How to Use an On-Screen Ruler and Calibrate It With a Bank Card
Measure small objects with your screen in centimetres or inches, and calibrate once using a standard bank card (85.6 mm wide).
Every screen has a different pixel density, so a ruler drawn with fixed pixels would be wrong on most devices. A short calibration with something of known size fixes that.
Quick answer
Hold a bank card against the screen, adjust the on-screen card width until it matches, and the ruler is calibrated. The setting is remembered on your device.
Why a bank card
Standard ID-1 cards (credit, debit, many ID cards) are 85.60 mm wide and 53.98 mm tall, the same all over the world.
Steps
- Open the tool and choose centimetres or inches.
- Place a bank card on the screen over the card outline.
- Adjust the slider until the outline matches the card width exactly.
- Use the ruler to measure small objects placed against the screen.
How accurate is it?
If calibrated carefully, within about 1 mm for objects up to the screen size. Differences come from viewing angle and from the card edge position. Do not use it for precise engineering.
Tips
- Place the object flat against the glass and look straight down.
- Calibrate again if you change the browser zoom.
- Large phones can measure up to about 14 cm; a laptop up to 30 cm.
- For longer lengths, use a real tape measure.
Good for
Jewellery, a screw, a coin, a passport photo size or checking a picture on a form.
