About MM Ruler Online
MM Ruler Online is a free tool that turns your screen into a ruler, so you can measure small things in millimeters, centimeters and inches without a physical ruler.
What This Site Does
You open the mm ruler online, calibrate it once with a credit card, and measure objects by holding them against the screen. It runs entirely in your browser on a phone, tablet or computer. There is nothing to install and no account to create.
We also offer a printable mm ruler you can download as a PDF, and a PPI calculator that helps you calibrate the ruler.
How It Works
A website cannot know how big your screen really is, so the ruler asks you to match it to an object of known size, such as a credit card (85.6 mm wide). Once calibrated, it draws every millimeter at its real size. Your calibration is saved only in your own browser.
Accuracy and Limits
After calibration the ruler is typically accurate to about 1%, but screen glass, cases and viewing angle add small errors. It is a convenient tool for everyday measuring, not a replacement for a calibrated instrument. Please do not rely on it for medical, safety or engineering decisions.
Questions or Feedback
If you find a bug, have a suggestion or noticed an accuracy problem on a particular device, we would like to hear about it. Visit the contact page. You can also read our privacy policy to see what we store, which is very little.