Tire tread depth chart, 2/32 to 12/32

A full tread depth chart from 2/32 to 12/32, in inches and millimeters, with shop sell lines, the penny test, and the FMCSA 4/32 steer rule.

The blunt answer

2/32" is the legal minimum tread depth in most states. Below that line, a tire is illegal to sell and unsafe to drive on. Full stop. Above it, how deep is "enough" turns into a business decision, not a legal one, and different shops draw that line in different places.

Here's the full table, in 32nds and in millimeters, with how most shops like ours treat each depth.

Why 4/32" matters even off the highway

49 CFR 393.75 sets 4/32" as the federal minimum for steer tires on commercial trucks, and 2/32" for the rest of the axles. That's a trucking regulation, not a passenger-car law. But it's a useful line for any shop. Wet-braking distance grows noticeably once tread drops below 4/32", which is exactly why many shops treat that number as their own "don't sell" line for used tires, even on cars that will never see a DOT inspection.

We like to stock 6/32" and up. Anything shallower goes in the discount bin with a clear warning, or it doesn't go out the door at all.

What tread depth actually buys a driver

Deeper grooves push water out from under the tire faster. That's the whole idea behind tread in wet weather: channels for water to escape so rubber can touch pavement instead of skating across a film of it. As those grooves get shallower, less water moves out per second, and the tire starts to ride on water instead of road. That's hydroplaning, and it's why the drop-off in braking performance below 4/32" isn't gradual. It's steep.

Dry-weather grip holds up longer than wet-weather grip does. A bald-ish tire can still feel fine on a dry Tuesday and turn dangerous the first time it rains. That gap is exactly why we set our sell line above the legal minimum instead of right at it.

We explain this to customers who push back on a "don't sell" call. Nobody wants to hear their tire failed the eye test, especially when it still holds air and looks round. Point at the groove, show the gauge reading, and say plainly that it's below the floor. Most people accept it once they see the number for themselves instead of taking our word for it.

The penny test, for when there's no gauge handy

Turn Lincoln's head upside down and push it into the groove. If the top of his head disappears, there's more than 2/32" left. If his whole head is visible, the tire's at or below the legal floor. It's not precise. But it's fast, and any customer standing at the counter can do it themselves while you finish the ticket in front of them.

Measuring it right

Always check three spots across the tread, not one. Inner edge, center, and outer edge wear at different rates depending on alignment and inflation history. Take the lowest of the three readings, not the average and not the highest. A tire that's 8/32" in the center and 3/32" on one edge is a 3/32" tire for grading purposes, because that's where it'll fail first.

Write the lowest number on the tire and in the record. It's the number that matters when someone asks later.

Uneven wear across those three spots tells its own story, separate from the raw depth number. A tire that's noticeably shallower on one edge usually means an alignment problem, wrong pressure over time, or worn suspension parts on the vehicle it came off. Log the pattern, not just the lowest figure. It helps the next person understand why a tire wore the way it did, and it can be worth mentioning to a customer buying that vehicle's old tire as a used unit for a different car.

A gauge does this in seconds, but a lot of shops still eyeball edge wear wrong. For how we grade a whole tire, tread included, see the used tire grading checklist, and to check how old a tire is before you even measure the tread, see how to read a DOT date code.

Converting 1/32" to mm

One thirty-second of an inch equals 0.79 mm. If you'd rather work in metric, the conversion above holds at every depth on the table, not just the ones we listed. Multiply the 32nds figure by 0.79 and round to one decimal.

Keep both units on the receipt when a customer asks, especially if their previous shop or a manual quoted the other one. Some import owners are used to seeing millimeters on their service records and get confused by 32nds alone. A quick note like "6/32\" (4.7 mm)" clears it up in three seconds and avoids a phone call later asking what the number even means.