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 (or use a coin, a banknote or a real ruler). Then press Save calibration. You only do this once per screen.
- The Ruler view shows centimeters along the top and inches along the bottom, like a real ruler. Line your object up with the left edge, then click or drag to its other end to read the length. Full screen makes the ruler as long as your display.
- Switch to Measure to drag a line between any two points, at any angle. cm and mm or Inches chooses which unit is shown first. Arrow keys fine-tune the end of the line.
Reading the ruler
On the top edge of the ruler the longest lines are centimeters, the middle ones half centimeters, and the short ones millimeters. On the bottom edge the lines mark inches, halves, quarters, eighths and sixteenths, from longest to shortest, like a standard imperial ruler.
When you click or drag on the ruler, a line marks the spot and the text below gives the length in centimeters, millimeters and inches. In Measure mode, the text also gives how far the line goes across and down, 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 millimeters.
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 millimeter is on your display. To see what your screen reports about itself, open screen resolution.