How to use the online ruler
- Press Calibrate, hold a bank card against the screen and match the outline to it with the slider or the outline’s edge. Then press Save calibration. You only do this once per screen.
- Choose cm and mm or Inches. The ruler runs along the top and left edges; Full screen makes it as long as your display.
- Hold the object against the screen at the top-left corner, or drag from one end to the other to measure it. Arrow keys fine-tune the end of the line.
Reading the ruler
In cm and mm mode, the longest lines are centimetres, the middle ones half centimetres, and the short ones millimetres. In Inches mode, the lines mark inches, halves, quarters, eighths and sixteenths, from longest to shortest, like a standard imperial ruler.
When you drag on the ruler, the line shows its length, and the text below gives the length plus how far it goes across and down, in both units, so you can measure at an angle too.
Getting the best result
- Calibrate on the screen you’ll measure on. A laptop screen and an external monitor have different pixel sizes.
- Look straight at the screen. Viewing from an angle makes it easy to misjudge where an edge lines up.
- Put small objects right on the glass. A coin, screw, button or ring held flat against the screen lines up most clearly.
- For a ring size, measure the inside diameter of a ring that fits, in millimetres.
Why screen sizes are never exact
Browsers measure in CSS pixels, which are meant to be about 1/96 of an inch but are set by the device maker to keep text readable, so the real size of a pixel varies from screen to screen. Web pages can’t ask the screen for its physical size, so a ruler page needs a known reference, like a card, to work out how big a millimetre is on your display. To see what your screen reports about itself, open screen resolution.