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About this tool
Actual-size on-screen ruler in centimetres, millimetres and inches, calibrated with a bank card, coin or screen size, with a draggable caliper.
This online ruler draws a centimetre scale with millimetre ticks along its top edge and an inch scale with 1/16-inch ticks along the bottom. Because every screen packs pixels differently, a web page cannot know the physical size of a pixel on its own — so you calibrate it once: match an on-screen outline to a bank card (85.60 mm wide), a coin or a sheet of A4, or enter your screen's diagonal and the ruler computes its pixel density from the screen resolution. The calibration is saved in this browser. Drag the two caliper jaws to measure a span, rotate the ruler vertically, or go fullscreen for longer objects.
Open Online Ruler on AltFTool — it loads instantly in your browser.
Press Calibrate and choose Bank card: hold a card (85.60 × 53.98 mm) against the screen and use the slider and + / − buttons until the outline matches, then press Save calibration.
No card? Use Coin or paper to match a US quarter, 1 euro coin, ₹10 coin or A4 sheet, or Screen size to enter your diagonal in inches.
Line your object up with the 0 mark, then drag the caliper jaws — the Caliper span shows the length in cm, mm and inches.
Switch Horizontal or Vertical (V key), choose cm / mm or inches for snapping (U key), and use Copy measurement to copy the result.
Three ways to calibrate — a bank card, a coin or paper of known size, or your screen diagonal — with fine adjustment down to 0.005 pixels per millimetre.
Centimetres and millimetres on one edge, inches in sixteenths on the other, and a read-out that gives the caliper span in mm, cm and inches with the nearest fraction.
Two jaws mark the start and end of what you measure. Drag them with mouse or finger, or nudge them by 1 mm or 1/16 inch with the arrow keys.
The page tells you when it is not calibrated, warns if your screen or zoom changed since calibrating, and remembers your calibration only on this device.
| Object | Millimetres | Centimetres | Inches |
|---|---|---|---|
| Bank / credit card (long side) | 85.60 | 8.56 | 3.370 |
| Bank / credit card (short side) | 53.98 | 5.398 | 2.125 |
| A4 paper (short side) | 210 | 21.0 | 8.27 |
| US Letter paper (short side) | 215.9 | 21.59 | 8.50 |
| US dollar bill (long side) | 155.96 | 15.60 | 6.14 |
| US quarter (diameter) | 24.26 | 2.426 | 0.955 |
| 1 euro coin (diameter) | 23.25 | 2.325 | 0.915 |
| UK £1 coin (diameter) | 23.43 | 2.343 | 0.922 |
| India ₹10 coin (diameter) | 27.00 | 2.700 | 1.063 |
Coin sizes vary between series and issue years; a bank card is the most consistent reference.
It can be accurate to about a millimetre over short lengths once it is calibrated, but never without calibration. Browsers assume 96 pixels per inch, while real screens range from about 90 to over 450, so an uncalibrated on-screen ruler is often 10 to 40 percent off. Calibrate with a bank card, and check against a physical ruler when precision really matters.
Press Calibrate and choose Bank card. Any standard bank, ID or loyalty card is 85.60 by 53.98 millimetres. Hold it flat against the screen in the top-left corner of the outline, then move the slider and the + and − buttons until the outline's edges line up exactly with the card's edges. Press Save calibration.
Calibrate this on-screen ruler with a card or coin, then hold the object against the screen at the 0 mark and drag the caliper to its far end. Other quick references: a standard bank card is 8.56 cm long, an A4 sheet is 21 cm wide, and a US dollar bill is about 15.6 cm long.
One inch is exactly 2.54 centimetres, or 25.4 millimetres, by international definition since 1959. So 1 centimetre is about 0.394 inches. On this ruler the inch scale on the bottom edge and the centimetre scale on the top edge line up, so you can read the conversion directly.
On the centimetre edge, every small tick is 1 millimetre, the medium tick is 5 millimetres and the numbered long tick is each centimetre. On the inch edge, the longest numbered ticks are whole inches, then come half inches, quarters, eighths and the shortest ticks at sixteenths. The caliper read-out gives the nearest sixteenth for you.
Yes. Choose Screen size and enter the diagonal in inches — it is printed in your device's specifications. The ruler divides the diagonal of the screen resolution reported by the browser by that number. It works when the browser zoom is 100% and the operating system is not scaling the display; otherwise the card method is more reliable.
Yes. Phones have very dense screens, so calibration is essential — the card method works well because a card fits across most phones held sideways. The horizontal ruler is limited by the screen's width, so switch to Vertical to measure longer objects along the length of the phone.
Browser zoom changes how many screen pixels make up one CSS pixel, so the calibration no longer matches. Reset the zoom to the level you calibrated at, or calibrate again. The ruler warns you when it detects that the screen resolution or pixel ratio is different from when you calibrated.
Standard credit, debit and ID cards follow the ISO/IEC 7810 ID-1 format: 85.60 × 53.98 millimetres, which is 8.56 × 5.398 centimetres or about 3.37 × 2.125 inches. Because almost everyone carries one, it is the most convenient reference for calibrating an on-screen ruler.