Tyre tread depth: the UK legal limit, the 20p test and when to replace
The legal minimum is 1.6 mm. The safe minimum is higher than that, and the gap between the two is where most of the risk sits. Here is what the law actually requires, how to check in under a minute, and why the tyre industry says replace sooner.
What the UK legal limit really says
For cars, vans and light trailers, the legal minimum tread depth in the UK is 1.6 mm. Two details in the wording matter more than the number, and both are routinely missed:
- It must be at least 1.6 mm across the central three-quarters of the tyre’s width. Deep tread at the edges does not compensate for a worn middle.
- It must be at least 1.6 mm around the entire circumference. One worn patch makes the whole tyre illegal, even if the rest looks fine.
That is why a tyre can look perfectly serviceable from where you are standing and still be illegal. Wear is often uneven, and the worn area may be on the inner shoulder facing the car, where you cannot see it without turning the wheel on full lock.
The fine is per tyre, not per car
Driving with a tyre below the legal limit carries a fine of up to £2,500 and three penalty points. That applies to each illegal tyre, not to the vehicle as a whole.
With all four tyres below the limit, that is a theoretical maximum of £10,000 and twelve penalty points — which on its own is enough to put a driver in front of a court facing disqualification. In practice most cases do not reach the maximum, but the multiplication is real and it is what catches people out.
Your insurance is the bigger risk
The fine is not usually the most expensive consequence. If you are involved in a collision on illegal tyres, your insurer may reduce or refuse a claim on the grounds that the vehicle was not roadworthy. That can leave you personally liable for damage to your own car and, more seriously, to other people’s. Set against the cost of a tyre, it is a poor trade.
The 20p test, in under a minute
The outer band of a 20p coin is a close enough stand-in for the legal minimum, which makes it the quickest check available to anyone with a pocket.
- Take a 20p coin and push it into one of the main tread grooves.
- If the outer band is hidden, the tread is above the legal limit at that point.
- If you can see the outer band, the tyre may be below 1.6 mm and needs checking properly.
- Repeat in at least three places around the tyre, and across the inner, middle and outer grooves.
- Do all four tyres. They rarely wear at the same rate, and fronts usually wear faster.
Checking one groove on one tyre is the mistake worth avoiding, because it misses exactly the uneven wear that makes a tyre illegal. Every couple of weeks, and before any long journey, is a sensible habit.
Tread wear indicators
Every modern tyre has small raised bars moulded into the bottom of the main grooves, set at 1.6 mm. Find the small triangle, TWI marking or brand logo on the sidewall and follow it inwards to locate them. When the surrounding tread has worn down level with those bars, the tyre is at the legal limit and needs replacing.
A depth gauge
A tread depth gauge costs very little and removes the guesswork. Push the probe into a main groove, press the base flat against the tyre and read the measurement, then repeat at several points across and around the tyre. It is the only method that gives you an actual number, which matters if you are trying to judge whether you have a season left.
Why the industry says 3 mm, not 1.6 mm
1.6 mm is the point at which a tyre becomes illegal. It is not the point at which it stops performing well. Tread exists to clear water out from under the tyre, and as it wears, that ability falls away sharply rather than gradually.
Testing by the UK research organisation MIRA found that wet stopping distances increase dramatically once tread drops below 3 mm. The difference in wet braking between a tyre worn to 3 mm and one worn to the legal 1.6 mm can be as much as 44 per cent. At 60 mph in the rain, that difference is measured in car lengths — frequently the difference between stopping short of something and not.
In a wet part of the country, that matters more than average. Most tyre manufacturers and safety organisations recommend replacing at around 3 mm for exactly this reason. Running to 1.6 mm is legal, and it is a long way from ideal.
What about the MOT?
The MOT applies the same 1.6 mm standard across the central three-quarters and the full circumference, and a tyre below it is a failure. The MOT also fails tyres for cuts, bulges, exposed cords, and incorrect or badly mismatched fitment. Passing an MOT means the tyre was legal on that day — it is not a prediction that it will still be legal in six months, and a tyre at 2 mm can pass and be illegal well before the next test.
Uneven wear tells you something is wrong
How a tyre wears is a useful diagnostic in its own right. Replacing a tyre without addressing the cause simply wears the new one out the same way.
- Both outer edges worn, centre fine — persistent under-inflation. The tyre is running on its shoulders. This also costs you fuel.
- Centre worn, edges fine — over-inflation. The middle of the tread is carrying the load.
- One edge only — usually wheel alignment or tracking, often after a kerb strike or a bad pothole.
- Patchy or scalloped wear — commonly worn suspension components or unbalanced wheels.
- Feathered edges across the tread blocks — typically a tracking problem.
If a tyre is worn on the inner shoulder only, it is worth knowing that this is the hardest pattern to spot and the most common way people fail an MOT on a tyre they had genuinely checked.
Age matters as well as depth
Rubber hardens and cracks with age regardless of mileage. A tyre with plenty of tread can still be unfit if it is old, which particularly affects caravans, motorhomes, trailers and second cars that cover very few miles. Check the sidewall for cracking, and look for the four-digit date code on the sidewall — the first two digits are the week, the last two the year, so 3419 means the thirty-fourth week of 2019.
When to replace, and what to fit
- Replace at 3 mm if you can, rather than running to the legal limit.
- Replace immediately for any bulge, split, exposed cord, or damage to the sidewall — regardless of how much tread is left. Sidewall damage cannot be safely repaired.
- Fit the correct size and speed rating for your vehicle. It is on the sidewall and in the handbook.
- Replace in pairs across an axle where you can, so grip is balanced left to right.
- Fit the better pair to the rear on most cars. Losing rear grip in the wet causes a much harder skid to recover than losing front grip.
If a puncture is in the central tread area, is small, and the tyre has not been driven on flat, it can often be repaired to the relevant British Standard. Sidewall or shoulder damage cannot, and neither can a tyre that has been run flat, because the internal structure will already have been destroyed.
Mobile tyre fitting across North Wales
If a check has turned up a tyre that needs replacing, our mobile tyre fitting service supplies, fits and balances new tyres at your home, work or the roadside, so you do not have to drive on it to get it sorted.
If a tyre is badly damaged or has a bulge in the sidewall, it is worth not driving on it at all. Call us and we will come to the vehicle.
Call 07768 491888More guides and tools
Questions, answered
What is the legal tyre tread depth in the UK?
The legal minimum for cars, vans and light trailers is 1.6 mm. It must be at least 1.6 mm across the central three-quarters of the tyre's width and around its entire circumference, so a single worn patch or a worn inner shoulder makes the whole tyre illegal.
Is 2 mm of tread legal?
Yes, 2 mm is above the 1.6 mm legal minimum, so it is legal. It is below the 3 mm that most tyre manufacturers and safety organisations recommend, however, because wet braking performance falls away sharply under 3 mm.
How do I do the 20p test?
Push a 20p coin into a main tread groove. If the coin's outer band is hidden, the tread is above the legal limit at that point. If you can see the band, the tyre may be below 1.6 mm and should be checked properly. Repeat at several points around each tyre and across the inner, middle and outer grooves.
What is the fine for illegal tyres in the UK?
Up to £2,500 and three penalty points per tyre. Because it applies per tyre rather than per vehicle, four illegal tyres could in theory mean £10,000 and twelve penalty points. Your insurer may also reduce or refuse a claim if you have a collision on illegal tyres.
Why do experts recommend 3 mm rather than 1.6 mm?
Because wet grip falls off sharply below 3 mm. Testing by MIRA found wet braking distance can be up to 44 per cent longer on a tyre worn to 1.6 mm compared with one at 3 mm. The legal limit marks where a tyre becomes illegal, not where it stops performing well.
Will my car fail its MOT on tyre tread?
Yes, a tyre below 1.6 mm across the central three-quarters or around any part of the circumference is an MOT failure. Tyres also fail for cuts, bulges, exposed cords and incorrect fitment. Passing an MOT only confirms the tyre was legal on the day of the test.
Why is only one edge of my tyre worn?
Wear on one edge alone usually indicates a wheel alignment or tracking problem, often following a kerb strike or a pothole. Wear on both outer edges suggests under-inflation, and wear in the centre suggests over-inflation. Fitting a new tyre without correcting the cause will wear the new one the same way.
Can a punctured tyre be repaired?
Sometimes. A small puncture in the central tread area can often be repaired to the relevant British Standard if the tyre has not been driven on while flat. Punctures in the sidewall or shoulder cannot be repaired safely, and neither can a tyre that has been run flat, because the internal structure will already be damaged.
How old is too old for a tyre?
Rubber hardens and cracks with age regardless of tread depth. Check the four-digit code on the sidewall: the first two digits are the week and the last two the year, so 3419 means week 34 of 2019. Age matters most on caravans, motorhomes, trailers and low-mileage cars, where tyres often age out long before they wear out.