mm Ruler Online
A free online ruler in millimetres — useful for small, precise measurements. Auto-detects your screen, or calibrate exactly with a card.
Works best on a laptop or desktop with a mouse. On a phone, calibrate with a card or pick your exact model for the most accurate reading — auto-detect is a best guess.
Keyboard: C crosshair · A angle · D dark mode · F fullscreen · H hide controls · 1-4 units · Esc clear/close
More ruler units
Frequently asked questions
How accurate is the mm reading?
Millimetre precision depends heavily on calibration — auto-detect is a rough guide, but for genuinely accurate mm readings, calibrate with a standard card first. Everything — detection, calibration and measuring — happens entirely in your browser. Nothing is uploaded.
What is mm useful for?
Millimetres are handy for small, precise measurements — ring sizing, screws, jewellery, small parts — where a cm or inch scale is too coarse.