Ruler on Screen: Real Size on Any Phone or Laptop

0.0 cm

Put one end of the thing you're measuring on the zero line, then drag the other handle to its far end. Drag the zero line too, to measure between two marks. Calibration is saved on this device.

About the ruler on screen

A ruler on screen is only useful if a centimetre on it is a real centimetre. That depends on the screen: a phone may pack 180 pixels into a centimetre where a laptop fits 50, and web pages are told neither. That is why many "actual size" rulers online are wrong on your device.

This one gets it right by asking once. Hold a bank card to the screen and size the drawn card to match: cards are always 85.60 mm wide, so the page can work out exactly how many pixels make a millimetre on your screen.

How to use it

Tap Calibrate and match the drawn card to a real bank, ID or library card.
Turn the phone sideways for a longer ruler; the ruler follows the screen.
Line up one end of the object with the zero line and drag the coloured handle to the other end.
Switch between cm and in at the top. Your calibration stays saved for next time.

Why screens need calibrating

Browsers size pages in CSS pixels. A CSS inch is defined as 96 CSS pixels, but on a real screen that "inch" can be anything from about two thirds of an inch to well over an inch, depending on the screen's size, resolution and zoom. Calibrating against a known object replaces the guess with a real measurement for your screen.

If you change the browser zoom or move to another device, calibrate again. With Bubble Level Pro you can keep several calibrations, for example one for your phone and one for your laptop.

Questions

Can I trust a ruler on my phone screen?

Yes, once it is calibrated against a known size. Without that, no web page can know how big a centimetre is on your screen.

What can I use instead of a bank card?

Any card of the standard ID-1 size: a debit or credit card, a driving licence photocard in many countries, or most loyalty and library cards.

Does zooming the page affect the ruler?

Yes. Browser zoom changes how big everything is drawn, so calibrate at the zoom level you will measure with.

Ruler on screen

Match a bank card