Bubble Level

How to use the bubble level

1. Tap Start level. On an iPhone, allow motion access when asked.
2. To check a flat surface such as a shelf, table or worktop, lay the phone on its back. To check that something is plumb, hold a long edge of the phone against it.
3. Read the large number: it is the tilt away from level in degrees. Pitch is the front-to-back tilt and Roll is the side-to-side tilt, which tells you which way to adjust.
4. Adjust until the bubble sits in the centre and the reading is close to 0°.

Calibrate it first with Set zero

A phone is not perfectly flat: the camera bump and the case both tip it slightly. Put the phone on a surface you trust — a kitchen worktop checked with a real level, or a sheet of glass — and tap Set zero. Every reading afterwards is measured from that position. Reset zero returns to the sensor's own zero.

A quick test of any surface: take a reading, turn the phone half a turn on the same spot and read again. If the two readings differ, the difference comes from the phone, not the surface, and it needs zeroing.

What an angle means in practice

Degrees are hard to picture, so it helps to turn them into a drop over a length. One degree of tilt is about 17.5 mm of rise or fall for every metre.

So a one-metre shelf reading 0.5° is nearly 9 mm lower at one end — enough for a pencil to roll.

What it is good for, and what it is not

The tilt sensor in a phone resolves a few tenths of a degree, which is plenty for hanging pictures, levelling a washing machine or a fridge, setting up a tripod or a camping table, or checking a shelf. A phone is short, though, so it measures only the patch it sits on. For a long run, rest it on a straight length of wood or a ruler that spans the surface. For building work that has to meet a tolerance, use a proper spirit or laser level.