Brake pad thickness is important because, over time, the friction between the brake pads and the rotors wears down the pads, necessitating replacement. When brake pads are less than 3 mm thick, they should be replaced with fresh pads. Since they are always replaced in pairs, only one pad needs to be worn down to necessitate the replacement of both pads.
Every time a driver depresses the brake pedal, the brake pads are ground against the rotor, which brings the vehicle to a stop. If the brake pads are worn too thin, the brakes might squeal and the driver's ability to stop the vehicle is compromised. Eventually, the pads will wear down to the metal studs, which will carve grooves into the rotors if the vehicle continues to be driven.
To measure brake pad thickness, the driver positions two points of a compass on either edge of the brake pad. Since pads often wear unevenly, it is important to measure the thickness at both ends to ensure an accurate assessment. Significant differences between the top and bottom of the pad might indicate the calipers require service.
If either end of the pad measures less than 3 mm, the brake pads should be replaced. Some drivers prefer to do this themselves, while others take their vehicles to certified mechanics. Cars can contain anywhere from four to eight brake pads in total.