Need to measure something in centimeters? Our free online ruler displays accurate CM markings directly on your screen. After a one-time calibration, you get true-to-life centimeter and millimeter measurements on any device.
Open CM Ruler →The centimeter ruler displays tick marks at every millimeter (1mm), with medium ticks at 5mm intervals, and large numbered ticks at each full centimeter. This mirrors a physical metric ruler exactly. Each centimeter contains 10 equal millimeter divisions for precision down to 0.5mm.
To use, click "cm" in the toolbar or press the "1" key. Then select which screen edge to place the ruler on — top, bottom, left, or right. Place your object flat against the screen edge and read the measurement from the numbered markings.
Every screen has a different pixel density (PPI). Without calibration, 1cm on screen might actually be 8mm or 12mm physically. Our 3-method calibration (auto-detect, screen diagonal, credit card) ensures that when the ruler shows 1cm, it truly IS 1cm on your specific display.