Inertial measurement unit (IMU): accelerometer and gyroscope in one

An inertial measurement unit (IMU) is a tiny block of sensors that measures acceleration (accelerometer), rotation (gyroscope) and often the direction of the magnetic field (magnetometer). From this data a device knows how it is moving and how it is tilted.

An IMU senses tilts and turns

In plain words

Close your eyes and spin around - you can still feel which way you are leaning and whether you are falling. That is the balance organ in your inner ear at work. An IMU is the "inner ear" of devices: thanks to it, a smartphone, drone or robot feels tilts, turns and bumps even in the dark.

How it works

The accelerometer measures acceleration and gravity - so it knows which way is "down".
The gyroscope measures rotation speed around three axes.
The magnetometer finds the direction of magnetic north - like a compass.
These sensor chips (MEMS) are so small they fit into a watch.
IMU data is combined with other sensors because the gyroscope drifts over time.

The IMU is the "inner ear" of devices

Where you'll see it

In every smartphone (screen rotation, tilt games, camera stabilisation), watches and bands (steps, sleep, fall detection), drones and gimbals, humanoid robots (balance), VR headsets and controllers, and car navigation.

Does it matter when buying

Rarely directly, but the quality of the IMU affects step accuracy, smooth video stabilisation and VR responsiveness. In watch and phone specs it shows up as "accelerometer, gyroscope, compass" - in our watch comparisons, that is the Sensors row.

Myths

Myth: A gyroscope and an accelerometer are the same thing. Reality: An accelerometer senses acceleration and tilt; a gyroscope senses rotation.
Myth: An IMU alone can navigate accurately for hours. Reality: Without corrections from GPS or cameras, the error builds up quickly.

FAQ

What are 6-axis and 9-axis IMUs?

A 6-axis IMU is an accelerometer plus gyroscope; a 9-axis one adds a magnetometer.

How does a watch detect a fall?

The IMU registers a sharp acceleration and impact, followed by stillness.

Why does the compass on my phone need calibrating?

Metal and magnets nearby interfere with the magnetometer.