How to check your car's tyre tread depth safely

Why tread depth deserves your attention

British weather rarely gives you a dry, grippy road for long. That thin layer of rubber between your car and the tarmac is doing an enormous amount of work, and the grooves cut into it are there for one main reason: to clear water out of the way so the rubber can stay in contact with the road.

As tread wears down, the tyre has less capacity to shift standing water. Braking distances stretch out, cornering feels vaguer, and in heavy rain you run a much greater risk of aquaplaning – where the tyre rides up on a film of water and you lose steering and braking altogether. None of that happens with much warning. A tyre that looked fine last month can be well past its best by the time autumn rain arrives.

The legal minimum, and what actually counts

In the UK, the legal minimum tread depth for cars is 1.6mm across the central three-quarters of the tyre's width, around the entire circumference. Miss that and you're not just unsafe, you're breaking the law. Each worn tyre can attract a fine of up to £2,500 and three penalty points, and if you're stopped with more than one below the limit, those penalties stack up. You could also find your insurance cover affected after a claim.

Two things catch drivers out. First, the limit applies around the whole tyre, not just the bit you can easily see at the bottom. Second, many tyres wear unevenly – the inner shoulder can be down to the cords while the outer edge still looks healthy. If you never look at the inside edge, you're only getting half the picture.

One more point: 1.6mm is a legal floor, not a target. Most motoring bodies suggest thinking seriously about replacement at 3mm, because wet-weather performance starts dropping off noticeably below that.

The 20p test, step by step

The 20p coin is a genuinely useful tool here, and it costs you nothing. Here's how to use it properly:

• Park on level ground, switch off the engine, and apply the handbrake. If you're on a road, make sure you're well away from traffic – ideally on a driveway or in a car park.
• Find the main grooves running around the tyre. On most tyres there are three or four wide channels across the tread.
• Push the coin into a groove with the outer band of the coin facing you.
• If you can see the outer band of the 20p above the tread, the groove is shallower than roughly 1.6mm and the tyre needs attention.
• If the band is hidden by the rubber, there's more than the legal minimum in that spot – but keep testing.
• Repeat at three points across the width of the tyre (inner, centre and outer) and at several points around the circumference. Rotate the wheel a quarter turn at a time so you're not just testing one patch.

Do this on all four tyres, and don't forget the spare if your car carries one. A space-saver or full-size spare that's been quietly losing tread for years is no help at the roadside.

Checking that bit more thoroughly

A coin gives you a rough answer. If you want a precise figure, a tread depth gauge costs a few pounds and takes seconds to use – push the probe into the groove, press the collar flat against the tread, and read the measurement. Many tyres also have tread wear indicators: small raised bars inside the main grooves, set at 1.6mm. When the tread surface sits level with those bars, the tyre is at the legal limit and must be replaced.

Check tyres when they're cold and the car hasn't just been driven hard, and take the opportunity to look at the sidewalls too. Cuts, cracks, bulges, or objects such as screws and nails all mean a trip to a tyre fitter, even if the tread is fine.

When to replace rather than wait

Replace a tyre if any of the following apply:

• Tread is at or below 1.6mm anywhere across the central three-quarters.
• Tread is between 1.6mm and 3mm and you do a lot of motorway miles or regularly drive in wet conditions.
• Wear is noticeably uneven across the tyre, which often points to alignment or suspension issues that need checking too.
• There are bulges, deep cuts, exposed cords, or signs of cracking in the sidewall.
• The tyre is old – rubber hardens with age, so anything over five to six years old is worth having inspected even if it looks healthy.

It's also worth checking pressures at the same time. Under-inflation wears the shoulders, over-inflation wears the centre, and both shorten the life of a tyre that might otherwise have plenty of tread left.

Making it a habit

Set a reminder to check your tyres every two weeks and before any long journey. It takes five minutes, needs no special equipment beyond a 20p coin, and it's the single easiest way to keep yourself legal and, more importantly, to keep your car predictable when the road turns wet. If anything looks borderline, get a second opinion from a qualified tyre technician – fresh rubber is far cheaper than an incident you could have prevented.