Online Ruler
A real-size ruler on your screen, in centimetres or inches.
About this ruler
A ruler drawn at true physical size on your screen โ 30 centimetres or 12 inches, with millimetre and sixteenth-inch markings. Useful when the real one is in another room and you need to check a length, a screw, a photo print or a piece of jewellery.
Why it has to be calibrated
A browser genuinely cannot tell how physically large your display is. A CSS pixel is a convention rather than a measurement, and the DPI a device reports is frequently wrong or scaled by the operating system. Any on-screen ruler that skips calibration is guessing, and on a modern laptop or phone it will be out by a considerable margin.
So this one calibrates against something you physically have: a bank, credit or ID card. Every one of them is 85.60 mm wide, fixed by the ISO/IEC 7810 ID-1 standard and identical worldwide. Hold one against the box, drag until they match, and the ruler above is then accurate. The setting is saved, so it is a one-time job per screen.
Keep browser zoom at 100% โ zooming changes the scale and invalidates the calibration.
Frequently asked questions
Is an online ruler accurate?
Only once calibrated. Browsers cannot detect physical screen size, so an uncalibrated ruler is a guess. Matching it against a bank card, which is 85.6 mm wide worldwide by standard, makes it genuinely accurate.
How do I calibrate it?
Hold any bank, credit or ID card flat against the box on screen and drag the slider until the two are exactly the same width. That is all โ the setting is saved for this device.
Why does the ruler look wrong after zooming?
Browser zoom rescales everything, including the ruler, which breaks the calibration. Reset zoom to 100% and it will be correct again.
Can I measure in inches?
Yes โ switch units and the ruler redraws in inches with sixteenth markings, using the same calibration.