Online Ruler (Actual Size): cm, mm and Inch Ruler
This online ruler shows centimetres, millimetres and inches on your screen. Calibrate it once with a bank card or a US quarter so the scale matches real life, then slide the line to measure, switch to a vertical ruler, or print a true-size ruler on paper.
Step 1. Calibrate once. Hold the object against the screen and move the slider until the outline is exactly as wide as the real thing.
Step 2. Measure. Slide the line along the ruler, or tap a quick button.
Measure with the ruler in three steps
- Calibrate. Pick a bank card or a US quarter, hold it against the screen and move the scale slider until the dashed outline is exactly the size of the object.
- Measure. Slide the orange line along the ruler. The readout gives millimetres, centimetres and inches, and the nearest sixteenth of an inch.
- Use the quick buttons to jump to 1 to 6 inches, switch to the vertical ruler, or print a true-size ruler. See also the time now clock for another quick tool.
Why on-screen rulers are wrong without calibration
A web page measures in CSS pixels, and the CSS standard defines 96 pixels to an inch. That is a convention, not the real size of a pixel. Real screens pack different numbers of pixels into an inch, and zoom and scaling change it further. The table shows the pixel density of four sample screens, worked out from their resolution and diagonal.
| Screen | Pixel density | One physical pixel is |
|---|---|---|
| 24 inch monitor, 1920 × 1080 | 92 ppi | 0.277 mm |
| 27 inch monitor, 2560 × 1440 | 109 ppi | 0.233 mm |
| 13.3 inch laptop, 1920 × 1080 | 166 ppi | 0.153 mm |
| 6.1 inch phone, 2532 × 1170 | 457 ppi | 0.056 mm |
Reference objects of known size
Use something you can hold against the screen. A bank card is the easiest, because its size is fixed by an international standard. A US quarter is a handy second choice.
| Object | Size in millimetres | Size in inches | Source |
|---|---|---|---|
| Bank card | 85.60 × 53.98 mm | 3.370 × 2.125 in | ISO/IEC 7810 ID-1 |
| US quarter | 24.26 mm across | 0.955 in | US Mint |
| A4 paper | 210 × 297 mm | 8.27 × 11.69 in | ISO 216 |
| US Letter paper | 215.9 × 279.4 mm | 8.5 × 11 in | US paper size |
Print a true-size ruler
Printing is more reliable than the screen, because paper has a fixed size. The Print true size button draws the ruler in millimetres and inches at their real length. Print at 100% scale, turn off fit to page, and check the result against a card before you rely on it.
Limits
- Calibration is saved in your browser only, so each device needs its own.
- Changing browser zoom or moving the window to another screen changes the scale. Calibrate again.
- The ruler can be off by a millimetre or more, so it is not suitable for work that needs precision.
Accuracy and device check
We checked the ruler in Chrome on a computer and have not yet tested it on iPhone Safari or Android Chrome. Tell us how it works on your device at [email protected]. Back to OxoApp.
Frequently asked questions
Why must I calibrate an on-screen ruler?
Screens have different pixel densities, and browser zoom and display scaling change the size of a pixel. A ruler that assumes 96 pixels to the inch is wrong on most screens. Calibration measures your own screen against an object of known size.
How accurate is it?
It depends on how carefully you match the outline to the object. It can be off by a millimetre or more, and we have not measured it on every device. Do not use it where precision matters.
What is the size of a bank card?
A standard bank card is 85.60 mm wide and 53.98 mm tall, which is 3.370 by 2.125 inches. The size comes from the ISO/IEC 7810 ID-1 standard.
How do I get a real-size ruler on paper?
Press Print true size and print at 100% scale, with no fit to page. Check the printed ruler against a real ruler or a card before you use it.
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