Online Ruler in MM: Measure in Millimeters

Need millimeter precision without a physical ruler? This online ruler in mm is ideal for small or detailed measurements such as jewelry, electronics components, screws, or craft materials. It's calibrated to your device so the millimeter markings scale to true size.

If your device isn't recognized, calibrate manually with your screen's diagonal size or a standard credit card, 85.6 mm wide.

How to Use the Online Ruler

1. Choose an edge — click Left, Right, Top, or Bottom above to pin the ruler to that edge of your screen.
2. Switch units — use the "Also measure in" links to open the cm, inch, mm, or px version of this ruler.
3. Check the status banner — green means your device was auto-detected and the ruler is accurate. A yellow banner means calibration is needed.
4. Calibrate if needed — click Calibrate and pick your device, enter your screen's diagonal size, or use a standard credit card.
5. Measure — place any object against the ruler edge. Hover for a live coordinate readout. Use Fullscreen for maximum measuring area.

How to Calibrate for Accurate Measurements

Accuracy depends on your screen's PPI being correctly set. There are three reliable ways to calibrate:

1. Pick your device — select your exact phone, tablet, or laptop model for the factory-precise PPI.
2. Screen diagonal — find your screen's diagonal size (printed on the box or searchable by model name) and enter it in the Calibrate panel. Works for any screen.
3. Credit card calibration — place any standard bank or credit card flat against the blue guide. All ISO 7810 cards are exactly 85.6 mm wide. Drag the slider until the guide matches the card edge.

Your calibration is saved in your browser and restored on your next visit.