What a wear rate is good for, and what it is not
Pad wear over the middle of a pad's life is close enough to linear that dividing material used by miles driven gives a usable planning figure. That is genuinely useful: it tells you whether to buy parts now or in six months, whether the pads will survive a long trip, and whether one axle is due before the other. It is a scheduling tool.
It is not a substitute for looking. Linear wear assumes the caliper is releasing properly, both pads on a wheel are working, the slides move, and the rotor face is flat. When any of those stops being true the wear stops being linear and goes vertical, and the calculation from three months ago is describing an engine that no longer exists. The thickness that matters is the one you measured this morning.
The numbers this page refuses to supply
Two figures in a brake job are absolute limits, and neither of them is something a website can tell you. The pad replacement thickness is set by the pad or vehicle manufacturer, and it varies by pad compound, by caliper design and by whether the figure quoted includes the backing plate. The rotor discard thickness is cast or stamped into the rotor itself, usually marked MIN TH, and it is specific to that part number. Whatever this page's default field says, the number that counts is the one on your parts and in the manufacturer's documentation. The default here is a placeholder to make the arithmetic run, nothing more.
The same goes for the machining question. Whether a rotor may be machined at all, and how much may come off, is decided by the discard figure and by the actual measured thickness across the whole face, not by whether the surface looks bad. Machining removes material from both sides, so a cut of two tenths of a millimetre per side takes four tenths off the thickness.
Why an axle at a time
Pads and rotors get replaced across an axle, both sides together, because braking force has to be symmetrical. Fresh friction material on one side and worn material on the other gives you a car that pulls under braking, and the pull is worst exactly when the braking is hardest. The same logic covers rotors: two different surfaces bed two sets of pads differently. Hardware — the clips, shims and abutment plates the pads slide on — is a wear item as well, and reusing corroded hardware is a common reason a new set of pads binds and cooks itself in a few thousand miles.
| Per axle | Quantity | Why |
|---|---|---|
| Pad set | 1 set (4 pads) | Both wheels together, always |
| Rotors | 2 | Matched pair, or the car pulls |
| Hardware kit | 1 | Clips and shims are consumable |
| Fluid | As needed | Pushing pistons back displaces it, and fluid absorbs water over time |
Before anything on this page happens under a car
A jack lifts a vehicle. It does not hold one. Jacks fail, they sink into asphalt on a warm day, they slip off a pinch weld that was already bent, and people are killed every year by vehicles that came off them. Whatever the work is, the vehicle sits on rated stands on hard level ground, with the wheels that stay on the ground chocked, before any part of a person goes under it. This page is arithmetic and gives no lifting procedure, because a written procedure is not how anyone should learn that. If you have not been shown in person by someone competent, the work belongs at a shop.
When to stop reading and call somebody
A brake pedal that goes further than it used to, a pedal that sinks under steady pressure, a noise that changes with braking, a pull to one side, a vibration through the pedal or the wheel, a warning light, a fluid level that dropped without pads wearing — none of those are planning problems. They are inspection problems, and this page cannot see any of them. There is no procedure here for doing the work, deliberately: a brake job is a thing to be shown in person by someone competent, and the consequence of a mistake is not a mistake you find out about in the driveway.
For the wider picture on what falls due when, see the car maintenance schedule, and the car ownership cost calculator puts a wear item like this into the running total. If the vehicle also tows, the towing and payload calculator covers loads that change how hard the brakes work.
Questions people ask
How thin is too thin for a brake pad?
The manufacturer decides, and the figure is specific to the pad and the vehicle. What a page can honestly tell you is how the measurement works: it is the friction material only, not the steel backing plate, taken on the thinnest pad on the axle rather than the healthiest one. If you are measuring through a wheel with a torch and guessing, you are not measuring. Get the actual figure from the pad manufacturer or the vehicle documentation and compare it to a real measurement.
Do I have to replace rotors every time I do pads?
Not automatically, but the decision is made with a micrometer rather than an opinion. A rotor that is above its discard thickness with margin to spare, running flat and true, with a surface a new pad can bed to, can stay. A rotor near the discard figure will not survive another set of pads and machining it only accelerates that. Thickness variation and runout matter as much as the raw number, and both take measuring equipment. The cost of finding out later is a job done twice.
Why did the inside pad wear out and the outside one look new?
That is the signature of a caliper that is not releasing — commonly seized slide pins on a floating caliper, sometimes a sticking piston, sometimes a collapsed flexible hose acting as a one-way valve. The piston pushes the inner pad hard against the rotor and the caliper body never slides back to relieve the outer one. A wear rate calculated from that axle is meaningless, because the real problem is not the pads. It also cooks the rotor and the fluid on that corner. This needs someone to look at the caliper before any new parts go on.
Can I just measure pads through the wheel spokes?
It is a useful screening look and not a measurement. You can usually see the outer pad on the front, which is the pad most likely to be the healthy one, and you cannot see the inner pad at all on most wheels. Uneven wear between inner and outer is exactly the fault you are trying to catch, so the look that misses it is the look that matters. A real check has the wheel off, both pads measured, the rotor faces inspected on both sides and the caliper slides checked.
Does my wear rate carry over to the next set of pads?
Only loosely. A different pad compound wears at a different rate and is easier or harder on the rotor. Towing, hills, city traffic, a heavier load and a change in how you drive all move it. Treat the figure from the last set as an opening estimate for the next one, then recalculate once you have a real thickness and a real mileage on the new pads. That second calculation is the one worth trusting.