How to check your tyre tread
The UK legal minimum tyre tread depth for a car is 1.6mm, measured in the primary grooves across the central three-quarters of the tread and all the way around the tyre. The fastest home check is the 20p test: push a 20p coin into a main groove, and if the coin’s outer band disappears you are probably above the limit. Two details catch people out. Only the deep primary grooves count, so a coin sitting in a shallow sipe can read as legal when the tyre is not, and tread below the limit is graded Dangerous in the MOT rather than a simple fail.
Your tyres are the only part of the car that touches the road, and the tread on them is what clears standing water, grips in the wet and lets you stop. Checking them takes a couple of minutes and needs nothing more than a coin. The catch is that most guides stop at "use a 20p", and the detail that decides whether your check is meaningful sits in the DVSA inspection manual rather than on the coin. This guide covers the check itself, and then exactly what an MOT tester is looking at when they get to your tyres.
The UK legal limit, and the part people get wrong
For a car, the minimum legal tyre tread depth is 1.6mm. The MOT inspection manual is precise about where that 1.6mm has to be. The primary grooves of the tread pattern must be at least 1.6mm deep within the central three-quarters of the breadth of the tread, and around the entire outer circumference of the tyre. Two conditions, both of which have to hold.
The circumference part is the one drivers usually understand: it is not enough for the tread to be legal at the point you happened to look. It has to be legal all the way round. The other part surprises almost everyone. The manual states plainly that either side of the central three-quarters of the tyre can be devoid of tread, which is to say bald. Only the middle band is measured.
That is a legal fact rather than a recommendation. A tyre worn through on its outer shoulder is still passing on tread depth, but it is telling you something has gone wrong with pressures, alignment or suspension, and the wear will not stay politely on the edge. Treat a bald edge as a fault to investigate, not a technicality to lean on.
Different vehicles carry different limits. The 1.6mm figure covers cars, most vans and light goods vehicles up to 3,500kg. Vehicles with more than eight passenger seats, and cars first used before 3 January 1933, work to 1.0mm instead. The Highway Code sets out the same limits in Annex 6.
Primary grooves versus sipes, and why the coin can lie
This is the single most useful thing on this page. The manual splits the tread into primary and secondary grooves. Primary grooves are the deep, continuous ones that run around the tyre, the ones with the tread wear indicator bars moulded into the bottom of them, or grooves cut as deep as those when the tyre was new. Everything else, the shallower channels and the fine slits called sipes, counts as secondary and is specifically excluded when tread depth is assessed.
A 20p coin dropped into a shallow secondary groove sits high, hides its outer band, and tells you the tyre is fine. The tyre may not be fine at all. On modern tread patterns with a lot of siping, this is an easy mistake to make, and it is the reason a home check and a tester's gauge sometimes disagree. Find the deepest continuous grooves, look for the small raised bars sitting in the bottom of them, and use those.
Those bars are useful on their own. When the surface of the tread has worn down level with the tread wear indicator bars, the tyre is at or about 1.6mm. No coin required.
How to do the 20p test properly
A 20p coin has a raised band running around its edge, and that band sits roughly at the height that matters. TyreSafe, the UK tyre safety charity, sets out the method:
| Step | What to do |
|---|---|
| 1 | Clean any stones or debris out of the tread grooves |
| 2 | Insert a 20p coin into a main tread groove, the deepest and most continuous ones |
| 3 | Look at whether the outer band of the coin is visible |
| 4 | Repeat at several points across and around each tyre |
If the outer band of the coin is hidden, the tread is above the legal limit at that spot. If you can see the band, the tyre may be illegal and should be measured properly by a tyre professional. TyreSafe recommends checking at least three locations around each tyre, and doing the whole check at least once a month or before any long journey.
Check the inner, middle and outer primary grooves too, not just the one facing you. Tyres wear unevenly, and the inner shoulder, the part you cannot see without turning the wheel on full lock, is very often the worst. If any part of a tyre fails the test, treat the whole tyre as suspect.
What your tread depth is actually worth
TyreSafe puts a new car tyre at roughly 7mm to 9mm of tread, varying by brand, model and whether it is a summer, winter or all-season design. Set against that, the legal limit is a sliver.
| Tread depth | Where that leaves you |
|---|---|
| 7mm to 9mm | New. Full wet-weather performance |
| 4mm to 5mm | Roughly half worn. Nothing to do yet, but worth tracking |
| 3mm | The point many drivers choose to start pricing up replacements |
| 2mm | Close enough to the limit that an MOT advisory is likely |
| Below 1.6mm | Illegal, and a Dangerous defect at the MOT |
Nothing changes at 1.6mm except the law. Wet grip falls away steadily as tread wears, which is why replacing at around 3mm is a common choice rather than a cautious one. It also buys you time to shop around instead of accepting whatever is in stock the week your MOT is due. Our guide to what new tyres cost covers what a fair price looks like.
Reading uneven wear
How a tyre wears is a free diagnostic. Even wear across the tread is what you want. Wear concentrated in one place points at something specific.
| Wear pattern | Common cause |
|---|---|
| Both outer edges worn, centre fine | Under-inflation, the tyre running with too little pressure |
| Centre worn, edges fine | Over-inflation, the tyre running with too much pressure |
| One inner or outer edge worn | Wheel alignment or a suspension component out of specification |
| Patchy or scalloped wear | Worn suspension parts, or wheels out of balance |
| Flat spot in one place | A heavy braking event, or a wheel that locked |
| Feathered edges you can feel one way but not the other | Toe setting out, usually alignment |
Fitting a new tyre without fixing the cause simply buys you the same wear again. If one edge is going, get the geometry checked; our guide to wheel alignment costs covers what that involves. Vibration through the wheel alongside patchy wear is usually balancing or a warped disc, which why is my steering wheel shaking goes into.
TyreSafe notes there are well over 25 different variables that affect how long a tread lasts, from the compound and the tread pattern to road surface and driving style. Two identical cars can wear a set of tyres at very different rates, so go by what you measure rather than by mileage.
Everything else the MOT checks on your tyres
Tread depth is one line in a much longer list. This is the full set of tyre defects from section 5.2.3 of the inspection manual, with the grade each one carries. A Minor is a pass with a note, a Major is a fail, and a Dangerous is a fail plus a warning not to drive the car away.
| Defect | Grade |
|---|---|
| Tread depth not in accordance with the requirements | Dangerous |
| A lump, bulge or tear caused by separation or partial failure of the tyre’s structure, or exposed or damaged cords | Dangerous |
| A valve seriously damaged or misaligned, likely to cause sudden deflation | Dangerous |
| Load capacity insufficient for the weight on the axle | Dangerous |
| A cut deep enough to reach the ply or cords, beyond the permitted limits | Major |
| Tyres of different sizes on the same axle | Major |
| Tyres of different structure on the same axle, such as radial next to cross-ply | Major |
| A tyre fouling part of the vehicle | Major |
| A tyre not fitted in line with the manufacturer’s sidewall instructions | Major |
| A tyre incorrectly seated on the wheel rim | Major |
| Tyre pressure monitoring system malfunctioning or obviously not working | Major |
| Load capacity or speed rating below the minimum requirement | Major |
| A tyre obviously under-inflated | Minor |
Notice which faults sit in the Dangerous band. Worn tread keeps company with a sidewall bulge and a failing valve, which is a fair reflection of what a tyre at 1.4mm does in standing water. If any of these come back on your certificate, our guide to what an MOT advisory means explains the difference between a note and a fail, and why cars fail the MOT covers the wider picture.
Cuts and bulges, and how a tester judges them
Not every mark in a sidewall is a fail, and the manual sets out how the call is made. Any ply or cord visible without touching the tyre is a fail. If folding back the rubber or opening a cut with a blunt instrument exposes ply or cord, that is a fail regardless of the size of the cut. A cut longer than 25mm, or more than 10% of the section width if that is greater, where cords can be felt but not seen, is also a fail. If the tester cannot be sure it is cords rather than a stone lodged in there, the manual tells them to pass and advise.
Bulges are treated differently and more harshly, because a lump caused by structural separation means the casing has started to come apart internally. That is a Dangerous defect, and it is one of the few tyre faults where the sensible response is not to drive the car at all. Testers are also told to distinguish a genuine bulge from the normal undulations left in a carcass by manufacturing, so a slight ripple is not automatically a problem. If you find a distinct bulge, get it looked at the same day.
The tyre pressure warning light that fails the MOT
Tyre pressure monitoring is inspected on M1 vehicles first used on or after 1 January 2012, which covers most cars on the road today. A system malfunctioning or obviously not working is a Major defect, so it fails.
The distinction that matters is between a system reporting low pressure and a system that is broken. The manual explains that the warning lamp normally lights and goes out when the ignition is switched on, that it stays on if the system has detected a pressure loss, and that a genuine malfunction often makes the lamp flash a number of times before staying on. If it is not clear which is happening, the tester adds an advisory rather than failing the car.
In practice, correct the pressures first. If the light clears, it was doing its job. If it stays on with all four tyres set correctly, a sensor has usually failed, and those sit inside the wheel on a sealed battery that lasts a few years, so on an older car it is a wear item. Fix it before the test rather than after. Our guide to preparing for your MOT lists the other easy wins.
Mixing tyres, and the ten-year rule that does not apply to you
Two things are commonly misunderstood. First, mixing. Tyres of different sizes on the same axle are a Major defect, and so are tyres of different structure, meaning you cannot put a cross-ply next to a radial. Steel and fabric radials count as the same structure. Run flat and conventional tyres on the same axle are allowed by the manual, which adds that it is not recommended. An asymmetric tyre fitted the wrong way round is not a reason for rejection, though the tester will point it out.
Second, tyre age. There is a widely repeated belief that tyres over ten years old are illegal. That rule is real, but it is not about your car. The age check applies to vehicles with more than eight passenger seats: a tyre over ten years old must be failed on any front steered axle, or any axle of a minibus with a single wheel fitment. The Highway Code adds goods vehicles over 3.5 tonnes. For an ordinary car there is no ten-year cut-off in the MOT.
Age still matters though, just through condition rather than the calendar. Rubber hardens and cracks with time and sunlight, and a tyre with plenty of tread and a network of fine cracks in the sidewall has lost grip you cannot measure with a coin. You can read the date on any tyre: find the four-digit code in a window on the sidewall, where the first two digits are the week of manufacture and the last two are the year. A code reading 3516 means week 35 of 2016. A three-digit code means the tyre is more than ten years old.
Your spare is not being checked by anyone else
The inspection manual is explicit that the tester only needs to inspect the tyres fitted to the road wheels at the time of the inspection, and that if they notice a defect on a spare wheel or tyre they should advise the presenter rather than record it. So your spare is not tested, and it never has been.
That matters because a spare sits untouched for years, slowly losing pressure, and a full-size spare fitted to a car after a puncture has to be legal like any other tyre. Check its pressure and its tread when you do the other four. If your car has a space saver instead, note the speed limit printed on it, usually 50mph, and treat it as a way of getting to a garage rather than a tyre.
Pressure, the other half of the job
Correct pressure keeps the tread in proper contact with the road, keeps the wear even and protects your fuel economy. Under-inflation wears both shoulders and is a Minor MOT defect in its own right. The right figures for your car are in the handbook, and usually on a label inside the driver’s door frame or the fuel filler flap.
Check pressures cold, before the car has been driven far, because a warm tyre reads high. Once a month and before long trips is the sensible rhythm, and it is the same trip to the driveway as the tread check, so do both together. The long drive checklist covers the rest of what is worth a look before a big journey, and winter car checks deals with the colder months, when pressures drop and wet grip matters most.
The law, and what a bald tyre actually costs
Using a vehicle with a defective tyre is endorsement code CU30 on the DVLA's published list, and it carries 3 penalty points. The points apply per tyre. A car on four illegal tyres can therefore collect 12 points from a single stop, and 12 points inside three years normally means disqualification. Fines can reach £2,500 per tyre on top of that.
Set that against the cost of checking. A coin, two minutes, once a month. Tyres are also among the most common MOT failure items, which is frustrating precisely because they are the easiest thing on the whole test to check yourself in advance. If you want to know what else the tester looks at, what is checked in an MOT test walks through it, and what an MOT should cost explains the fixed maximum fee.
When to replace, and how many
Replace when the tread reaches the point you have chosen, not when the law forces you. If you are replacing, do it in pairs across the same axle so grip is balanced side to side. A new tyre next to a half-worn one gives the two ends of an axle different grip, which shows up under braking in the wet and is exactly the sort of imbalance the MOT brake test is designed to catch.
New tyres normally go on the rear, whichever wheels drive the car, because a car that loses grip at the back first is much harder to correct than one that understeers. Any decent tyre fitter will do this without being asked, but it is worth knowing why if they move your wheels around.
Make it a two-minute monthly habit
Keep a 20p coin in the door pocket. Once a month, put it in the deepest groove on each tyre at three points around, look for cuts, bulges and anything embedded in the rubber, check the pressures cold, and glance at the spare. Anything that looks off gets booked in rather than watched.
Tyres wear gradually, which is why they catch people out. A regular quick look means you replace them on your terms, at a price you chose, rather than the week your MOT is due. If a check turns something up, you can check when your MOT is due and compare local garages on YourCarGarage to get it looked at.
Key takeaways
- The legal minimum is 1.6mm in the primary grooves, across the central three-quarters of the tread, around the whole tyre.
- The MOT manual allows the outer edges either side of that central band to be completely bald, so where you measure changes the answer.
- Only primary grooves count. A 20p coin resting in a shallow sipe will read as legal on a tyre that is not.
- Worn tread is graded Dangerous in the MOT, in the same band as a sidewall bulge or a damaged valve.
- A tyre pressure monitoring fault is a Major defect on cars first used on or after 1 January 2012.
- New car tyres start at roughly 7mm to 9mm, so 1.6mm is the last sliver of the tyre’s life, not a target.
- The ban on tyres over ten years old applies to buses, coaches and minibuses, not to your car.
- Your spare is not inspected at the MOT, so nobody else is checking it for you.
- A defective tyre is 3 penalty points under DVLA code CU30, and the points apply per tyre.
Frequently asked questions
What is the legal tyre tread depth in the UK?
For a car it is 1.6mm. The MOT inspection manual is specific about where: the primary grooves of the tread pattern must be at least 1.6mm deep within the central three-quarters of the breadth of the tread and around the entire outer circumference of the tyre. Below that the tyre is illegal and the MOT records it as a Dangerous defect.
Can part of my tyre be bald and still be legal?
Yes, technically. The MOT manual states that either side of the central three-quarters of the tyre can be devoid of tread. Only the central band and the full circumference are measured. That does not make a bald edge a good thing, because it almost always points to pressures, alignment or worn suspension, and the tyre is likely to be near the limit in the middle before long.
Which grooves should the 20p coin go in?
The primary grooves, meaning the deepest continuous grooves that run around the tyre and contain the tread wear indicator bars. Shallower grooves and the fine slits called sipes are secondary and are not counted when tread depth is assessed. Put the coin in a shallow groove and it can hide the outer band on a tyre that is actually below the limit.
Should I really wait until 1.6mm to replace a tyre?
No. It is the legal floor, not the ideal. New car tyres start with roughly 7mm to 9mm of tread according to TyreSafe, so a tyre at the limit has almost nothing left. Grip in the wet drops off well before 1.6mm, which is why many drivers replace at around 3mm to keep a sensible margin.
How much tread does a new tyre have?
TyreSafe puts new car tyres at roughly 7mm to 9mm, varying by brand, model and whether the tyre is a summer, winter or all-season design. That is a useful reference point, because it shows that a tyre at 3mm has already used most of its usable depth.
Does the MOT check my spare tyre?
No. The inspection manual says the tester only needs to inspect the tyres fitted to the road wheels at the time of the inspection, and that if they notice a defect on a spare they should advise the presenter. So a spare or space saver in the boot is never formally tested, which is exactly why it is worth checking yourself.
Will a tyre pressure warning light fail the MOT?
It can. On M1 vehicles first used on or after 1 January 2012, a tyre pressure monitoring system that is malfunctioning or obviously not working is a Major defect, which is a fail. A light on simply because a tyre is genuinely low is a different matter: an obviously under-inflated tyre is recorded as a Minor defect. Either way, put the pressures right and clear the fault before the test.
Are tyres over ten years old illegal?
Not on a car. The age rule in the MOT applies to vehicles with more than 8 passenger seats, and tyres over ten years old must be failed on any front steered axle or any rear axle of a minibus with a single wheel fitment. The Highway Code adds heavy goods vehicles over 3.5 tonnes. Age still matters on a car, because rubber perishes and cracks, but there is no ten-year MOT cut-off for ordinary cars.
Can I mix different tyres on the same axle?
Not freely. Tyres of different sizes on the same axle are a Major defect, and so are tyres of different structure, such as a radial-ply next to a cross-ply. The manual does allow run flat and conventional tyres on the same axle, while noting that it is not recommended. Fitting an asymmetric tyre the wrong way round is not a reason for rejection, but the tester will advise you.
What is the penalty for driving on a bald tyre?
Using a vehicle with a defective tyre is endorsement code CU30 on the DVLA list and carries 3 penalty points. The points apply per tyre, so a car on four illegal tyres can reach 12 points, which normally means disqualification. Fines can reach £2,500 per tyre on top.