How do I check tyre tread depth and know if a tyre is legal?
The 1.6mm legal minimum and where it has to be met, the moulded wear indicators that do the measuring for you, and why 3mm is the sensible replacement point.
- Difficulty
- beginner
- Time
- 10 min
- Read
- 4 min
- Safety
- warning
Short answer
The legal minimum is 1.6mm across the central three quarters of the tread, all the way round. Every tyre has small raised bars moulded into the bottom of the main grooves at exactly that depth — when the tread wears level with them, the tyre is illegal. Most specialists advise replacing at around 3mm.
Tread does one job: it clears water out of the way so the rubber can touch the road. Below about 3mm it stops doing that well, and the wet braking distance grows sharply long before the tyre becomes illegal.
Safety
What you'll need
- A 20p coin, or a tread depth gauge
- A torch (optional)
Step by step
- Find the wear indicator bars.Look along the main grooves for small raised blocks bridging the groove. Their tops sit at exactly 1.6mm. Many tyres also mark their position with a small triangle, the letters TWI or the maker's logo on the shoulder.
- Compare the tread surface with the bars.If the tread has worn down level with a bar, that part of the tyre is at the legal limit and the tyre is illegal to use. This is the check, and it needs no tools at all.
- Use the 20p test as a quick check.Push a 20p into a main groove. If the raised outer band of the coin is visible above the tread, the tyre is close to or below the limit and needs proper inspection.
- Check in several places, not one.Legal minimum has to be met across the central three quarters of the width and around the whole circumference. Tyres often wear unevenly, so check inner, middle and outer grooves at three points around the tyre.
- Turn the steering to full lock to see the fronts.The inner shoulder of a front tyre is where wear hides, and it is invisible with the wheels straight. It is also where alignment problems show first.
- Run a hand across the tread.A sawtooth feel in one direction, or a scalloped edge, indicates an alignment or geometry problem that will keep eating tyres until a garage corrects it.
- Inspect the sidewalls at the same time.Bulges, cracks, cuts deep enough to show cord, or damage from kerbing all condemn a tyre whatever the tread. A bulge means the internal structure has failed and the tyre can burst.
- Replace at around 3mm rather than waiting.Wet braking distances lengthen considerably between 3mm and 1.6mm. Running to the limit saves a little money and costs a lot of stopping distance in the rain.
- Check the spare too.Spares age quietly in the boot. A spare with cracked sidewalls is no use, and a space saver has its own speed and distance restrictions printed on it.
Tips
- The wear bars do the measuring for you — you never need to know a number at the roadside, only whether the tread is level with them.
- Uneven wear across one tyre is a geometry problem, not a tyre problem. New tyres alone will wear out the same way.
- Buy tyres in pairs across an axle so both sides behave the same.
Common mistakes
- Checking the middle groove only — Tyres commonly wear on the inner shoulder first, especially at the front. The middle can look fine while the inner edge is already down to the cords.
- Running to 1.6mm to get the last of the value — The wet performance loss between 3mm and 1.6mm is large. The saving is a few weeks of tread and the cost is a markedly longer stopping distance in rain.
If it doesn't work
Inner edges worn out, rest of the tread fine
Cause: Wheel alignment or suspension geometry out — Fix: Have the alignment checked and corrected before fitting new tyres, or the new ones will wear the same way.
Tread fine but the sidewall has a bulge
Cause: Internal structural damage, usually from a pothole or kerb — Fix: Do not drive on it. Fit the spare or call for assistance and have the tyre replaced.
Tread deep but the rubber is cracked
Cause: Age and ultraviolet exposure, common on little-used cars — Fix: Have a tyre specialist assess it. Old rubber hardens and grips poorly even with full tread.
One tyre wearing far faster than the others
Cause: Under-inflation, alignment, or a worn suspension component — Fix: Check pressures first, then have the alignment and suspension inspected by a garage.
Questions people ask
What is the legal tread depth in the UK?
1.6mm, and it must be met across the central three quarters of the tread width and around the entire circumference. A tyre that meets it in one place but not another is still illegal.
How old is too old for a tyre?
There is no age limit in law for a car, but rubber hardens with age and many manufacturers suggest considering replacement after five to ten years regardless of tread. The four-digit DOT code on the sidewall gives the week and year of manufacture.
Can a punctured tyre be repaired?
Sometimes. A repair is only permitted within a defined central area of the tread and within size limits, and never in the sidewall or shoulder. A tyre specialist decides — a plug pushed in at home is not a legal repair.