How to get a ruler on your phone
- Open this page on your iPhone or Android phone. Hold the phone upright: the ruler runs down the left side.
- Press Calibrate, lay a bank card on the screen and drag the yellow dot until the outline matches the card.
- Lay the object along the yellow ruler and drag the other red jaw to its end.
Does my phone have a ruler built in?
iPhones have the Measure app, and many Android phones have an AR measuring app. Those use the camera and are fine for furniture, but for small things like screws, rings and beads a ruler laid against the screen is more exact.
How long can I measure on a phone?
About as long as your screen, usually 12 to 15 cm (5 to 6 in) when the phone is upright.
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.
Questions
How do I get a ruler on my iPhone?
Open this page in Safari, calibrate with a card and measure. Add it to your home screen from the share button to open it like an app.
Does my Samsung phone have a ruler?
Some Samsung phones include a measuring app that uses the camera. This page works in Samsung Internet or Chrome without installing anything.
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.