How to track your period: what to log and what it tells you
This article is general information, not medical advice. It is not a substitute for a doctor, dietitian or other qualified professional who knows your circumstances. If you have a medical condition, are pregnant, or are changing your diet or exercise significantly, talk to one first.
Most people only think about their cycle when a period arrives at an awkward moment. A few months of simple notes changes that: you learn your own pattern rather than the textbook 28 days, you can plan around it, and you have real dates to show a doctor if something changes. I built the period tracker around the smallest useful log, and this guide explains what to record and how the numbers are worked out.
Why tracking helps
Memory is a poor record of cycles. Ask anyone when their last period started and the answer is often a guess within a week. A written log replaces the guess with dates, which gives you three things: a personal average cycle length to plan holidays, exams and events around; an early warning when a period is unusually late or a pattern shifts; and accurate history for appointments, where the first questions are usually "when was the first day of your last period?" and "how long are your cycles?".
What to log
The essential entry is the first day of bleeding each cycle. That single date per month is enough to calculate cycle length and predict the next period. Add the last day if you want your typical period length. Beyond that, optional notes are useful when they answer a question you actually have:
- Flow — light, medium or heavy, and whether you are soaking through protection every hour or two.
- Pain — cramps, back pain or headaches, and whether painkillers help.
- Spotting between periods, which is worth recording separately from a period.
- Mood, sleep and energy, if you suspect they follow your cycle; the mood journal is built for that.
Keep it light. A log you fill in for two years is far more valuable than a detailed one you abandon after a month.
How cycle length is counted
A cycle runs from the first day of one period to the day before the next one starts, so cycle length is simply the number of days between two start dates. Day 1 is the first day of real bleeding, not the spotting that sometimes comes before it. If your periods started on 2 August and 30 August, that cycle was 28 days. Cycles vary from month to month, so trackers average several: the period tracker uses your last six cycles, which smooths out a single odd month while still following changes over a year. It also ignores gaps that look like a forgotten entry rather than a real cycle.
How predictions work
The next period is predicted by adding your average cycle length to the start of your last period. Ovulation is estimated at about 14 days before the next expected period, because the phase after ovulation is fairly constant from cycle to cycle while the phase before it varies; that is why ovulation in a 32-day cycle is around day 18, not day 14. The fertile window is the five days before ovulation and the day after, explained in The fertile window: which days you can get pregnant. These are averages, so the more regular your cycles, the closer the predictions. They are not reliable enough to use as contraception.
Changes worth noticing
Adult cycles commonly run between about 21 and 35 days (the NHS describes a wider normal range of 21 to 40), with periods lasting two to seven days. A log makes it easy to spot the patterns doctors ask about:
- cycles that are regularly shorter than 21 days or longer than 35, or that suddenly change;
- cycles that vary by more than about a week from one month to the next;
- periods that last longer than seven days, or bleeding between periods or after sex;
- very heavy bleeding, or pain that stops you doing normal things;
- no period for three months when you are not pregnant.
None of these is a diagnosis on its own, but they are good reasons to book an appointment, and your log turns a vague description into dates. How long is a normal menstrual cycle, and what changes it? covers what changes cycle length.
Tracking privately
Cycle data is sensitive health information, and many people prefer it not to sit on a company's servers. You can track with a paper calendar, a note on your phone, or a tool that keeps everything on your device. The period tracker has no account and sends nothing anywhere: entries are saved in your browser only, you can export a backup file, and Clear all deletes them. The trade-off is that clearing your browser data or changing phones loses the log unless you export it first. If you are also trying to conceive, the ovulation calculator gives a quick estimate from a single date, and the due date calculator takes over once you are pregnant.
Sources and further reading
The claims in this guide rest on these references, which were checked when the guide was last updated. Spotted an error? The contact page says how to report it.