Turn your screen into an accurate ruler. Calibrate it once with any card in your wallet, then measure anything you can hold up to the glass.
Take any card from your wallet — bank card, ID, gift card, they're all the same size — and drag the slider until the yellow outline matches its width exactly.
Hold the object against the left edge of the ruler (or the top edge, in Long mode) so its starting point touches the zero mark.
Drag the red line to the object's other end. The measurement updates live above the ruler, in centimeters or inches.
Want the full picture? Read our complete measuring guide, including when to use Tracer and Diagonal mode.
As accurate as your calibration. Every screen packs pixels at a different density, so a ruler that assumes a fixed number of pixels per inch is wrong on most devices. Calibrating against a real card fixes that for your specific screen, typically to within a millimeter. Skipping calibration falls back to an estimate that can be off.
Bank cards, ID cards, and most gift or membership cards are all manufactured to the same international standard size (85.6 × 53.98mm). That makes them a reliable, everyday reference object — almost everyone has one within reach, so there's no need to dig out an actual ruler just to calibrate a digital one.
Yes. Use "Wide" mode for short, flat objects and "Long" mode to make use of a phone's full screen height for longer items. Calibration is saved on your device, so you only need to do it once per device.
Yes. Grab the grip bar at the top of the ruler — the one marked "Drag to move" — and drag it anywhere on the page. This is useful when the object you're measuring isn't near the top of the screen, or when you want to keep the ruler in view while scrolling. A "Reset position" link appears once you've moved it, to snap it back into place.
Yes. There's a small strip at the far end of the ruler — the right edge in Wide mode, the bottom edge in Long mode. Drag it to extend or shrink the ruler in real time. A "Reset length" link appears once you've resized it, to return to the default size.
Your calibration value is stored only on your own device and isn't shared or linked to any account. Nothing about the object you're measuring is ever recorded.
It's built for quick, everyday measuring — furniture, packages, craft projects — not for precision or safety-critical work. For anything that needs guaranteed accuracy, use a physical ruler or tape measure.