The minimum thickness allowed for a disc brake pad is approximately one millimeter. A brake pad's standard thickness is between 10 and 12 millimeters. Because disc brake pads and rotors are vehicle parts that will wear down, it is important to inspect and replace these items when necessary.
For driving safety reasons, the brake pad wear indicator will emit a squealing sound when the brake pad thickness is close to 2.5 millimeters. This squealing sound indicates that the pad wear plate and the rotor disc are in contact.
To check if the disc pad is worn, look through the inspection hole and measure the thickness of the pad with an instrument called a machinist's rule. If the pad is worn, then it is advisable to have the disc pad replaced by a qualified mechanic.