How to use the online ruler
- First calibrate: hold any bank card, credit card or ID card against the screen and drag the slider until the dashed box is exactly as wide as the card. Save it; the setting is remembered on this device.
- Choose centimetres/millimetres or inches (marked in sixteenths).
- Place the object against the screen at the 0 mark and read its length. Scroll sideways for longer rulers.
Why calibration is needed
Web browsers measure in CSS pixels, nominally 96 per inch, but real screens have very different pixel densities, and operating systems scale them differently. Without calibration an on-screen ruler can be off by 20% or more. Bank cards are made to an international standard (ISO/IEC 7810 ID-1), exactly 85.60 mm wide, so matching one gives your screen's true pixels per millimetre. After calibration, the ruler is typically accurate to about half a millimetre.
It is handy for measuring small items when you do not have a ruler nearby: screws, buttons, jewellery, ring sizes, paper, photos or packaging. Calibration is specific to each device and display (and to the browser zoom level, so keep zoom at 100%). Nothing is uploaded; the calibration is stored only in this browser.
Is this ruler actual size?
After calibration with a card, yes, to within about half a millimetre.
Does it work on phones?
Yes. Calibrate on the phone itself; the ruler then covers the width of the screen and scrolls.
Can I measure my ring size?
Measure the inner diameter of a ring that fits, in millimetres, and compare it with a ring size chart.