A ruler that shows centimetres and inches at real size on your screen. Calibrate it once with a bank card (Step 1), then hold any small object against the ruler (Step 2).
Every screen shows a slightly different size, so take ten seconds to calibrate. A bank or ID card is exactly 85.60 mm wide everywhere in the world. Hold your card against the outline below and move the slider until the outline matches your card exactly.
Scale: 100%. Once set, both rulers below are actual size on this screen.
Place the object's edge at the 0 mark and read where it ends. For very small things, our 2 mm visual guide shows sizes against a coin, and how to read a tape measure covers the physical tool.
After card calibration, this ruler is accurate to about ±0.5 mm — plenty for jewellery, screws, SIM cards, coins, craft paper and checking sizes before you buy. It is not a replacement for a caliper in engineering work, and if you change screens or zoom your browser, calibrate again.
Screens have different pixel densities, so "10 cm" drawn by a browser is not 10 cm on every display. Matching a bank card — a worldwide standard size — fixes the scale for your exact screen.
Anything of exactly known size works: a US quarter is 24.26 mm wide, an A4 sheet is 210 mm wide, and most ID cards share the 85.60 mm card standard.
Printing is possible but printers often scale pages to fit, which ruins the accuracy. If you print, set scaling to 100% / "actual size" and check the print against a real card before trusting it.
Yes — the rulers adapt to your screen width and the same card calibration applies. A phone screen fits about 15 cm of the centimetre ruler at once.