How to use an online ruler in cm, mm and inches

MeasureKite turns your screen into a horizontal and vertical ruler. Match its scale to a real object first, then measure small, flat objects without installing an app.

Calibrate before measuring

A browser cannot reliably determine your screen's physical size. A centimeter drawn using an estimated scale may not be a real centimeter. Calibration matters more than the number of decimal places in the result.

1. Open the ruler and select Calibrate.
2. Hold a standard payment card gently against the screen. Its long edge is 8.56 cm (85.6 mm). Do not enter or photograph any card details.
3. Adjust the slider until the outline's left and right edges match the card. If the card cannot fit on the screen, try landscape orientation or a larger display.
4. Select Aligned, start measuring. Check the scale against a physical ruler if one is available.

Using “estimated scale” is only an approximation. Recalibrate after changing displays or browser zoom. Avoid using this tool for work where measurement errors could cause harm.

Measure width and height

Place a small object gently against the screen. Align one edge with the ruler's zero mark, or drag from one corner to the opposite corner. The selection shows width × height in your chosen unit. On a touchscreen, use a finger to drag the measurement box.

Keep sharp, heavy or wet objects away from your screen. Do not press an object into the display. An on-screen ruler cannot measure the real-world size of an object inside a photograph unless that image has a known scale.

Switch between centimeters and inches

Use the cm, mm and in buttons. Switching units changes the readout, not the object's physical size.

Common questions

Is this an actual-size ruler?

Only after you have correctly calibrated it for the current display and zoom level. Screen scaling, alignment and object placement can all affect the result. The displayed decimal places are not a guarantee of accuracy.

Can I use it on a phone?

Yes. Calibration, units and theme controls are available on small screens. Some reference sizes may be too large to fit; MeasureKite warns you instead of shrinking them. Fullscreen availability depends on your browser.

Why does a ruler look different on another monitor?

Monitors have different pixel densities and operating-system scaling. A saved calibration belongs to the setup on which it was made. Calibrate again after moving the window to another screen.

What are reference sizes and recent measurements?

Reference sizes draw guides for a payment card, a photo or one inch. Recent measurements let you recall your last four selections during the current session. They are not a permanent measurement log.

What is stored?

The ruler stores calibration, language and theme preferences in this browser when storage is available. Measurement calculations run locally. The site also includes third-party advertising; those services may receive browser and connection information. Do not treat local measurement processing as a promise that the entire website makes no network requests.