How to Calibrate a Screen Ruler With a Credit Card
A screen ruler cannot know how big your screen is. A credit card fixes that, because every bank card in the world has the same size.
Why a credit card works
Bank cards, debit cards, driving licences in most countries and most loyalty cards follow the ISO/IEC 7810 ID-1 standard: 85.60 x 53.98 mm, or 3.370 x 2.125 inches. When the on-screen outline matches the card, the ruler knows exactly how many pixels make one millimeter on your screen.
Step by step
- Set browser zoom to 100% (Ctrl+0 or Cmd+0).
- Open the online ruler and press Calibrate. The Card tab opens with a dashed outline.
- Hold the card flat against the screen, with its top left corner on the top left corner of the outline.
- Drag the slider until the right edge of the outline lines up with the card edge. Use the arrow keys for 0.1 steps when you get close.
- Check the bottom edge too. If width and height both match, press Save.
The value is saved in your browser. Next time you open the ruler it is already calibrated.
Check your calibration
Measure the card with the ruler itself. It should read 8.56 cm or 3 and 3/8 inches. A coin also works: a US quarter is 24.26 mm and a 1 euro coin is 23.25 mm.
No card nearby?
- Device tab: pick your phone, tablet, laptop or monitor model.
- Diagonal tab: type your screen size in inches. The screen size calculator explains the math.
- Other ID-1 cards: library cards, gift cards, hotel key cards and many ID cards are the same size.
When to calibrate again
- You changed browser zoom or Windows or Mac display scaling.
- You moved the window to a different monitor.
- You switched browser or cleared browser data.