cm Ruler Online
A free online ruler in centimetres. Auto-detects your screen, or calibrate exactly with a card, then measure small objects directly on screen.
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 cm reading?
It depends on your screen being calibrated correctly. Auto-detect gives a good starting point for common devices; calibrating with a card gives a genuinely accurate cm reading on any screen. Everything — detection, calibration and measuring — happens entirely in your browser. Nothing is uploaded.
Can I switch to inches or pixels?
Yes — tap in or px at the top any time.