Actual Size Ruler
A real-size on-screen ruler. Calibrate it once to your screen and it stays accurate — measure in cm, mm or inches on desktop, or go full-screen to use your phone's edge as the ruler.
Calibrate for exact accuracy — screens vary in pixel density.
Come funziona
Screens differ in pixel density, so a ruler drawn at a fixed number of pixels is the wrong size on most devices. This tool fixes that with a one-time calibration: hold a standard credit or ID card (ISO ID-1, 85.60 mm wide — the same everywhere in the world) against the screen and slide the control until the on-screen outline matches it exactly. From then on every tick is drawn at its true physical size, and the calibration is remembered in your browser.
Switch between centimetres, millimetres and inches at any time. On a computer the ruler runs across the page; on a phone, tap Full screen to turn the long edge of your screen into the ruler — perfect for measuring something you can lay against the edge in portrait or landscape.
Esempi
- Calibrate once with a bank card, then the ruler is accurate on that device forever.
- Switch to inches for imperial measurements with 1/16" ticks.
- On a phone, go full screen and measure along the edge in portrait or landscape.
Domande frequenti
- Why do I need to calibrate?
- Because CSS pixels do not correspond to a fixed physical size — pixel density varies by device. Calibrating against a credit card (a worldwide standard size) makes the ruler accurate to your specific screen.
- How accurate is it?
- After calibration it is accurate to within the precision of your slide adjustment — typically well under a millimetre. Without calibrating, it uses a best-guess default that may be off by 10–30% on high-density screens.
- Does it remember my calibration?
- Yes. Your calibration and unit choice are saved in your browser's local storage, so the ruler stays correct the next time you open it on that device.
- Can I use my phone's edge as the ruler?
- Yes. Tap Full screen and the ruler is pinned to the long edge of your screen — vertical in portrait, horizontal in landscape — so you can lay an object against the edge and read its size.