Online Ruler
A real-size ruler in cm and inches on your screen.
Screens differ in size, so calibrate once for accurate measurements.
How to use
- 1Open the ruler on the device you want to measure with.
- 2Click Calibrate with a card and match the box to an ATM or ID card.
- 3Place the object on or next to the screen.
- 4Read the size in centimetres (top) or inches (bottom).
Frequently asked questions
Why do I need to calibrate?
Every screen has a different pixel density, so a browser can't know the real size of a centimetre. Matching a standard 85.6 mm card fixes that.
Is my calibration saved?
Yes, in this browser, so the ruler stays accurate next time on the same device.
How accurate is it?
After calibration it's usually within about 1 mm, good enough for rings, screws, photos and small items.
Can I measure my ring size?
Yes. Measure the inside diameter of a ring that fits, then use our Size Converter to find the ring size.
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