Online Measure ruler | Measure Accurately and Easily
The Online Measure Ruler lets you measure objects on your screen accurately in inches or centimeters. Align your screen with the object for instant measurements on any device.
Calibrate this screen
A browser is never told how big your screen physically is, so no ruler can be accurate until you tell it. This takes about ten seconds and is remembered on this device.
Hold any bank, credit or ID card flat against the screen, then drag the slider until the picture is exactly the same width as the real card.
Every one of those cards is 85.60 mm wide worldwide, which is what makes this reliable. A driving licence or an old hotel key card works just as well.
No card to hand? Type the diagonal size of your monitor or laptop screen instead. It is usually printed on the box, in the model name, or in your system display settings.
Print a paper ruler instead
This one needs no calibration at all. Printers work in real physical units, so as long as you print at 100% and not fit to page, the paper ruler comes out exact: 19 cm and 7 inches on one strip.
How to use the online ruler
The ruler measures real lengths against your screen, so it has to be told the size of that screen once. Calibrate, then drag the marker to measure anything you can hold against the glass.
- Hold a bank card flat against the screen and drag the slider until the picture matches its width exactly.
- Check the green line: it tells you the screen size that implies, which is a quick way to spot a mistake.
- Press Done, then hold the object you want to measure against the ruler.
- Drag the red marker to the end of the object, or nudge it with the arrow keys, and read the length.
- Use Rotate ruler for tall objects and Full screen for longer ones.
Why calibration is not optional
A web page has no way to find out the physical size of your display. It is often claimed that a browser can measure this, usually by drawing a box one CSS inch wide and reading its width back, but CSS defines one inch as exactly 96 pixels regardless of the hardware, so that measurement always returns the same number and tells you nothing. Device lists have the same problem from the other side: a phone quoted at 460 pixels per inch reports only about 153 to a web page, because it packs three hardware pixels into every CSS pixel. The only honest way is to compare against something whose real size is known, which is why this tool asks for a bank card.
How accurate is it
Once calibrated it is as accurate as your eye was when you matched the card, which in practice is within about a millimetre over a 10 cm span. You can check it: measure the card itself and it should read 8.56 cm. Two things still limit it. Browser zoom changes the size of everything, so recalibrate if you have zoomed. And a curved or very glossy screen makes it harder to line an object up squarely. For anything where a millimetre matters, print the paper ruler instead, since that one is exact by construction.
Key features
Best use cases
Frequently asked questions
Often used together with: Aspect Ratio Calculator · What’s my screen size?