Accelerometer: how your phone knows it has been turned or shaken

An accelerometer is a sensor that measures a device's acceleration along three axes, including gravity. Thanks to it, a phone knows how you are holding it, counts your steps and recognises shaking and falls.

An accelerometer is like a ball on springs

In plain words

Imagine a ball in a little box held by springs. Tilt the box and the ball shifts down; jerk it and the ball "lags behind". An accelerometer is a microscopic box like that with a ball on a silicon chip: from how the "ball" shifts, it knows which way gravity is pulling and how you are moving.

How it works

Inside the chip is a tiny movable mass on flexible supports.
When the device accelerates, the mass shifts relative to the casing.
The shift is measured by a change in electrical capacitance.
The sensor gives values along three axes: X, Y and Z.
Apps use this data to rotate the screen, count steps and power games.

Counting steps with an accelerometer

Where you'll see it

Every phone, tablet, watch, fitness band, game controller and drone has an accelerometer. In our watch and phone comparisons it appears in the Sensors row. Together with a gyroscope and magnetometer it forms an inertial measurement unit.

Does it matter when buying

Accelerometers are everywhere, so you do not choose a device for one. But their quality and the algorithms behind them affect step accuracy, fall detection in watches and crash detection in phones.

Myths

Myth: An accelerometer and a gyroscope are the same thing. Reality: An accelerometer measures acceleration and tilt; a gyroscope measures rotation speed.
Myth: A pedometer only counts footsteps. Reality: It can count arm movements too - which is why steps sometimes "add up" on public transport.

FAQ

Why won't my screen rotate?

Auto-rotate is probably turned off in settings.

How does a watch count steps?

The accelerometer picks up rhythmic movement, and an algorithm turns it into steps.

How do I calibrate the accelerometer?

Usually you do not need to; some phones have service menus for testing it.