Calibrate
For accurate results, calibrate once using the panel under the ruler - pick a credit card or any object you have, then adjust the PPI until it fits.
Free online tool · no sign-up
Measure anything, right on your screen
A real-size ruler in centimeters, millimeters, and inches. Calibrate it once to your display and it ‘just works’ - on your laptop, desktop, or phone.
cm & mm scaleinch scaleclick to markremembers your settings
Online ruler
…PPI (pixels per inch)
Calibrate to actual sizerecommended
Pick an object you have at hand. Place its left edge on the ruler’s 0 mark, then adjust the PPI until the object’s right edge lines up with the red band.
What is an online ruler?
An online ruler is a browser-based measuring tool that renders a centimeter/millimeter and inch scale at actual size, calibrated to the pixel density (PPI) of your screen. It needs no download or sign-up: open the page, calibrate once against any standard object, and use it like a physical ruler.
2.54cm in 1 inch - a 12-inch ruler is 30.48 cm
85.6mm - the exact width of any credit or bank card (ISO/IEC 7810 ID-1)
96PPI - the browser default; real screens range ~80-460, so calibration matters
How to use it
Measure
Place your object next to the screen. The top scale is centimeters and millimeters; the bottom scale is inches.
Mark & read
Hover to see the size under your cursor, or click to drop a pin that shows the measurement - handy when the object blocks your view.
Calibrate to actual size
The ruler guesses your screen’s PPI, but every display is a little different. A 30-second calibration makes it match a physical ruler.
- Take any object whose size you know - a credit card (85.6 mm), a banknote, or a sheet of A4 paper (210 mm wide).
- Select it in the calibration panel. The ruler now highlights a band matching that object’s real length.
- Hold the object against the screen, align its left edge to the 0 mark, and use the -/+ buttons until its right edge lines up with the red band.
- Press Save PPI - the setting is stored on your device and reused next visit.
Guides
Frequently asked questions
Is an online ruler accurate?
Yes - the ruler is drawn from your screen’s PPI, not from a fixed image. After a quick calibration it’s as accurate as the display allows (displays are rarely perfect, so a visible object reference is the best check).
How do I check that the ruler is correct?
Calibrate it against any standard object. If a credit card’s width matches its 85.6 mm band on screen, your ruler is set.
How do I read the centimeter and millimeter scale?
The top half of the ruler is metric. Long ticks are centimeters (numbered 0, 1, 2…); the short ticks between them are millimeters (10 per centimeter).
How do I read the inch scale?
The bottom half of the ruler is inches. Each inch is split into eighths, sixteenths, or thirty-seconds - pick a finer graduation to measure smaller increments.
How do I convert centimeters to inches?
Divide the number of centimeters by 2.54. For example, 10 cm ÷ 2.54 = 3.94 in. The ruler shows both units at once, so no conversion is needed.
Does the tool remember my settings?
Yes. Your PPI is saved in your browser’s local storage. You can reset it anytime with the “Restore” button.
Free, fast, and always at hand
No downloads, no accounts, no cost. Open it on any device and you have a ruler in your pocket. If you find it handy, share it with a friend.