How to use
- The first time, calibrate. Choose A bank card, lay the card flat on the screen with its long edge along the dashed box, adjust until the box is as long as the card and press Save calibration.
- Put the thing you want to measure on the screen with one end at 0.
- Tap the ruler or drag to the other end. The length shows below in mm, cm and inches.
- Phone screens are narrow, so press Full-screen ruler to lay the scale along the long edge of the screen and measure further.
Options explained
- Calibrate with a bank card
- Bank cards, credit cards, ID cards and most driving licences are ISO/IEC 7810 ID-1 size, 85.60 mm long and 53.98 mm wide. Once the box matches the card's long edge, the tool knows how many pixels make a millimetre on your screen.
- Calibrate with the screen size
- Enter the advertised diagonal of a laptop or monitor in inches and the pixel density is worked out from the screen resolution. Phone screens have rounded corners, so their advertised size is not precise enough; use a card instead.
- Scale
- The centimetre scale has a tick every millimetre and a longer one every 5 mm; the inch scale goes down to 1/16 inch. Show one or both at once.
- Saved calibration
- Stored only in this browser on this device. If you change monitor, screen resolution or browser zoom, the page asks you to calibrate again.
Which cards work for calibration
Any ISO/IEC 7810 ID-1 card: bank cards, credit cards, transit cards, most national ID cards and driving licences — 85.60 mm long and 53.98 mm wide. Loyalty cards and business cards vary in size, so don’t use them.
Tips for calibrating:
- Lay the card flat on the screen and look straight at it; looking at an angle adds parallax.
- On a phone held upright the box is vertical, so hold the card upright too; on a computer the box is horizontal.
- Card corners are rounded, so line up the ends of the long edge, not the corners.
Frequently asked questions
Can I use it without calibrating?
It will draw, but not accurately. Browsers assume 96 pixels per inch, and most phones and high-resolution screens differ from that, often by 50% or more. Calibration takes half a minute.
How long can I measure?
Up to the length of your screen — usually 11 to 16 cm on a phone in full screen and around 30 cm on a laptop. Measure longer things in sections, or use a tape measure and convert units with the Length Converter.
How accurate is it?
Calibration moves in 0.5-pixel steps, about 0.1 mm on common screens; in practice accuracy depends on how carefully you line things up. It is fine for rings, screws and photos; use calipers for precision work.
Why do I have to calibrate again in another browser?
The calibration is saved in the current browser only. It is not shared between browsers or devices and is never uploaded.
Privacy
This tool runs entirely in your browser. Your files and text are processed on your device and are never uploaded to our servers.