Actual Size Ruler

A real, calibrated on-screen ruler. Match a bank card once to set your screen's true size, then measure accurate centimeters and inches — and drag across the ruler to measure any length.

How to use it

First calibrate: hold a bank card to the screen and size the box to match. That's stored, so you only do it once per device. Then the ruler shows true centimeters along the top and inches along the bottom. Lay a small object against the screen to measure it, or click and drag across the ruler to measure a span — the readout shows cm, inches and mm.

One calibration, many tools. Your screen size is shared with the Ring Sizer and Eye Test, so calibrate once and they're all accurate. Nothing is uploaded — it runs entirely in your browser.

Why most online rulers are wrong

A ruler drawn at a fixed number of pixels only looks right on the screen it was designed for. A 4K laptop, a budget monitor and a phone all pack pixels at different densities, so the same image is a different physical size on each. Calibrating to a fixed-size real object — the humble bank card — is the only way to get it right, and it's exactly what opticians' apps do.

Frequently asked questions

How can an online ruler be accurate?

By calibrating to physical size. You match a fixed-width bank card, which gives true pixels-per-mm for your display, so the scale is correct.

Do I have to calibrate every time?

No — it's saved in this browser and shared with the ring sizer and eye test. Recalibrate only if you change screens or zoom.

Does it work on phones?

Yes, calibrate the same way. Avoid browser zoom, which changes the scale.

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