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.
Millimetres, centimetres and inches
There are 10 mm in a centimetre and 25.4 mm in an inch. The ruler shows the millimetre reading in big numbers and the same length in cm and inches underneath, so you do not need to convert.
Small things people measure in mm
- Screw and bolt length and thread size (M3 is 3 mm, M4 is 4 mm).
- Bead and button size, usually 2 to 20 mm.
- Ring width and stone size.
- Wire, cable and nozzle size.
- Print and sticker sizes.
Questions
How many mm are in an inch?
25.4 mm exactly.
How do I read the mm marks?
Every short mark is 1 mm, the middle marks are 5 mm and the numbered marks are every 10 mm (1 cm).
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.