A brake wear indicator is used to warn the operator of a vehicle that the brake pad is in need of replacement. The main area of use for this is on motor vehicles with more than three wheels. However brake wear indicators are also useful for brake pads in industrial applications, including wind turbines and cranes.

This article refers to disc brakes as an example, but the principle is the same for other types of friction brakes.

There are different types of wear indicators for brake pads:

Of the alternatives above, the first three are simple and cheap, Their lifetime coincides with the service life of the brake pad.

A position sensor is more expensive to implement and designed to survive the service life of the vehicle. It is usually only seen on heavy duty vehicles.

Pads B are mounted on carriers G. These are pushed against the rotor A by the piston D which is pushed by the brake fluid E. This induces wear on the brake pads. The rotor A also experiences some wear, but to a lesser extent than the brake pads. The modules C are joined to the cylinder that houses the piston D and acts as counter-force to the piston D.

Sufficient wear to validate a change of brake pads is considered when one of the following cases are applicable:

F can either be an analog sensor, with an electronic threshold value set to signal an alarm when it has been reached, or an on/off switch, activated at a certain distance.