Online Ruler — Inches

A 12 inch on-screen ruler marked in sixteenths, for measuring something on the fly against a real imperial scale — useful anywhere you think in inches rather than centimetres and do not have a tape measure to hand.

Calibrating against a bank card

Screens vary too much in actual pixel density for the ruler to be trustworthy out of the box — the web's assumed 96 pixels per inch does not match most real displays, especially high-density laptop and phone screens. A standard bank card is manufactured to an internationally fixed width of 85.6 mm, so holding one against the calibration strip and adjusting the setting until they line up gives the ruler an accurate real-world scale for your specific screen.

Why sixteenths

Sixteenths of an inch is the standard fine subdivision on a real imperial ruler or tape measure, which is why the marks follow that pattern here rather than a metric-style tenth. The longer tick marks land on the quarter-inch points, which makes the ruler easy to read at a glance even without counting every sixteenth individually.

Browser zoom needs to stay at 100% after calibrating — zooming changes the effective scale of every pixel on the page, which throws the calibration off until it is redone.