On-screen ruler
Measure small things in centimetres and inches on your screen. Match a standard-size card once and the ruler becomes accurate on this device.
Put one end of the object at 0 (the left edge), then tap or drag on the ruler to set the marker at the other end. Arrow keys move it by 1 mm.
Runs on your phone. Nothing is uploaded. Only the calibration size is remembered, on this device.
Calibrate
Any card in the standard bank-card shape works, e.g. a library card. Only its outline matters: never type or show any card numbers, and you can turn it face down. If you zoom the page or change the display size, calibrate again.
Why a screen needs calibrating
Web pages measure in CSS pixels, defined so that 1 inch is 96 pixels. Phones and monitors pack pixels at different densities, so a "1 cm" line can come out much bigger or smaller than a real one.
Bank cards, ID cards, licences and many membership cards share one international size, ID-1: 85.60 × 53.98 mm. Matching it gives the ruler the true size of your screen's pixels.
The 1 dirham coin in use since 1995 is 24 mm across. The large older coin (28.5 mm) and special coins differ, so use a card if you are unsure.
With a careful match, expect about ±1 mm over 10 cm. For anything that must fit exactly, use a real ruler or tape.
Sources: ISO/IEC 7810 (ID-1 card size 85.60 × 53.98 mm, as summarised on Wikipedia's ISO/IEC 7810 article); W3C CSS Values and Units (1 in = 2.54 cm = 96 CSS px, which is not tied to a screen's physical size); United Arab Emirates dirham coin details from Numista (1 dirham, nickel-plated steel, 2012 onwards: 24 mm) and Wikipedia's UAE dirham article (reduced to 24 mm in 1995; 28.5 mm before). 1 inch = 25.4 mm exactly.