Why an online ruler needs calibrating
Web pages don't know the real size of your screen. Browsers assume 96 pixels to the inch, but a laptop, a phone and a big monitor all pack their pixels differently, so an uncalibrated on-screen ruler can be out by a third or more. Calibrating tells the ruler how many pixels make a millimetre on your screen. You only need to do it once per screen.
How to calibrate
With a bank card (most accurate)
- Choose With a bank card. Any standard-size card works: a debit or credit card, an ID card, a library or loyalty card. They are all 85.60 × 53.98 mm.
- Hold the card flat against the screen, lined up with the top-left corner of the outline.
- Drag the slider, or use the − and + buttons for fine steps, until the outline is exactly the size of the card.
- Press Save calibration.
With your screen size
If you don't have a card to hand, choose With your screen size and type the screen's diagonal in inches, such as 15.6 or 27. You'll find it in the device's specifications. This is quick, but slightly less accurate, and it only works when the browser zoom is 100%.
How to measure
- Place the object against the screen with one end at 0, and read the length from the scale.
- Click or drag on the ruler to place a marker: the exact distance from zero appears in centimetres and inches. Arrow keys move the marker by a millimetre (with Shift, a centimetre).
- Switch between cm, inches or both, and turn the ruler vertical on a phone held upright to get more length.
Be gentle with the screen: don't press hard, and don't measure anything sharp, wet or dirty against it.
How accurate is it?
Calibrated carefully with a card, the ruler is usually accurate to within about half a millimetre over 10 cm. Accuracy depends on your calibration, so recalibrate if you change the browser zoom, move the window to another screen, or use another device. For anything important, check with a real ruler.
The calibrated ruler also helps with the DPI analyzer: open this ruler on a phone, lay it flat on your desk and move your mouse along it. To check the screen itself, try the dead pixel test.