Tire Tread Depth: How Many Miles Are Left on Your Tires?

The short answer

Work out your wear rate first: tread worn so far ÷ miles driven. Then divide the tread you have left above the minimum by that rate: (current depth − minimum) ÷ wear rate = miles left. A tire that has gone from 14/32 in to 10/32 in in 40,000 miles has worn 0.1/32 in per 1,000 miles; against a 2/32 in minimum it has 8/32 in left, which is 80,000 miles at the same rate. On a commercial truck or bus the federal minimum is 4/32 in on front-wheel tires and 2/32 in on every other tire (49 CFR 393.75), so the axle the tire sits on changes the answer before any wear is involved. The tire tread life calculator does this arithmetic for you.

How to measure tread depth properly

A tread depth gauge reads directly in 32nds of an inch, millimeters, or both, and is far more useful than a coin, because a coin only tells you whether you are already below one threshold.

1. Measure in a major tread groove — one of the main circumferential channels, not a narrow sipe or a shoulder slot.
2. Stay off tie bars, humps and fillets. The federal rule says the measurement shall not be made where those are located, because they sit higher than the groove floor and read shallow. The same goes for the raised tread wear indicators (wear bars) molded into the groove.
3. Take several readings: each major groove across the tire, and at a few points around its circumference.
4. Use the lowest reading. For front-wheel tires on trucks and buses the federal rule applies at any point on a major groove, so the shallowest spot is the one that decides compliance.
5. Write down the odometer reading with the depth, every time.

Tread depth in 32nds of an inch and millimeters

One inch is exactly 25.4 mm, so 1/32 in is exactly 0.79375 mm. To convert, multiply 32nds by 0.79375 to get millimeters, or divide millimeters by 0.79375 to get 32nds. The values below are exact, not rounded:

Going the other way, 1.6 mm is about 2.02/32 in, 2 mm is about 2.52/32 in and 3 mm is about 3.78/32 in. Note that 2/32 in (1.5875 mm) is very slightly less than 1.6 mm, so the two common thresholds are close but not identical.

Federal minimum tread depth for trucks and buses

In the United States, 49 CFR 393.75 sets the tire rules for commercial motor vehicles. The tread-depth part is short, and the wording matters: the 4/32 rule is written for tires on the front wheels of a bus, truck or truck tractor, and the 2/32 rule covers everything else.

The same section also bars operating on any tire with ply or belt material exposed through the tread or sidewall, any tread or sidewall separation, a flat or audibly leaking tire, or a cut that exposes ply or belt material — regardless of tread depth. It also restricts regrooved and retreaded tires on front wheels. Passenger cars are not covered by 393.75; their minimum is set by state law and varies, so check your own state's vehicle code. The U.S. Tire Manufacturers Association's consumer advice is that once tread is down to 2/32 in, it is time to visit a tire shop.

Worked example: a front (steer) tire

More scenarios, each one reproducible

Every row below is the calculator's own output for the inputs shown, entered in US units. The original depths are example inputs, not specifications — use the new-tire depth published for your exact tire, or your own first measurement.

Two things stand out. First, the minimum you choose moves the answer as much as the wear does: the first two rows are the same tire, and changing only the minimum from 2/32 to 4/32 takes 20,000 miles off the estimate. Second, the last row shows how little is left near the limit — 1/32 in at 0.15/32 per 1,000 miles is 6,667 miles, so a small measuring error near the end is a large share of what remains. With no wear recorded yet, or a reading already at the minimum, the calculator gives a message instead of a mileage.

What changes how fast tread wears

The estimate is a straight line: it assumes the rest of the tread wears at the rate you have recorded so far. Anything that changes that rate makes the estimate wrong, and these are the usual culprits.

• Inflation pressure. The U.S. Tire Manufacturers Association (USTMA) says under-inflated tires build up excessive heat and stress, causing irregular wear and internal damage. Check pressure cold — USTMA defines that as parked for at least three hours — and expect it to move one to two psi for every 10 °F of temperature change. On commercial vehicles, 49 CFR 393.75 also prohibits running a tire below the cold inflation pressure specified for the load it carries.
• Alignment. USTMA notes that misaligned wheels can lead to uneven, rapid tread wear, and that striking a pothole or other road hazard can cause alignment problems.
• Load. 49 CFR 393.75 prohibits a tire carrying more weight than the load marked on its sidewall (outside of a state special permit at reduced speed). A rate measured on light freight will not hold if the work gets heavier.
• Position and rotation. Front tires steer, drive tires carry engine torque and trailer tires do neither, so a rate measured on one position does not transfer to another. USTMA says rotating tires as the owner's manual recommends can prevent irregular wear. After a tire changes position, start a new wear-rate measurement.
• Route and driving. Hard braking, fast cornering and route changes all shift the rate, and the calculator only sees the readings you give it.

Replacing before the legal minimum

The legal minimum is the point where a tire must come off, not a target to run down to. USTMA puts it plainly: tread equals traction, especially in bad weather, and losing too much tread can cost you control. In its advice on replacing tires it also notes that newer tires have better grip, particularly on wet roads. If you want to plan replacement above the legal floor, the calculator lets you set that point yourself with the Custom minimum — the pickup rows above show that choosing 4/32 instead of 2/32 cuts that tire's estimate from 45,000 to 30,000 miles. The federal rule sets no recommended replacement depth, only the minimum.

Where this goes wrong

• Guessing the original depth. The wear rate hangs on it. If the real new depth was deeper than you assume, you underestimate the wear rate and overestimate the miles left.
• Measuring on a wear bar or tie bar, or taking one reading per tire. Both overstate the depth that counts — use the lowest major-groove reading.
• Picking the wrong minimum. 4/32 on a front wheel, 2/32 elsewhere. On the same tire, that choice alone moved the estimate by 20,000 miles in the table above.
• Entering the vehicle's miles instead of the tire's. If tires were fitted used, rotated, or moved between vehicles, enter the miles those tires have run in their current position.
• Treating one estimate as final. Re-measure every few thousand miles and run the numbers again. If the estimate keeps shrinking faster than the miles you drive, the wear rate has gone up — check inflation and alignment.
• Ignoring everything that is not tread. Exposed belts, separations, cuts and leaks rule a tire out under 49 CFR 393.75 however much tread is left.