Method 3 · Mobile & display
Hold a bank card to the screen and match the bar to its edge, and this screen ruler becomes accurate for your exact display, in inches and centimetres.
Runs in your browser. Nothing is uploaded.
Hold a bank card to the screen along the grey bar and drag the slider until the bar matches the card edge exactly.
Your browser zoom or display scaling changed since calibration. Calibrate again for accurate results.
Hold an object against the screen, lining it up with the zero edge.
Centimetres on the top edge, inches on the bottom edge. Millimetre and eighth-inch ticks included.
How it works
FAQ
Browsers cannot know the physical size of your pixels. Matching a card of known size gives the exact scale.
Within about a millimetre for most screens when calibrated carefully. Calibrate again if you change zoom or display resolution.
Yes, once it is calibrated. Hold a bank card against the screen and match the on-screen bar to its edge. The ruler then knows the real size of your pixels and is accurate to about a millimetre on most screens, in both centimetres and inches.
20 cm is about 7.87 inches, roughly the length of a standard school ruler's first two thirds, or a bit shorter than an A4 sheet's long edge (29.7 cm). Calibrate this ruler with a bank card, then measure 20 cm on the screen to see the true size.
Yes. This page shows a horizontal and a vertical ruler on your screen in centimetres and inches. Calibrate it first with a bank card so the marks match real-world size, then use it to measure small objects held against the screen.
Yes. Open this page on your phone, calibrate with a bank card and hold small objects against the screen. It is handy for items shorter than your screen, such as a coin, a ring or a small part. For anything longer than the screen, use a physical tape measure.
Guide
This screen ruler windows users can open in any browser needs no download. Browsers cannot know how large your pixels are, so you calibrate once by matching the bar to the edge of a bank card. After that the ruler is accurate for your display, and your setting is saved on the device.
The top edge of the horizontal ruler shows centimetres with millimetre ticks, and the bottom edge shows inches with eighth-inch ticks. That makes it a handy screen ruler for inches, for checking small objects, jewellery, or print sizes. The estimator also converts your calibration into the diagonal size of your screen.
Calibrate again whenever you change browser zoom or display scaling, because both change the size of a pixel. On Windows, display scaling of 125 percent or 150 percent will change the result, so recalibrate after altering it. For display faults, use the dead pixel test.
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