Measuring a small object without a ruler at hand is a common problem, and the screen in front of you is always within reach. This online ruler draws a scale in centimeters or inches that you can hold objects against, with a vertical mode for tall things and a measuring marker that reads the distance in both units. To make the numbers true, it needs to be calibrated once.
Measure in three steps
- Calibrate: hold a bank card against the screen and drag the slider until the dashed rectangle is as wide as the card.
- Choose centimeters or inches and the horizontal or vertical ruler.
- Place the object along the ruler, or click and drag on the ruler to read a distance below it.
Why calibration is needed
A web page measures in CSS pixels, and a CSS pixel is defined as one 96th of an inch only on paper. On a real screen the size of a pixel depends on the panel, the scaling in the operating system and the zoom of the browser, and the browser cannot see the physical size of the display. A bank card is the same width everywhere, 85.6 millimeters under the ISO standard, which makes it a handy reference. You can also enter the diagonal of your screen in inches, which the ruler combines with the number of pixels to work out the scale.
Getting the best accuracy
- Set the browser zoom to 100 percent before calibrating, and calibrate again after changing the zoom.
- Hold the card flat against the glass and look straight at the screen to avoid parallax.
- Use the sixteenth-inch marks for woodworking sizes and the millimeter marks for small parts.
- Check the result against a known object, such as a coin or a sheet of A4 paper that is 297 millimeters long.
Limits to keep in mind
A screen ruler is for rough work: crafts, shopping and school projects. It is no replacement for a real measuring tool when the precision matters. The scale is stored in this browser only, so another device needs its own calibration. Nothing is uploaded.