Online ruler (actual size)

Browsers don't know the physical size of your screen's pixels. CSS assumes 96 pixels per inch, but real displays range widely: a phone may pack well over 400 pixels into an inch, a large monitor fewer than 100. Display scaling settings change things further. Without calibration, the ruler is only an estimate.

Calibration fixes this by comparing the screen with an object of known size. The tool uses a standard ID-1 card, the size of credit cards, debit cards and most driver's licenses: 3.370 × 2.125 inches (85.60 × 53.98 mm), defined by the ISO/IEC 7810 standard. Once you match the rectangle to a card, the tool knows exactly how many pixels make an inch on your display.

1. Set browser zoom to 100% (Ctrl + 0 or Cmd + 0).
2. Click Calibrate with a credit card.
3. Hold the card flat against the screen, with its long edge on the left edge of the rectangle.
4. Drag the slider until the rectangle is exactly as wide as the card.
5. Click Save.

The calibration is stored in this browser, so next time the ruler is ready to use. If you change the zoom level, display resolution or scaling, or move to another monitor, calibrate again. A card with its magnetic stripe or chip side toward you is easiest to line up.

Any card in the standard ID-1 size works: a debit card, a gift card, a hotel key card, a library card or a US driver's license. A dollar bill is another option for a rough check: US paper currency measures about 6.14 × 2.61 inches, so with the ruler in inch mode, the long edge of a bill should end just past the 6 1/8 inch mark. If it doesn't, recalibrate with a card.

In inch mode the ruler is divided into sixteenths of an inch, with longer ticks for halves and quarters and numbers at every whole inch. In centimeter mode it shows millimeters, with longer ticks every 5 mm and numbers every centimeter.

Click anywhere on the ruler to drop a marker. The distance from the left edge to the marker is shown in inches (to two decimals) or centimeters (to one decimal). Click again to move it.

An on-screen ruler is handy for small, flat things: screws, washers, jewelry, a ring's inner diameter, a photo print, a button, a SIM card, the width of a cable. It is limited by your screen width, so a phone can measure only a few inches. For anything bigger, use a tape measure.

Don't press hard on the screen, especially on laptops and OLED phones, and avoid placing sharp or metal objects directly on the glass.

After careful calibration, the ruler is typically accurate to within about a millimeter (1/32 inch) across its length. Errors come from imprecise slider matching, viewing the screen at an angle and parallax from the glass thickness. Look straight down at the object, and for the best results, measure near the left end where the calibration was done.

Need to convert the result? Use the inch to cm converter.