5 min read · about Average CalculatorStandard deviation explained: what it measures, how it's calculated, and the 68–95–99.7 ruleWhat standard deviation tells you that an average cannot, how to compute it, population versus sample, the 68–95–99.7 rule, and reading it in results.6 min read · about Standard DeviationFractions, decimals and percentages: the same number three ways, and how to convertWhy 3/4, 0.75 and 75% are one number, how to convert in every direction including recurring decimals, and where each form is the natural one.5 min read · about Fraction CalculatorRatios explained: how to simplify, scale, and divide an amount in a given ratioWhat a ratio says, how it relates to fractions, simplifying and scaling (recipes, maps, mixes), dividing an amount in a ratio, and aspect ratios and odds.5 min read · about Ratio CalculatorHow body fat percentage is estimated — and how much to trust each methodHow the US Navy tape formula, BMI, bioimpedance scales, calipers and DEXA estimate body fat, their error margins, and healthy ranges by sex and age.6 min read · about Body Fat CalculatorHeart rate zones explained: how they're calculated and what each one trainsHow maximum heart rate is estimated, what the five zones are for, why zone 2 gets so much attention, and how to find your zones without a lab.6 min read · about Heart Rate ZonesHow much water you should drink a day — what the evidence says, and where 8 glasses came fromWhere the 8-glasses rule came from, what health bodies recommend, how weight, exercise, heat and diet change your needs, and when more is harmful.5 min read · about Water IntakeHow daily calorie needs are calculated — BMR, activity multipliers and what a deficit meansWhat BMR and TDEE are, the Mifflin-St Jeor formula behind most calculators, how activity multipliers work, what a deficit does, and the limits.6 min read · about Calorie CalculatorRunning pace explained: converting pace, speed and race times, and what pace to train atWhat pace means and how it converts to speed and race times, min/km versus min/mile, predicting a marathon from a 10K, splits, and training paces.5 min read · about Running Pace CalculatorHow to set a savings goal: the arithmetic, the interest, and the habits that make it stickHow to turn a target and a date into a monthly amount, what interest and inflation change, emergency funds first, and the habits that make saving stick.6 min read · about Savings GoalGross vs net salary: what comes off your pay, and how to compare offersThe difference between gross and take-home pay, what deductions sit between them, how hourly, monthly and annual figures convert, and comparing offers.5 min read · about Salary ConverterMargin vs markup: why 50% markup is 33% margin, and how to price from eitherThe definitions, the formula that converts between them, why confusing the two under-prices products, gross versus net margin, and pricing from a target.5 min read · about Margin CalculatorHow to calculate fuel cost for a trip, per mile, and per year — and what really changes itThe formula for fuel cost from distance, consumption and price, converting mpg, L/100 km and km/L, why official figures flatter, and what changes the number.5 min read · about Fuel Cost CalculatorHow working days are counted — and why 'within 5 business days' means different thingsThe counting conventions for business days (start day, weekends, public holidays), how contracts and SLAs define them, and how many working days a year has.5 min read · about Working Days CalculatorISO week numbers explained: why week 1 isn't 1 January and some years have 53 weeksHow the ISO 8601 week year works, the rule that decides week 1, why 30 December can be in week 1, which years have 53 weeks, and where week numbers are used.5 min read · about Week NumberHow moon phases work — the geometry behind the 29.5-day cycleWhy the Moon shows phases, the eight named phases in order, why a lunar month is 29.5 days, how to tell waxing from waning, and what phases affect.5 min read · about Moon PhaseHow sunrise and sunset times are calculated — latitude, the seasons, and the equation of timeWhat sunrise means precisely, how latitude and the date set day length, why the earliest sunset is not the shortest day, and the three twilights.5 min read · about Sunrise & SunsetDice probability explained: why 7 is the most common roll, and how to work out any oddsHow to count outcomes for one or more dice, why two dice make a triangle of sums, the odds of doubles and at least one six, and how to tell a loaded die.5 min read · about Dice RollerLottery odds explained: how the jackpot probability is calculated, and what it meansHow combinations give the odds for a 6-from-49 or 5+2 lottery, why every ticket has the same chance, expected value versus price, and comparisons that make it real.5 min read · about Lottery Number GeneratorWhat a passphrase is, why four random words beat P@ssw0rd!, and how to make oneThe difference between a passphrase and a complex password, how entropy is counted, why random word lists work, how many words you need, and where they fit.5 min read · about Passphrase Generator