Online ruler

A ruler in centimetres, millimetres and inches drawn on your screen. Calibrate it once with a standard bank or ID card and the markings match real size on that screen, so you can measure small objects held against the display.

Not calibrated: the scale is a guess (96 CSS pixels per inch). Calibrate with a card below before measuring.

Pointer position: —
Screen pixels per inch: —
Screen diagonal (estimate): —
Device pixel ratio: —

Calibrate with a bank or ID card

Hold a standard card (bank, ID, loyalty or driving licence card: 85.60 × 53.98 mm) flat against the screen and adjust the outline until its width matches the card exactly, then save.

85.60 × 53.98 mm

Calibrate with the screen size

Enter the diagonal of the display in inches and its native resolution. The resolution is prefilled from what the browser reports, which may differ from the native one; check your display settings.

Browsers do not know the physical size of your screen. Without calibration the ruler uses the CSS standard of 96 pixels per inch, which is rarely exact. Browser zoom and moving the window to another monitor change the scale; the calibration is stored only in this browser.

How to use the ruler

Open Calibrate with a bank or ID card and hold a card flat against the screen, with its left edge on the left edge of the dashed outline.
Move the slider, or press − and + for single-pixel steps, until the outline is exactly as wide as the card. Then press Save card calibration.
Choose cm / mm or inches (1/16), and Horizontal or Vertical.
Place the object against the screen with one end on the zero mark. Move the pointer, or tap, over the ruler to read the exact position under Pointer position.

Why a card is used for calibration

Bank cards, ID cards, driving licences and most loyalty cards share the ID-1 size from the ISO/IEC 7810 standard: 85.60 × 53.98 mm (3.370 × 2.125 in). Almost everyone has one at hand, and it is long enough to set the scale to within a fraction of a millimetre.

If you have no card, open Calibrate with the screen size and enter the diagonal of your display in inches and its native resolution. This works well when you know the exact figures, but a rounded diagonal (15.6 in that is really 15.5 in) or a scaled resolution shifts the result, so check it against a real ruler when you can.

Why an uncalibrated ruler is not real size

A web page cannot read the physical size of your screen. Browsers measure in CSS pixels, defined as 96 per inch, and each device maps them to its panel differently: a phone, a laptop and a 27-inch monitor all show “1 cm” at different real lengths. That is why the ruler starts in a clearly marked uncalibrated state.

Browser zoom and the operating system display scaling change the size of a CSS pixel. The ruler notices a change in the device pixel ratio and adjusts, but recalibrating is more reliable.
Another monitor: the calibration belongs to the screen it was made on. Moving the window to a different display needs a new calibration.
The calibration is saved only in this browser on this device.

Readouts

Pointer position: the distance from zero to the pointer, in cm and mm, or in inches with the nearest 1/16 fraction and the decimal value.
Screen pixels per inch: the physical pixel density implied by your calibration.
Screen diagonal (estimate): the diagonal that follows from the calibration and the screen size the browser reports. A large mismatch with the real diagonal points to a calibration error.
Device pixel ratio: physical pixels per CSS pixel. It changes with browser zoom and display scaling.

Accuracy and limits

With a careful card calibration, expect accuracy of about half a millimetre over the length of the card, and proportionally less over longer distances.
The ruler is as long as your screen. For longer objects, measure in steps or use a physical tape measure.
It is not a certified measuring instrument: do not rely on it for engineering, medical or legal measurements.
To print a ruler on paper, a printed page depends on the printer's scaling; print at 100% (Actual size) and check it with the printer alignment test page.

For other screen checks, see the dead pixel test and the lightbox for tracing.

Questions

Is this ruler actual size?

Only after calibration. Without it, the ruler uses the CSS standard of 96 pixels per inch, which rarely matches the real size of a screen. Calibrate with a bank card and it shows actual size on that screen.

What size is a bank card?

Bank, ID and most loyalty cards use the ID-1 format: 85.60 × 53.98 mm, about 3 3/8 × 2 1/8 inches.

Does it work on a phone?

Yes. Calibrate on the phone itself, since its pixel density is different from your computer's. Turn the phone sideways or switch to the vertical ruler for longer objects.

Why did the scale change?

Browser zoom, the operating system display scaling or moving the window to another monitor change the size of a CSS pixel. The ruler adjusts for zoom, but recalibrate if the readings look off.