Tyre Tread Depth Law in the UK: The 1.6mm Rule Explained

Tread depth is the most measurable thing on your car. There is a number, the number is 1.6mm, and either the tyre meets it or it does not. Despite that, tyres remain one of the largest sources of MOT failures and one of the most common reasons drivers are stopped at the roadside — largely because the rule is more specific than most people realise, and because the part of the tyre that fails is rarely the part anyone looks at.

This guide sets out exactly what the law requires, how a tester measures it, what an illegal tyre actually costs you, and how to check your own tyres properly in a couple of minutes.

The legal minimum, stated precisely

The requirement comes from the Road Vehicles (Construction and Use) Regulations 1986. For a car, the wording that matters is this: the grooves of the tread pattern must be at least 1.6mm deep throughout a continuous band comprising the central three-quarters of the breadth of the tread and round the entire outer circumference of the tyre.

Three separate conditions are buried in that sentence, and all three must be satisfied at once:

1. A depth of 1.6mm. Measured in the principal grooves of the tread pattern, not on the sipes or the fine decorative cuts.
2. Across the central three-quarters of the width. The outermost eighth on each shoulder is excluded from this particular measurement.
3. Around the entire circumference. Not at one point. Everywhere.

The limit is not the same for every type of vehicle:

For almost every reader, 1.6mm is the number that matters.

What "central three-quarters" actually means

Imagine the tyre's tread as a strip. Divide that strip into eight equal columns running around the wheel. The six columns in the middle — the central three-quarters — are the band where 1.6mm is legally required. The single column at each shoulder sits outside that band.

This is where drivers get caught out in both directions. Someone with a tyre worn smooth on the inner shoulder assumes it is legal because the middle looks fine, when in fact the wear usually extends well inside the six-column band. Someone else assumes any shoulder wear is automatically illegal, which is not quite how the depth rule is written.

The outer quarter is not a free pass

The excluded eighth at each edge is exempt from the 1.6mm depth requirement only. It is not exempt from everything else. A tyre with the cords or ply showing at the shoulder is a straightforward major defect on the grounds of exposed structure, regardless of what the tread depth is doing elsewhere. The same applies to cuts, bulges and tears in that area.

So the practical answer to "can I have a bald edge?" is no. Either the wear reaches into the measured band and breaches the depth rule, or it exposes the carcass and breaches the damage rule. There is no useful gap between the two.

How the MOT tester measures it

A tester uses a tread depth gauge and checks the principal grooves at several points around and across the tyre. They are looking for the worst reading, not an average, because the law is written as a minimum that must hold everywhere in the band.

The outcomes fall into three groups:

• Below 1.6mm anywhere in the central three-quarters — major defect, and the vehicle fails.
• At or fractionally above 1.6mm — a pass, but almost always recorded as an advisory along the lines of "tyre worn close to the legal limit". You can read what that wording means in practice in the tyre close to limit defect entry.
• Tread depth fine, but the tyre is damaged, incorrectly sized or badly worn on the shoulder — a separate defect with its own grade.

Testers do not remove wheels. That is worth knowing, because the inner shoulder of a front tyre is the single hardest part of a car to see, and it is exactly where alignment-related wear appears first. A tester will inspect what is visible with the wheels turned, but if you want certainty you need to look yourself with the steering on full lock.

Penalties apply per tyre, not per car

An illegal tyre is an offence under the Construction and Use Regulations, and the penalty is up to £2,500 and three penalty points for each tyre. That per-tyre structure is the part that surprises people.

• One illegal tyre: up to £2,500 and three points.
• Two illegal tyres: up to £5,000 and six points.
• Four illegal tyres: up to £10,000 and twelve points.

Twelve points accumulated within three years triggers a totting-up disqualification, so a full set of worn tyres is not a trivial matter for anyone who already has points on their licence. The £2,500 figure is a maximum available to a court, not a standard charge; most roadside cases are resolved with a fixed penalty and points, and many result in a prohibition notice preventing the vehicle from being driven until the tyre is replaced.

There is a second cost that rarely gets mentioned. If you have a collision on tyres that were illegal at the time, your insurer can treat the vehicle as unroadworthy and decline to pay for your own damage. Third-party liability generally still has to be met because of the way motor insurance law works, but the insurer may then pursue you for what it has paid out.

How to measure tread depth at home

You need roughly five minutes and, ideally, a gauge that costs about the same as a coffee.

Use a proper gauge

A dial or digital tread depth gauge is the only method that produces a number. Push the probe to the bottom of a principal groove with the base flat against the tread blocks, and read it off. Take readings at three positions across the tyre — inner, centre and outer, all within the measured band — and repeat at three or four points around the circumference. Record the lowest figure you find. That is the one that counts.

The 20p test and its limits

If you have no gauge, insert a 20p coin into a main groove. The coin has a raised outer band, and if that band is hidden by the tread on all sides, you have a reasonable margin above the limit. If the band is clearly visible, the tyre is at or near the point where it needs replacing and should be measured properly.

This is a screening tool. It tells you "check this" or "probably fine". It does not tell you whether a tyre is legal, and it is easy to fool yourself by testing only the deepest groove in the middle of the tread.

Tread wear indicators

Every tyre sold in the UK has moulded tread wear indicators: small raised bars sitting in the base of the principal grooves at exactly 1.6mm. Find the small triangle, the letters TWI, or the manufacturer's logo on the shoulder of the sidewall, then follow that mark inwards to locate the bar.

When the surface of the tread has worn down level with those bars, the tyre is at the legal limit. Level with the bars means the tyre is finished, not that it has a little left.

Where to look

Check all four tyres, not the two you can see from the pavement. Turn the steering onto full lock so you can inspect the inner shoulders of the front tyres, which is where the majority of surprise failures hide. Look for uneven wear across the width as well as the absolute depth.

Why edge wear is a warning about something else

A tyre that wears evenly is simply doing its job. A tyre that wears unevenly is reporting a fault somewhere else, and replacing it without fixing the cause means the new one wears out the same way.

Alignment problems in particular are cheap to correct and expensive to ignore. A car that eats the inside edge of both front tyres can destroy a pair of otherwise healthy tyres in a few thousand miles, and the same worn bush causing it will eventually show up as a suspension defect at MOT time. Related failures such as worn suspension arm bushes frequently appear on the same certificate as tyre advisories for precisely this reason.

The other tyre rules that fail cars

Tread depth is only one of several tyre requirements checked during the test.

• Cuts, lumps and bulges. A cut deep enough to reach the ply or cord is a failure, as is any bulge or egg-shaped lump, which indicates internal structural damage from a pothole or kerb impact. These are graded under tyre damage.
• Exposed cord or ply anywhere on the tyre, including the sidewall.
• Incorrect size, load index or speed rating for the vehicle. Fitting a lower speed rating than the manufacturer specifies is a defect.
• Mismatched tyres across an axle. Tyres on the same axle must be of the same size and structure type. Mixing radial and cross-ply construction across axles is also restricted, with cross-ply not permitted on the rear when radials are fitted at the front.
• Perished or cracked sidewalls, common on cars that cover very low mileage and sit for long periods.
• Valve condition and security, including damaged or badly angled valve stems.
• Tyre pressure monitoring. On cars first used from 1 January 2012, an inoperative or missing TPMS is a major defect.
• Part-worn tyres, which are legal to sell but must carry at least 2mm of tread and be marked "PART WORN" in letters at least 4mm high.

There is no general legal age limit on car tyres, though there are age restrictions on the front axles of larger vehicles and on minibuses. Rubber degrades regardless of use, and a ten-year-old tyre with 6mm of tread can still be unsafe. The four-digit code on the sidewall gives the week and year of manufacture — 3121 means the thirty-first week of 2021.

Four persistent myths about the tread rule

"The limit is 1.6mm across the whole tyre." It is not. The depth requirement applies to the central three-quarters of the breadth. That said, the outer eighths are still governed by the damage rules, so the practical effect is close to the same.

"As long as one groove is above 1.6mm the tyre is legal." No. The requirement is a continuous band across the central three-quarters, so the shallowest point within that band is what determines legality. A single deep groove does not rescue a tyre that is worn flat either side of it.

"Winter tyres are exempt." They are not. Winter and all-season tyres must meet the same 1.6mm minimum, though their tread wear indicators sit at the same depth and they are widely considered to lose their cold-weather advantage well before that point — most manufacturers advise replacing winter tyres at around 4mm.

"An advisory means the tyre will last another year." An advisory means the tester measured a depth that is legal today. A tyre at 2mm on a car covering 12,000 miles a year will not still be legal at the next test. Advisories are a warning, not a reprieve.

There is a fifth belief that is not exactly a myth but is worth correcting: that tyre depth is checked only at the MOT. Roadside enforcement by police and DVSA examiners happens all year, and an illegal tyre found in March does not become less illegal because the certificate was issued in January.

When to replace: the case for going above 1.6mm

The legal minimum is the point at which a tyre becomes an offence, not the point at which it stops working properly. Wet grip declines sharply as tread depth falls, because the grooves have less volume available to clear water from the contact patch. Stopping distances in the wet lengthen noticeably between 3mm and 1.6mm, and aquaplaning resistance falls away at the same time.

Most tyre manufacturers and road safety organisations recommend replacing at around 3mm for this reason. A new tyre typically starts life with around 8mm of tread, so replacing at 3mm gives up a modest slice of the tyre's life in exchange for a meaningful safety margin — and it removes any risk of drifting below the limit between annual tests.

If your MOT certificate carries an advisory for a tyre close to the limit, treat it as a twelve-month countdown that has already started. Advisories are the tester telling you in writing what will fail next year.

Check what your car's record already says

Every MOT test result in Great Britain since 2005 is public, including all recorded advisories. If a tyre was flagged as close to the limit at the last test, or if the same corner of the car has attracted tyre comments year after year, that pattern tells you something an inspection on the day will not.

You can pull up any vehicle's full test history from the registration alone using the MOT history check, and if you are not sure when your next test falls due, the MOT due date lookup answers that in a couple of seconds. For a broader view of everything a tester will be looking at, the guide to what is checked on an MOT covers the full inspection.