Tire tread depth chart and converter

Tread depth chart

Usable tread left: the reading above 2/32 inch (the wear bars) as a share of a new tire's 10/32 or 11/32 inch above the same line (Bridgestone's typical new depths [Bridgestone penny test]). A tire that started deeper has more.

How much of a new tire's tread is left

A depth gauge tells you what is there; to judge a used tire you need to know what it started with. Bridgestone:

“New tires typically come with 10/32” or 11/32” tread depths, and some truck, SUV and winter tires may have deeper tread depths than other models.” [Bridgestone penny test]

Only the tread above the wear bars is usable, so measure both depths down to 2/32 inch. A used tire with 6/32 inch has 4/32 left above the bars; a new 10/32 tire has 8/32 and a new 11/32 tire 9/32. So the used tire has (6 − 2) / (10 − 2) = 50% to (6 − 2) / (11 − 2) = 44% of a new tire's usable tread. The value calculator turns that into a price per 32nd.

The penny and quarter tests

No gauge at hand? Bridgestone describes two coin tests. The penny checks for 2/32 inch:

“Turn the penny so that Lincoln’s head points down into the tread.” [Bridgestone penny test]

“If you can see his entire head, it may be time to replace the tire because it has reached the end of its useful life.” [Bridgestone penny test]

The quarter checks for 4/32 inch:

“The quarter test is a quick way to see if your tread is above or below 4/32”, a depth that’s vital for safe stopping in wet weather.” [Bridgestone penny test]

“If the tread covers part of Washington’s head, you have at least 4/32”. If you can see all of Washington’s head, your tread is dangerously shallow for wet conditions.” [Bridgestone penny test]

A coin test only says above or below one depth. On a used tire you are paying for, ask the shop to measure with a gauge.

The treadwear bars: 2/32 inch

New passenger and light-truck radial tires have raised bars across the bottom of the main grooves. Federal Standard No. 139 sets what they show:

“Except in the case of tires with a 12-inch or smaller rim diameter, each tire shall have not less than six treadwear indicators spaced approximately equally around the circumference of the tire that enable a person inspecting the tire to determine visually whether the tire has worn to a tread depth of one sixteenth of an inch. Tires with 12-inch or smaller rim diameter shall have not less than three such treadwear indicators.” [49 CFR 571.139]

When the tread is level with those bars, the tire has worn to 2/32 inch (one sixteenth of an inch, about 1.6 mm).

Trucks and buses: the federal rule

For commercial motor vehicles, the Federal Motor Carrier Safety Administration sets minimum depths in 49 CFR 393.75:

“Any tire on the front wheels of a bus, truck, or truck tractor shall have a tread groove pattern depth of at least 4 ⁄ 32 of an inch when measured at any point on a major tread groove.” [49 CFR 393.75]

“tires shall have a tread groove pattern depth of at least 2 ⁄ 32 of an inch when measured in a major tread groove.” [49 CFR 393.75]

The same rule says where to measure:

“The measurements shall not be made where tie bars, humps, or fillets are located.” [49 CFR 393.75]

Cars: state law, for example California

Limits for private cars come from state law and differ from state to state. California's Vehicle Code section 27465 is one example[California Vehicle Code section 27465 (tread depth)]:

“One thirty-second (1/32) of an inch tread depth in any two adjacent grooves at any location of the tire, except as provided in paragraphs (2) and (3).” [California Vehicle Code section 27465 (tread depth)]

“Six thirty-second (6/32) of an inch tread depth at all points in all major grooves on snow tires used in lieu of tire traction devices in posted tire traction device control areas.” [California Vehicle Code section 27465 (tread depth)]

The same section applies to sellers, which matters when you buy a used tire:

“No dealer or person holding a retail seller’s permit shall sell, offer for sale, expose for sale, or install on a vehicle axle for use on a highway, a pneumatic tire when the tire has less than the tread depth specified in subdivision (b).” [California Vehicle Code section 27465 (tread depth)]

Check your own state's vehicle code or motor vehicle agency for the rule where you drive.

Measuring a used tire

Measure in a main groove, away from the raised tie bars, at several points across the tread and around the tire: uneven wear shows up as different readings. Then read the date code and look the tire over for damage; the used tire checklist lists what the rules look for.