Measuring brake shoes comes down to three numbers: the drum diameter they’re designed to fit, the width of the lining, and the remaining thickness of the friction material. Getting these right ensures your replacement shoes match your drums and that your current shoes are still safe to use. Here’s how to take each measurement accurately.
The Two Key Dimensions for Replacement
Brake shoes are sized by two measurements that must match your brake drum: the arc diameter and the lining width.
The arc diameter corresponds to the inside diameter of your brake drum, which is the circular friction surface where the shoes press outward. A drum labeled “16.5 inch” has a friction surface roughly 16.5 inches across, and the shoes curved to fit it share that same designation. To confirm this measurement, use a brake drum gauge or large inside caliper placed across the drum’s inner surface. Take readings at several points about 90 degrees apart to check for uneven wear or out-of-round conditions.
The lining width is how tall the friction material is on the shoe, measured from top to bottom along the flat face. This corresponds to the depth of the friction surface inside the drum, sometimes called the brake surface width. If your drum has a 7-inch-wide contact area, you need shoes with 7-inch-wide linings. Shoes that are too narrow won’t make full contact with the drum, reducing braking power. Shoes that are too wide physically won’t fit inside the drum.
You can measure the lining width on your old shoes with a standard ruler or tape measure. For the arc diameter, measuring the drum directly with an inside caliper or drum gauge is more reliable than trying to measure the curved shoe itself.
How to Check Lining Thickness
If you’re inspecting shoes that are already installed, the critical measurement is how much friction material remains on the shoe’s metal table. Use a depth gauge, small ruler, or caliper to measure the lining thickness at its thinnest point, since shoes wear unevenly across their surface.
The legal minimum in the United States is 1/16 inch (1.6 mm) for drum brake linings. At or below that point, a vehicle is considered unsafe and would fail a roadside inspection. In practice, most mechanics recommend replacing shoes well before they reach that minimum, typically when the lining is down to about 1/8 inch (3 mm). Waiting until you’re at the bare minimum leaves almost no margin for the time between noticing and actually getting the work done.
Check the thickness at multiple spots along the shoe. The leading shoe (the one that contacts the drum first in the direction of rotation) almost always wears faster than the trailing shoe, so don’t assume both sides are the same.
Measuring the Drum for Wear
While you have the drum off, measure its internal diameter and compare it to the nominal size. Every drum has a maximum allowable diameter stamped or cast into its outer edge. As a drum wears, its inside diameter gets larger because material is being removed from the friction surface.
Place a drum gauge or inside micrometer inside the drum and take several readings at 90-degree intervals, both at the top and bottom of the friction surface. If any measurement exceeds the nominal diameter by 0.120 inch (about 3 mm) or more, the drum needs to be replaced. If readings vary significantly from one angle to another, the drum is out of round and also needs replacement, since out-of-round drums cause pulsation and uneven shoe wear regardless of how new the shoes are.
A worn drum with a larger-than-nominal diameter also means the shoes have to travel farther outward to make contact. This can affect pedal feel and reduce braking response, even with fresh shoes installed.
Using Part Numbers to Confirm Fit
The aftermarket brake industry uses a standardized numbering system maintained by the Friction Materials Standards Institute. Every brake shoe design has a unique FMSI number that serves as a universal identifier from manufacturer to installer. Rather than measuring everything yourself and hoping you’ve matched the right part, you can cross-reference your vehicle’s year, make, and model to its FMSI number, then verify that number matches the shoes you’re buying.
This is especially useful when your old shoes are too worn to measure reliably, or when you’re working with a vehicle where you can’t easily access the drums. Any auto parts store can look up the correct FMSI number for your application. The number encodes the shoe’s dimensions and design, so if the FMSI number matches, the physical measurements will too.
Tools You’ll Need
- Brake drum gauge or large inside caliper: for measuring the drum’s internal diameter accurately. A standard tape measure isn’t precise enough for this.
- Ruler or vernier caliper: for measuring lining width and remaining lining thickness on the shoes themselves.
- Depth gauge: helpful for checking lining thickness at specific points, especially in areas that are hard to reach with a caliper.
When measuring lining thickness, make sure you’re measuring only the friction material, not the metal backing plate underneath. The friction lining is typically a different color and texture from the steel table it’s bonded to, making the boundary easy to identify visually.