Online Ruler
Match the bar to a bank card to calibrate, then measure in cm, mm or inches.
A web page cannot know how big your screen really is, so this ruler needs one quick calibration. Hold any standard bank card (85.6 mm wide) flat against the screen and drag the slider until the bar below matches its long edge.
Click or drag on the ruler to place a marker. Your calibration is remembered in this browser, and nothing is uploaded.
What this tool does
A web page cannot tell how large your screen really is, so this ruler is calibrated with a bank card: hold a standard card (85.6 mm wide) against the screen and drag the slider until the bar is as wide as the card. The ruler then shows true millimetres, centimetres and inches, and you can click it to measure.
Draws a ruler scaled to your screen from a one-step calibration.
Measures a distance from the left edge in mm, cm and inches.
How to use it
- Hold a bank card flat against the screen.
- Drag the calibration slider until the bar matches the card's long edge.
- Choose centimetres or inches.
- Click or drag along the ruler to place a marker and read the length.
Why use this tool
Calibrates to your own screen using a standard bank card, 85.6 mm wide, so the centimetres and inches are real and not a guess based on a nominal 96 dpi. Browsers cannot read a screen's physical size, so most online rulers are wrong by a surprising amount. Matching a known object, a bank card of a fixed international size, gives a ruler that is accurate for your own display.
Why we built it
We built the online ruler to calibrate to your own screen with a bank card, so its centimetres and inches are real. Convert units with the cm to inches converter and check your display with the PPI calculator.
Who this tool is for
- People measuring small objects
- Crafters, jewellers and hobbyists
- Teachers showing units of length
When to use it
- Measuring a small object when you have no ruler.
- Checking the size of a part, a jewellery piece or a print.
- Teaching units of length.
- Comparing a size to a real-life object.
How it works
Bank cards follow ISO/IEC 7810 ID-1, which is 85.60 mm wide. When the on-screen bar equals the card's width, the number of pixels in the bar divided by 85.6 gives the pixels per millimetre. The ruler uses that scale for its ticks, and a click at x pixels reads x ÷ pixels-per-mm millimetres. There are 25.4 mm in an inch.
Example: if the bar is 342 pixels wide when it matches the card, the scale is 4.0 pixels per millimetre, so a marker 120 pixels from the edge reads 30.0 mm, or 1.18 inches.
Accuracy depends on how carefully you match the card and on your screen being viewed straight on. Browser zoom changes the scale, so calibrate again if you zoom.
What you give it, and what you get back
| Item | Details |
|---|---|
| Calibration | Drag the slider until the bar matches an 85.6 mm bank card. |
| Unit | Centimetres with millimetres, or inches with sixteenths. |
| Output | A ruler on screen and a marker readout in mm, cm and inches, with a copy button. |
Issues you may run into
| What you see | Why it happens | What to do |
|---|---|---|
| The ruler is wrong after zooming | Browser zoom changes pixel sizes. | Recalibrate at the zoom level you are using. |
| Different on my other screen | Each screen has its own size. | Calibrate again on each display. |
| No card to hand | Any item of known width works. | Use a card, then check against a real ruler. |
Assumptions and limitations
- Accurate only after calibration.
- Limited to the width of the browser window.
Frequently asked questions
How do I calibrate the ruler?
Hold a standard bank card against the screen and drag the slider until the bar is exactly as wide as its long edge.
How wide is a bank card?
85.6 mm (3.37 inches), the ISO ID-1 standard.
Is it accurate?
It can be within about a millimetre if you calibrate carefully, but it is not a precision instrument.
Will it remember my calibration?
Yes, in this browser. Zooming the page or using another screen needs a new calibration.
Can I measure in inches?
Yes. Switch the unit to inches.
Is anything sent to a server?
No. Everything runs in your browser and nothing leaves the page.