How to Check Tyre Tread Depth: The 20p Test and Legal Limit

Tread is what grips the road when it's wet, and it wears away so slowly that you'll never notice it going. One day it's perfectly legal, a few months later it isn't. The good news is that you can test it in under a minute with a coin.

I've based this on guidance from the TyreSafe charity and the Highway Code on GOV.UK. Both are clear on the numbers, and I'll point out where the legal limit and the advice differ.

The legal tread depth for cars in the UK

TyreSafe says the legal tread depth minimum for cars, light vans and light trailers in the UK is 1.6mm. The Highway Code adds that this must be present across the central three-quarters of the breadth of the tread and around the entire circumference of the tyre.

Motorcycles, large vehicles and passenger-carrying vehicles have a lower limit of 1mm, and the Highway Code says mopeds should have visible tread. The 1.6mm rule here is the one for ordinary cars.

Why 3mm is the tread depth experts suggest

Legal doesn't mean ideal. TyreSafe says the minimum tread depth commonly advised by tyre and safety experts is 3mm, and its checks page says to consider replacing tyres once tread falls below that.

The reason is stopping distance. TyreSafe's tread page quotes tests by the vehicle research company MIRA, which found cars with 1.6mm of tread had up to 44% longer braking distances in the wet than cars with 3mm. That's a big gap for a difference of only 1.4mm.

The 20p coin test for tyre tread

This is the quickest check and needs nothing but a 20p coin. TyreSafe describes it like this:

1. Take a 20p coin and slot it into one of the main grooves of the tyre.
2. Look at the coin's outer band.
3. If you can't see the outer band, the tread is above the legal limit.
4. If the band is visible, the tread may be at or below the limit, so get the tyre checked.

Repeat it in several places across the tyre, not just one spot, because tread can wear unevenly. The coin test only tells you whether you're above the legal limit. It won't show whether you're nearing the 3mm advice level.

Using a tread depth gauge for a precise reading

A tread depth gauge measures individual grooves in millimetres, so it's more precise than the coin. TyreSafe lists it as the best option if you want an exact number. Push the probe into a main groove in the central three-quarters of the tyre, press the base flat against the tread and read the measurement. Take readings in a few places around the tyre and across it.

Method	What it tells you	Good for
20p coin	Whether tread is above the legal 1.6mm	A quick check in the driveway
Tread depth gauge	The depth of each groove in millimetres	Spotting when you're nearing 3mm
Wear indicator bars	Whether the tyre is worn down	Spotting worn tyres at a glance

Tread wear indicator bars on your tyres

Raised bars sit inside the grooves. TyreSafe describes them as easy to spot and a good way of identifying worn tyres: when the tread wears down level with those bars, it's time to act. Look for the small marking on the sidewall that points to where they sit.

How much tread new tyres start with

TyreSafe says new tyres typically start with 7 to 9mm of tread. That puts the 3mm advice in context: by then a tyre has worn through most of its starting tread.

TyreSafe also says that tyres wear down even with careful driving, which is why monitoring them matters more than guessing from the age of the tyre or the miles you've covered.

Damage to look for alongside tread

Depth isn't the only thing that can make a tyre unsafe. TyreSafe recommends inspecting the sidewalls and tread regularly for cuts, bulges or other visible damage. The Highway Code adds that some defects can attract penalty points, and that the date of manufacture marking on the tyre should be legible because it shows how old the tyre is.

• Run your eye around the full sidewall on both sides of each tyre.
• Look for bulges, cuts or anything lodged in the tread.
• Check the date marking is readable.
• If anything looks wrong, have a tyre specialist inspect it rather than carrying on.

The penalties for illegal tyres

TyreSafe says that driving on tyres below the limit can mean substantial fines and three penalty points per tyre. With four tyres, that adds up quickly, and an illegal tyre is a safety problem long before it's a legal one.

Tread depth checks and the rest of your tyre routine

TyreSafe recommends checking your tyres once a month and before long journeys, and tread is only one part of it. You'll also want to look over the sidewalls for cuts and bulges, and make sure the air pressure is right. If you haven't done that before, this walk-through on checking tyre pressure takes you through it step by step.

Sharing your tyre and motoring knowledge

If tyres and everyday car safety are your thing, here's somewhere that welcomes people who can write about them.

Mechanics and car writers can check automotive blogs that welcome contributors here.

Frequently asked questions

What is the minimum legal tread depth for car tyres in the UK?

TyreSafe says it's 1.6mm for cars, light vans and light trailers, across the central three-quarters of the tread and around the whole circumference.

When should I replace my tyres?

TyreSafe says to consider replacing them once tread falls below 3mm, which is the minimum commonly advised by tyre and safety experts, well before the legal limit.

How does the 20p test work?

Put a 20p coin into the tread groove. If you can't see the coin's outer band, TyreSafe says your tyres are above the legal limit.

Is a tread depth gauge better than the 20p test?

A gauge is more precise because it reads each groove in millimetres. The coin is a quick pass or fail check against the legal limit.

Do I need to check all four tyres?

Yes. The penalty applies per tyre, so check every tyre, and check in several places on each one because wear can be uneven.