Open the Online Ruler
A free on-screen ruler in centimetres, millimetres and inches. Calibrate it once with a credit card or banknote for true actual-size measurement.
- Free to use
- No signup
- Nothing to install
- Runs in your browser
A standard bank card is 85.6 mm wide. After calibration, dragging the handles across the card should read 85.6 mm (3.370 in, about 3 3/8 in).Open the Online Ruler
Illustration. Open the tool for the real thing.
What is the Online Ruler?
Most online rulers are wrong because a web page cannot know how big a pixel is on your particular screen. This ruler fixes that with a ten-second calibration: hold a bank card (85.6 mm wide), a dollar bill or an A4 sheet against the screen and stretch the on-screen shape until it matches. From then on the ruler is drawn to your real screen scale, in millimetres, centimetres and inches (down to 1/16 inch).
Two red handles let you measure the length of any small object you hold up to the screen: drag them to its ends and read the distance in millimetres, centimetres, decimal inches and fractions. Your calibration is saved in your browser, so you only do it once per screen.
3 steps to your result
Open the tool and follow along. Nothing to install.
What you get
Match a bank card, banknote or A4 paper for true actual size.
Millimetre ticks on top and 1/16 inch ticks on the bottom.
Drag handles to measure a length and read four units.
Calibration is saved in your browser.
AZRS tools are free to open and use.
It opens in your browser, so there is nothing to download.
Used for
Pick one to see how it fits.
How it compares
A plain look at this tool next to the usual alternative.
| This utility | Desktop software | |
|---|---|---|
| Install | Nothing to install | Usually an app or program to install |
| Account | No signup | Depends on the product |
| Cost | Free to use | Free or paid, depends on the product |
| Where it runs | In your browser tab | On your computer |
Questions people ask
How accurate is the online ruler?
After calibrating with a bank card, it is typically within about half a millimetre for small objects — good for quick checks, not for precision work.
Why do I need to calibrate?
Screens differ in pixel density, and a website cannot read that value, so a physical reference is the only reliable way to set the real scale.
How big is a credit card?
ISO 7810 ID-1 cards, including credit and debit cards, are 85.60 × 53.98 mm.
Does it work on a phone?
Yes, but the ruler is longer than the screen, so scroll it sideways; recalibrate in each orientation.
Is my calibration saved?
Yes, in your browser only.
More free tools
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Try the Online Ruler now
A real-size on-screen ruler you calibrate with a credit card. Free to use.
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