Knowing how thick a brake pad is when new gives you a reference for judging wear. From full to worn-out is a measurable scale, and the starting point is where it begins. Here is what a new brake pad measures and how it wears.
New pad thickness
A brand-new brake pad has roughly 10 to 12mm of friction material on top of the steel backing plate, with some performance or truck pads a bit thicker. That full block of material is your 100% reference - all wear counts down from there.
How it wears down
The material wears away with use - fronts faster than rears since the front does most of the work. As a scale: around 6-7mm is roughly half-worn and fine; about 3-4mm means plan replacement; at 3mm or less it is time for new pads; see minimum pad thickness. The wear indicator usually squeals near 3mm.
Do not run to the backing plate
Never let the pad wear down to the steel backing plate - that means metal-on-metal grinding that ruins the rotor and cuts braking; see why brakes grind. Replacing at about 3mm costs only pads; running to zero costs pads and rotors.
Checking your pads
You can eyeball pad thickness through the wheel spokes against the rotor, or measure precisely with a gauge when the wheel is off - see when to replace pads. Replace as an axle set once any pad hits the limit, with quality pads and fresh hardware.