How to Check Tire Tread Depth With a Coin or Gauge

A tire with 2 mm of tread can feel perfectly normal on a dry morning commute. Put the same car on a wet motorway at 70 mph and it can need many extra metres to stop, because the grooves can no longer clear water fast enough. Most drivers only find out how worn their tires are at an MOT or service, often months later than they should.

Learning to check tire tread depth yourself takes five minutes, costs nothing with a coin or a few pounds with a gauge, and tells you far more than a glance at the sidewall. Here is how I do it on our own cars, once a month and before every long trip.

Why Tread Depth Matters More in the Wet

The grooves in a tire are channels. On a wet road they push water out from under the contact patch so the rubber can touch tarmac. As the grooves get shallower, less water escapes, and the tire starts to ride on a thin film instead of the road.

That is the first step towards aquaplaning, which we cover in detail in our guide on how to avoid aquaplaning in heavy rain. The grip loss is gradual, which is exactly why it catches people out. A tire at 3 mm still feels fine. It simply stops sooner in the dry than it does in the wet.

Legal Limits and When to Actually Replace

The legal minimum depends on where you drive, and it is lower than most tire experts would like.

Region	Legal minimum	Recommended replacement
UK and EU (cars)	1.6 mm across the central three quarters	3 mm
Most US states	2/32 in (about 1.6 mm)	4/32 in (about 3 mm)
Winter tires in snow	Varies by country	4 to 5 mm (5/32 to 6/32 in)

New car tires usually start at around 7 to 8 mm (9/32 to 10/32 in). My own rule is simple: start budgeting at 4 mm and replace at 3 mm. The cost of a pair of tires is small next to the cost of a rear-end collision on a wet roundabout.

How to Check Tire Tread Depth With a Gauge

A tread depth gauge costs a few pounds or dollars at any car parts shop and gives you a real number. It is the method I recommend.

Park on level ground with the wheels straight. Turn the steering to full lock if you want easier access to the front tires.
Push the gauge's probe fully in and place the flat base across a main groove, not a shallow sipe.
Press the base flat against the tread blocks so the probe drops to the bottom of the groove.
Read the depth in millimetres or 32nds of an inch.
Measure three points across the tire: inner, centre and outer groove. Repeat at three places around the tire, roughly a third of a turn apart.

Write the lowest reading for each tire on a note in your phone with the date and mileage. After a few months you will see how fast each one wears.

The Coin Test When You Have No Gauge

Coins are a rough check, useful for a quick look in a car park.

UK: the 20p test

Insert a 20p coin into the main grooves. If the outer band of the coin is hidden, your tread is above the legal 1.6 mm. If you can see the band, the tire is close to or below the limit and needs a proper check immediately.

US: the penny and quarter tests

Place a penny upside down in a groove, with Lincoln's head pointing into the tire. If you can see the top of his head, you are at or below 2/32 in. A quarter with Washington's head down does the same job at roughly 4/32 in, which is a better warning point.

The coin test only tells you whether you are near the line. It cannot show you the gradual difference between 5 mm and 3 mm, which is why a gauge is worth buying.

Tread Wear Indicators and Uneven Wear

Every modern tire has small raised bars at the bottom of the main grooves, usually marked by a small triangle or "TWI" on the sidewall shoulder. When the tread wears down level with these bars, the tire is at the legal minimum. If you can see them flush with the surface, replace the tire now.

Your readings across the tire also tell a story:

Both edges worn, centre fine: running underinflated.
Centre worn, edges fine: running overinflated.
Inner or outer edge only: wheel alignment or worn suspension parts.
Patchy, scalloped wear: worn shock absorbers or wheels out of balance.

If one edge is much lower than the other, get the alignment checked when you replace the tire, or the new one will wear the same way. Pressure problems often go hand in hand with a tire pressure warning that keeps returning, and our guide to the maintenance musts for your vehicle lists the other checks worth doing on the same morning.

Making Tires Last Longer

Most tread loss comes from habits you can change.

Check pressures monthly, when the tires are cold, against the figures on the door pillar sticker.
Rotate front and rear tires every 5,000 to 8,000 miles on front-wheel-drive cars, where the fronts wear faster.
Avoid hard launches and late braking, which scrub rubber off quickly.
Do not overload the car; check the load index before a fully packed road trip.
If you replace only two tires, fit the newer pair on the rear axle for better stability in the wet.

Remember that age matters as well as depth. Rubber hardens over time, so a tire more than six to ten years old should be inspected by a fitter even if the tread looks deep. The four digit date code on the sidewall shows the week and year it was made. More car care basics are in the car care section.

Tire Tread Depth FAQ

What is the minimum tread depth for car tires?

The legal minimum is 1.6 mm in the UK and EU, and 2/32 in (about 1.6 mm) in most US states. Most safety experts recommend replacing tires at 3 mm for good wet weather grip.

How often should you check tire tread depth?

Check tread depth once a month and before any long journey. It takes only a few minutes when you check your tire pressures at the same time.

Is the 20p test accurate?

The 20p test is a reliable quick check for the 1.6 mm legal limit, but it cannot measure exact depth. Use a tread depth gauge to track wear and plan replacement before you reach the limit.

Bottom Line

Buy a gauge, check every tire at three points once a month, and replace at 3 mm rather than waiting for the legal limit. It is the cheapest safety upgrade your car will ever get.