Screen Ruler & Measure

An on-screen ruler you can calibrate with a real card, plus a pixel ruler for measuring anything on the page.

1. Calibrate with a real card

Place a standard card (bank card, ID card: 85.60 × 53.98 mm) against the outline below and move the slider until the outline matches the card exactly. The calibration is saved in this browser and is also used by the Vision Check tool.

Outline width: 324 px · Not calibrated — assuming 96 DPI, which is approximate and can be off by 20% or more.

2. Screen ruler

3. Drag to measure

Press and drag inside the strip below to measure a distance on the screen. Distances are exact in pixels; millimetres depend on the calibration above.

A screen ruler can only be as accurate as its calibration. Different monitors, browser zoom and display scaling all change the physical size — recalibrate after changing any of them, and do not use this for machining or construction tolerances.

How to use

Hold a standard card (85.60 × 53.98 mm, such as a bank card) against the on-screen outline and move the slider until they match exactly.
Once calibrated, the ruler and the drag measurement show millimetres that match the real world.
Drag inside the measuring strip to read a distance in pixels and millimetres.

Notes

A browser does not know your screen's physical size, so accurate millimetres are impossible without calibration. Until you calibrate, the page assumes 96 DPI and labels that as approximate. The calibration is stored in this browser's localStorage and is also used by the Vision Check (vision.yunedia.com) to size its optotypes. Recalibrate after changing browser zoom or display scaling.

FAQ

What if I have no card to calibrate with?: Pixel measurements work as-is. Millimetre values then rely on the 96 DPI assumption, so treat them as a rough guide only.
Is calibration per device?: Yes. The value is stored per browser, so each device and browser needs its own calibration.