Digital Ruler: Actual Size on Your Screen

How to use the online ruler

Calibrate first, once. Hold any bank card, debit card, ID card or loyalty card flat against your screen with its left edge on the left edge of the dashed box. Tap + Bigger or − Smaller until the box is exactly as wide as the card, then tap Save calibration. These cards are all made to one international size, 85.60 mm wide, which is why they make a reliable reference. Your device remembers the setting.

Now lay a small object on the screen with one end at the zero mark, and tap where the other end sits. The big number shows the length, with the other unit underneath. Drag along the ruler to fine-tune. Switch between cm / mm and inches at any time.

Why a screen ruler needs calibrating

Web pages are measured in CSS pixels, and the number of real pixels per inch varies hugely between screens: a phone may pack 400 to 500 real pixels into an inch, a laptop 120 to 220, a large monitor under 100. Browsers do not tell websites the physical size of the screen, so any online ruler that claims to be actual size without calibration is guessing. Uncalibrated, this ruler assumes 96 CSS pixels per inch, which is right on many desktop monitors at 100% scaling but wrong on most phones.

If you do not have a card, enter your screen's diagonal size in inches, for example 6.1 for many phones or 15.6 for a common laptop. The tool works out the scale from the screen resolution. This is less accurate than the card method because rounded corners, notches and browser zoom all get in the way, so check it against something you know the size of.

Accuracy

With a careful card calibration, the ruler is accurate to about half a millimetre over the length of the card. Keep browser zoom at 100%, since zooming in or out changes the scale. Rotating a phone keeps the scale correct, but on some laptops changing the display scaling in system settings means you need to calibrate again.

Handy sizes to check against

A bank card is 85.60 by 53.98 mm. A sheet of A4 paper is 210 by 297 mm. A one euro coin is 23.25 mm across, a UK one pound coin 23.43 mm, a US quarter 24.26 mm and an Indian ten rupee coin 27 mm. A standard paper clip is about 33 mm long.

What it is good for

Measuring screws, bolts, buttons, jewellery, ring sizes and small parts; checking a print size before you send a file to the printer; or measuring something when there is no ruler nearby. For anything longer than your screen, measure in sections or use a tape measure.