How to measure with it
- Calibrate once. Hold a bank card (or a coin) on the screen and drag the yellow dot until the dashed outline matches it. Every bank, ID and loyalty card is the same size, 85.6 by 53.98 mm, so the ruler learns exactly how big your screen's pixels are. It remembers this on this device.
- Put your object on the screen, touching the left (or top) red jaw. Press Start at 0 to move that jaw to the end of the ruler.
- Drag the other jaw to the far end of the object, or just tap there. The size shows in big numbers, with the other units underneath.
Why most online rulers are the wrong size
A browser does not know how big your screen really is. It draws 96 "CSS pixels" to an inch, but on a real phone or laptop that can be off by a third or more. That is why a ruler that is not calibrated can show 1 inch as 0.8 inch. Matching a card you already own fixes this in a few seconds, without knowing your screen size.
If you have no card, pick a coin, or type your screen size in inches (it is written in your device's specs). The card is the most exact, because it is the largest.
Reading mm, cm and inch marks
On the metric ruler the long marks with numbers are centimetres, the middle marks are 5 mm and the short marks are single millimetres. On the inch ruler the numbered marks are whole inches, and the marks between get shorter for 1/2, 1/4, 1/8 and 1/16 inch. The readout also gives the nearest 1/16 inch, which is how most US tape measures are read.
| Inch fraction | Decimal | Millimetres |
|---|---|---|
| 1/16 | 0.0625 in | 1.59 mm |
| 1/8 | 0.1250 in | 3.17 mm |
| 3/16 | 0.1875 in | 4.76 mm |
| 1/4 | 0.2500 in | 6.35 mm |
| 5/16 | 0.3125 in | 7.94 mm |
| 3/8 | 0.3750 in | 9.52 mm |
| 1/2 | 0.5000 in | 12.70 mm |
| 5/8 | 0.6250 in | 15.88 mm |
| 3/4 | 0.7500 in | 19.05 mm |
| 7/8 | 0.8750 in | 22.22 mm |
| 1 | 1.0000 in | 25.40 mm |
Check a size before you buy
Press Show a size and type a number, for example 12 mm, to see exactly how big a bead, button, earring or screw from an online shop will be. The span between the red jaws is shown life size.
Everyday things to check your calibration
| Thing | Size |
|---|---|
| Bank, ID or loyalty card (ISO/IEC 7810 ID-1) | 85.60 x 53.98 mm (3.37 x 2.13 in) |
| US quarter | 24.26 mm across |
| Canadian 1 dollar coin | 26.5 mm across |
| UK 1 pound coin | 23.43 mm across |
| 1 euro coin | 23.25 mm across |
| Australian 1 dollar coin | 25.0 mm across |
| A4 paper | 210 x 297 mm |
| US Letter paper | 8.5 x 11 in (215.9 x 279.4 mm) |
Coin sizes are from the US Mint, Royal Canadian Mint, Royal Mint, European Central Bank and Royal Australian Mint, as published November 2026.
Questions
How accurate is an online ruler?
After calibrating with a card it is usually within about half a millimetre on a laptop and better on a phone, because phone screens have more pixels per millimetre. Without calibration it can be off by a lot.
Can I measure something longer than my screen?
Measure up to the screen edge, put a small mark on the object, slide it along and add the two readings. For long things a tape measure is quicker.
Why does my 1 cm look too small or too big?
Your ruler is not calibrated for this screen yet. Press Calibrate and match a bank card.
Does it work on a phone?
Yes. On a phone held upright the ruler runs down the side of the screen, which gives you the longest length to measure. Press Turn to switch.
Is anything sent or saved?
No. The ruler runs in your browser. The calibration is saved only in your browser on this device.