Online Ruler

How to use the online ruler

1. Open Calibrate for exact size and pick a method: match a bank card to the box, type your screen's diagonal in inches, or hold a real ruler against the screen and fine tune.
2. Choose cm / mm or inches. Press Vertical to turn the ruler for tall objects, or Full screen for the longest ruler.
3. Hold the object against the screen at the zero mark and read its length from the ruler.
4. Or drag along the ruler: the highlighted span shows the exact length between your start and end points.
5. Your calibration is saved in this browser, so the ruler stays accurate next time.

What the results mean

Without calibration, the ruler uses the web standard of 96 pixels per inch, which most desktop monitors are close to but phones, laptops and 4K screens are not. After calibration the ruler knows your real pixels per inch and the marks match a physical ruler to within about half a millimeter.

Ruler length is how much of a real ruler fits on your screen at the current size; a 24 inch monitor shows about 50 cm in full screen, a phone around 6-7 cm in portrait. Measured is the length of your last drag.

Accuracy depends on your calibration. A bank card is the easiest reference: every card is 85.60 mm by 53.98 mm. If you have a real ruler, the fine tune buttons get the last fraction of a percent.

Why screen rulers need calibration

A web page measures everything in CSS pixels, and the browser assumes 96 of them make an inch. Real screens have anywhere from 90 to 500 physical pixels per inch, and operating systems scale pages by 125%, 150%, 200% or odd amounts, so "1 inch" on a page is rarely 1 inch of glass. The browser does not know the physical size of your screen, which is why every online ruler needs a reference object or your screen size to be exact.

The screen size method uses your diagonal and the screen resolution reported by the browser to calculate pixels per inch. It is accurate when the browser is at 100% zoom and the display scaling is a standard value. The bank card method works everywhere, because you match the real object directly. Keep browser zoom at 100% (Ctrl+0) after calibrating, or the ruler will be off by the zoom factor.

Common sizes to check against

Bank or ID card: 85.60 x 53.98 mm. A4 paper: 210 x 297 mm. US letter: 8.5 x 11 inches. A one euro coin: 23.25 mm; a US quarter: 24.26 mm; a UK pound coin: 23.43 mm. A standard pencil: about 7 mm thick. Hold one of these at the zero mark to double check your calibration before measuring something that matters, such as ring size, screw length or a phone case.

Common problems and fixes

The ruler is the wrong size

Calibrate it: open "Calibrate for exact size" and match a bank card, or enter your screen diagonal. Then check that browser zoom is at 100%.

It was right yesterday and is wrong today

Browser zoom or display scaling changed, or you are on a different screen. Press Ctrl+0 to reset zoom, or calibrate again. Each browser stores its own calibration.

The ruler is too short for my object

Use Full screen for the longest ruler, or switch to Vertical if your screen is taller than it is wide. Measure in sections by lining up the end of the first measurement with the zero mark.

The ruler looks blurry

Set browser zoom to 100% and avoid fractional display scaling (such as 125%) if you can. The ruler is drawn at your screen's pixel density, so blur usually means the page is being scaled after drawing.