Tread is what clears water out from under a tyre so the rubber can grip in the wet. As it wears down, that ability fades, which is why a quick monthly check is one of the most useful habits a driver can keep. There are three ways to do it, from a coin in the pocket to a proper gauge.
The legal minimum
In the UK, the legal tread depth is 1.6mm, measured across the central three-quarters of the tread and around the whole circumference. A tyre below that is illegal, and the penalty is steep: a fine and penalty points per tyre, as set out in the rules on tread depth and the law. Well before that point, wet grip has already dropped off, so 1.6mm is a legal floor rather than a sensible replacement target.
Three ways to check
The 20p test is the no-tools option. A 20p coin slotted into a main groove gives an instant rough answer: if the coin's outer band disappears, there is a healthy margin of tread; if it stays visible, the tyre is close to the limit and needs a proper look. The full method is in the guide to the 20p test.
A tread depth gauge gives an exact figure in millimetres. It is the way to know precisely where a tyre stands, useful for tracking wear over time or judging a borderline tyre, and using one correctly takes seconds.
The tread wear indicators are built into the tyre. These small raised bars sit at the bottom of the grooves at exactly 1.6mm; when the surrounding tread wears down level with them, the tyre is at the limit. They are covered in the guide to tread wear indicators.
Where to measure
Tyres rarely wear evenly, so where the reading is taken matters:
- Measure in the main circumferential grooves, never on a wear bar or in the fine sipes
- Check at several points around the tyre, not just one spot
- Check across the width too, inner, middle and outer
- Judge the tyre by its lowest reading, since that is the part that fails first
A tyre worn more on one side than the other is telling a story of its own, usually pressure or alignment, which is worth reading from the wear pattern rather than ignoring.
Don't forget the inner edge
The most commonly missed problem is wear on the inner shoulder, the edge facing under the car. It is hard to see without turning the wheel on full lock or crouching down, and it is exactly where alignment faults wear a tyre first. A tyre that looks fine across the visible face can be down to the cords on the hidden edge, so it pays to check there too.
From the workshop: the number of tyres that pass a glance from the front but are bald on the inner edge is the one that catches people out. Turn the wheel onto full lock and look at the inside shoulder, that's where the MOT tester looks, and that's where the surprises are.
Sources and accuracy. The legal minimum and checking methods here reflect UK law and TyreSafe guidance at the time of writing and can change. Anything safety-critical should be confirmed against current official guidance. If anything here looks wrong, get in touch and we will check it and put it right.
Common questions
What is the legal tread depth in the UK?+
1.6mm, measured across the central three-quarters of the tread and around the whole circumference of the tyre. Below that, a tyre is illegal and a serious safety risk, with fines and penalty points for each one.
How do I check my tyre tread at home?+
The quickest way is the 20p test: pop a 20p coin into the main grooves at several points. If the coin's outer band is hidden, the tread is above the limit. For an exact figure, a tread depth gauge is better.
Where should I measure tyre tread?+
In the main circumferential grooves, not on the raised wear bars or the fine sipes. Measure at several points across the width and around the tyre, and judge by the lowest reading, since tyres rarely wear evenly.
How often should I check my tread?+
Once a month is sensible, alongside a pressure check, and before any long trip. Catching low tread early gives time to plan a replacement rather than being caught out at an MOT or in the wet.