How to Check Tread Depth: Penny, Quarter or Gauge

Two coins, one gauge, five minutes

You can check your tread with a penny from the cup holder. It just won't tell you everything.

There are three ways to check tread depth at home: the penny test, the quarter test and a tread depth gauge. Each one answers a different question. Here's what each shows, how to do it, and which lines matter.

The two lines that matter: 2/32 and 4/32

In the US, tread depth is measured in 32nds of an inch, from the top of the tread to the bottom of the main grooves.

The 2/32 line is the floor. The 4/32 line is where AAA recommends replacing, based on its 2018 wet-braking study: at 60 mph, tires worn to 4/32 took an average of 87 feet longer to stop a passenger car than new tires did. Consumer Reports makes the same point about worn tires in the wet. In a South Florida rainy season, 4/32 is the line to plan around.

Our tread depth chart covers every depth in between. This guide is about measuring.

Method 1: the penny test (checks 2/32)

NHTSA and USTMA give the same instructions:

1. Hold a penny with Lincoln's head upside down, facing you.
2. Push it into a main groove until the edge touches the bottom.
3. Look at his head. If you can see the top of Lincoln's head, the tread is at or below 2/32, and NHTSA says to replace the tire.

If part of his head is hidden by the tread, you're above 2/32 at that spot. That's all it tells you: above the floor or not.

Method 2: the quarter test (checks 4/32)

Same move, bigger coin:

1. Hold a quarter with Washington's head pointing down into the groove.
2. Push it in until it touches the bottom.
3. If you can see all of Washington's head, you're at about 4/32 or less. Time to start shopping.

Bridgestone's tread guide describes the quarter test this way, as a check for the 4/32 line on wet roads. If the tread covers part of his head, you have more than 4/32 at that spot.

Coins are quick, but they're pass-or-fail checks at one line. Treat them as a screen, not a measurement.

Method 3: a tread depth gauge (gives you a number)

A tread depth gauge is a small probe that reads in 32nds of an inch, and often millimeters too. It's the only one of the three methods that gives you a number you can write down and compare next month.

How to use one:

1. Push the probe in so it sticks out past the base.
2. Set the tip into a main groove, not a thin slit in the tread block.
3. Press the base flat against the tread blocks on either side of the groove.
4. Read the depth, then pull the gauge out without touching the probe.

Drag the gauge below to see what each depth looks like, with the penny and quarter overlaid:

The built-in check: wear bars

You don't even need a coin to spot the 2/32 line. NHTSA describes treadwear indicators: small raised bars molded across the bottom of the grooves. Every tire has them.

• Look across the tread for short, flat bridges running between the ribs.
• If the tread surface is flush with a bar anywhere, the tire is at 2/32 there and it's time to replace it.

Wear bars only show the floor. By the time they're flush, the tire is already past AAA's 4/32 line.

How to check tread depth, step by step

1. Park on level ground and turn the front wheels fully to one side so you can see the whole tread.
2. Check three grooves on every tire: inside, center and outside.
3. Check at two or three spots around each tire. Roll the car a few feet if you need to.
4. Go by the shallowest reading. A tire is only as deep as its most worn groove.
5. Look for the wear bars while you're there.
6. Write it down by position: front left, front right, rear left, rear right.
7. Check the spare if it's a full-size tire that's part of your rotation.

Do it monthly, alongside the pressure check. Our guide on how to check tire pressure covers the other half of the routine.

What your readings are telling you

Even and above 4/32. Keep checking monthly.

At or near 4/32. AAA recommends replacing here. Start shopping before the rainy season, not during it.

At 2/32, or flush with the wear bars. NHTSA says to replace the tire.

Uneven across one tire. If the inside groove reads 3/32 and the outside reads 6/32, something is wearing the tire unevenly. Inflation, alignment and suspension are possible causes, and new tires will wear the same way if the cause isn't found. Have a technician inspect it.

Different from front to rear. Note it, and keep the tires on your rotation schedule. See the rotation pattern for your car.

Tread isn't the whole story

Deep tread doesn't clear a tire on its own. While you're down there, look for cuts, bulges and cracks in the tread and sidewalls; NHTSA's advice is to have any damage looked at by a tire professional. And check the date code, because a tire can have plenty of tread and still be old. Our guide on how long tires last covers miles and years together.

Run your numbers through Do I Need Tires Yet?. If it's time, shop tires by size; the Tire Size Finder reads your size off the door-jamb sticker. Free shipping covers the 48 contiguous states and DC. Extreme Tires installs at the Sunrise shop or by mobile van in Miami-Dade, Broward and Palm Beach. See how installation works, or call (954) 773-1896.