Donaldson R F Online Ruler
Measure small objects against your screen. Calibrate the ruler once with a bank card, then read centimetres, millimetres and inches at true size.
Step 1 — Calibrate with a bank card
Hold a standard bank or ID card (85.60 × 53.98 mm) against the screen and adjust the box until it is exactly the same size as the card.
Step 2 — Measure
How accurate is an on-screen ruler?
Web pages do not know the physical size of your screen, so a ruler can only be true to size after you calibrate it. A bank card is the same size worldwide (ISO/IEC 7810 ID-1: 85.60 × 53.98 mm), which makes it a handy reference. Your calibration is remembered in this browser only.
Zooming the page changes the size, so calibrate again after you zoom. Use this for quick, approximate measurements of small things such as a coin, a screw or a photo; use a real ruler or tape when precision matters.