About the Online Ruler
Screens vary so much in size and sharpness that a ruler on one cannot be correct on another without help. This online ruler fixes that by letting you calibrate it. A box the size of a standard bank card, 85.6 millimetres wide, appears on the screen: hold a real card against it and drag the slider until they match. From that moment, the centimetre and inch scales are drawn at the right size on your own screen, and the setting is saved in your browser. Hold a small object against the display, such as a coin, a key or a screw, to measure it. It suits short lengths, and it will not replace a proper tape for large jobs.
Updated October 2026
How to use the Online Ruler
- 1Choose centimetres or inches.
- 2Press Make it exact, and hold a bank card against the outline on the screen.
- 3Move the slider until the outline is the same width as your card.
- 4Close the calibration, and use the ruler to measure objects against the screen.
- 5Scroll sideways if the ruler is longer than the screen.
How to calibrate
- Open the calibration panel.
- Hold the card flat against the screen, over the dashed box.
- Slide until the box edge matches the card edge.
- Check the scale against a real ruler if you have one.
- Close the panel. The setting is remembered.
Why it needs calibration
Web pages are drawn in CSS pixels, and a CSS inch is defined as 96 pixels, but real screens have different pixel densities, and phones and browsers scale them again. The result is that a ruler drawn at 96 pixels per inch can be too big or too small. A card has a known size, so it provides a reliable reference on any display.
Using a screen ruler takes a steady hand. Place the object flat against the glass and line up one edge with the zero mark, then read the other edge from straight above to avoid parallax errors caused by the thickness of the glass. If your phone has a screen protector, it adds a little distance, so look straight down on the object. For tiny items, read to the nearest half millimetre, and repeat the reading to confirm it before you use it to order a part.
Other ways to measure
Use a physical ruler or tape wherever you can. A printed ruler from a trusted source at 100 percent scale also works, if you check its length against a real ruler. For phones, many have a measuring app that uses the camera to measure distances, which works for larger objects but is less precise than a card calibrated ruler for small ones.
Related tools
The Inches to CM Converter and MM to Inches Converter change units, the Screen Size Checker shows your display's size, the Unit Converter handles other lengths and the Image Resizer sets pictures to exact sizes.
Features
- Centimetre and inch scales
- Calibration with a standard bank card
- Setting saved in your browser
- Millimetre and sixteenth-inch ticks
- Works on phones and computers
- Free
Frequently asked questions
How accurate is an online ruler?
If you calibrate it with a card, it is usually within about 1 percent, which is a millimetre or so over a few centimetres. Without calibration it can be off by 10 percent or more, because pixel sizes differ between screens.
What card should I use?
Any ISO ID-1 card: bank cards, credit cards, driving licences and many ID cards are 85.60 by 53.98 millimetres. Do not use a thick or bent card, and do not bend the screen.
Will it stay accurate if I zoom the page?
No. Zooming changes the size of everything, including the ruler. Calibrate again after changing the zoom, or set the zoom back to 100 percent.
Can I use it on my phone?
Yes. Calibrate on the phone itself, and rotate to landscape for a longer ruler. The scale saved is for that browser on that device only.
What can I measure with it?
Small things that you can lay flat on the screen: a coin, ring, screw, button, stamp or photo corner. Take care not to scratch the display. For anything that is awkward to hold, measure with a tape.
Why is the ruler only 40 centimetres long?
That is longer than most phone and tablet screens, and you can scroll across it. If your screen is large, you will see only part of it at once. For long measurements use a tape measure.