How to use the online ruler
- Take a credit card, debit card or any card the standard ID-1 size (85.60 mm by 53.98 mm). Hold it flat against your screen next to the green box.
- Drag the Card width on screen slider, or use the Narrower and Wider buttons, until the long edge of the box matches your card exactly.
- Choose Inches or Centimeters, and Horizontal or Vertical.
- Click or tap the ruler to drop a red marker. The reading below shows the length from the start of the ruler to the marker.
- Your calibration is remembered in this browser. Redo it if you change browser zoom, resolution or monitor.
Why calibration is needed
Web pages measure in CSS pixels. By definition there are 96 CSS pixels in a CSS inch, but the device does not know how big its physical pixels are, and browser zoom, display scaling and different screens all change what an inch looks like. So a ruler drawn from the 96 per inch rule can be noticeably wrong.
The fix is to compare against something of known size. A bank card is made to the ISO/IEC 7810 ID-1 size, 85.60 by 53.98 millimeters, which is nearly universal. Once the green box matches your card on your own screen, the tool knows how many pixels make one millimeter, and every tick mark is placed from that number.
How the ruler is drawn
The metric ruler has a mark every millimeter, a longer one every 5 millimeters and a numbered one every centimeter. The inch ruler has a mark every sixteenth of an inch, longer ones at eighths, quarters and halves, and a number at every inch. The marker reading is computed from the pixel position and your calibration, and is shown in both units.
On a phone, switch to Vertical to use the longer screen dimension. A ruler can never show something longer than the screen itself, so measure larger objects in sections and add them up, or use a real tape measure.
Limits you should know
This is a convenient estimate for small items such as a coin, a screw, a photo or a ring size sketch. After calibration it can be close near the length of the card, and the error can grow farther from that length. Screens are not perfectly uniform, and curved or very wide displays distort more. Do not rely on it for fitting, medical, engineering or legal measurements.
The page never uses your camera. Nothing is uploaded, and the only thing stored is the calibration number, in your own browser.
Assumptions and limits
- The reference card is the ISO/IEC 7810 ID-1 size, 85.60 mm by 53.98 mm, which most bank and ID cards follow. Measure yours if it looks different.
- Calibration is by the long edge. The slider allows widths from 120 to 900 CSS pixels.
- The browser default assumes 96 CSS pixels per inch and is only right if your display scale happens to match.
- The calibration is saved in this browser only. A change in zoom or screen resolution requires redoing it.
- Accuracy depends on how carefully you match the card, and it is not guaranteed at other lengths.
Frequently asked questions
How accurate is an online ruler?
It is only as accurate as your match to the card. Careful matching can get close near the card width, but errors grow with distance and on unusual or curved screens, so use it for rough measurements only.
Why does my ruler look the wrong size before I calibrate?
A web page cannot read the physical size of your screen, so it assumes 96 pixels per inch. Zoom levels and display scaling change that, which is why the box rarely matches a real card until you adjust it.
What if I do not have a credit card?
Any card of the standard ID-1 size will do, such as a driver license, a student card or a gift card. The box is sized for 85.60 mm, so only use a card you know has that long edge.
Does the ruler work on a phone or tablet?
Yes. Calibrate the same way with a card held to the screen. Use the vertical mode for a longer ruler, and recalibrate if you rotate or change the display size setting.
Does the page use my camera or store anything about me?
No camera is used. The only thing saved is the card width number, in this browser, so you do not repeat the setup. For conversions between units see the unit converter.