Ruler calibration

Real length does not fall out of a resolution alone. 1920×1080 can be a phone or a television. The card fixes the scale. Until then, the page estimates it.

Match a card

1. Set zoom to 100% with Ctrl+0. Full screen helps.
2. Press Calibrate. Keep the 85.60 mm long edge selected. On a narrow phone choose the 53.98 mm short edge.
3. Hold the card flat on the screen. Drag the right edge of the blue frame until it meets the card edge. Minus and plus move one pixel.
4. Press Use this calibration. On the ruler, the long edge of the card should span 8.56 cm from zero.

From the screen size

Type the diagonal in inches from the device spec, plus the horizontal and vertical pixels. The site computes pixel density and divides by the device pixel ratio. This is less direct than the card, so check it afterwards.

When to do it again

You move to another monitor or phone
The system scale changes (125% or 150%)
The browser zoom changes
You clear site data

Which card works?

ISO/IEC 7810 ID-1 cards: bank cards, credit cards, and many national ID cards. The long edge is 85.60 mm and the short edge is 53.98 mm. A sleeve, a cut corner, or a warped card shifts the result.

Can software read the real size by itself?

Not exactly. The page estimates from known phone diagonals or from the CSS centimetre, and it does that as soon as the page opens. Only a physical object of known length, such as the card, measures this screen.

Where is the calibration stored?

Only in this browser’s local storage. Nothing is sent to the server. A private window or cleared site data means you match the card again.