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Online ruler in actual size

Screens differ in pixel density, so a ruler drawn without calibration is rarely exact. This ruler asks you to match a standard bank card once, then shows centimeters, millimeters and inches at true scale on your device.

Browsers don't report your screen's physical size, so until you calibrate we assume 96 px per inch. On phones and laptops that can be 10–40% off.

Move the pointer or tap the ruler to read a position. Reading: –

Keys: H / V orientation · C calibrate · F full screen · ← → move cursor (Shift = 1 cm) · Space sets a start point

Results are for information only.

How to use it

  1. Press “Calibrate” and hold any payment, ID or loyalty card (ID-1 format, 85.60 × 53.98 mm) flat against the screen.
  2. Drag the slider or the handle until the outline matches the card edge exactly, then save. The setting is stored only in this browser.
  3. Place your object against the screen at the zero mark and read the length. Switch to vertical mode for tall items.

How it works

A web page can count pixels, but it has no idea how big those pixels are on your glass. Browsers simply declare that 96 CSS pixels make an inch, and that is only roughly true on an old desktop monitor. Calibration fixes the gap: you match an outline to a real card, the page divides the outline's width in pixels by 85.60 mm, and every tick on the ruler is drawn from that number instead of the 96 px guess.

Worked example

Say your card lines up when the outline is 412 px wide. That gives 412 ÷ 85.6 = 4.81 px per millimeter. The default assumption is 96 ÷ 25.4 = 3.78 px per millimeter, so on this screen an uncalibrated “1 cm” would really be about 7.9 mm long, and a 10 cm object would read as nearly 12.7 cm. After calibration the same object reads 10 cm.

Assumptions and limits

  • Calibration belongs to one screen, one browser and one zoom level. Change any of those and recalibrate.
  • Expect about ±0.5 mm over 10 cm if you look straight down at the screen. Viewing at an angle (parallax) is the biggest source of error.
  • Under 2 mm, use a caliper. For anything longer than your screen, use a tape.

Frequently asked questions

How accurate is an on-screen ruler?

Once calibrated with a card, it is usually accurate to about half a millimeter, which is enough for screws, jewelry, buttons, and small pieces of paper. Without calibration it can be off by 10% or more.

Why do I need a bank card?

Almost every wallet card follows the ISO/IEC 7810 ID-1 size of 85.60 × 53.98 mm. That makes it a reliable reference you already own.

Do I have to calibrate again?

Only if you change device, browser, or the browser zoom level. The calibration is saved locally and nothing is sent to a server.