Something not covered? The ruler is simple on purpose: calibrate once, then measure.
Questions
How accurate is an online ruler?
After calibrating with a card it is usually within about half a millimetre on a laptop and better on a phone, because phone screens have more pixels per millimetre. Without calibration it can be off by a lot.
Can I measure something longer than my screen?
Measure up to the screen edge, put a small mark on the object, slide it along and add the two readings. For long things a tape measure is quicker.
Why does my 1 cm look too small or too big?
Your ruler is not calibrated for this screen yet. Press Calibrate and match a bank card.
Does it work on a phone?
Yes. On a phone held upright the ruler runs down the side of the screen, which gives you the longest length to measure. Press Turn to switch.
Is anything sent or saved?
No. The ruler runs in your browser. The calibration is saved only in your browser on this device.
Why a bank card?
Cards are made to one international size (ISO/IEC 7810 ID-1, 85.60 x 53.98 mm), almost everyone has one, and it is big enough to calibrate precisely.
I changed screens. Do I calibrate again?
Yes. The calibration belongs to one screen. Press Calibrate on the new screen.