How to Check Your Tyre Tread Depth
A practical guide to tyre tread depth for UK drivers, from the CarPal mobile tyre team.
Three ways to check
There are three practical methods, in ascending order of accuracy: the tread wear indicators moulded into the tyre, the 20p coin test, and a proper tread depth gauge.
All three answer the same question — is this tyre above 1.6mm — but only the gauge gives you a number you can track over time.
Tread wear indicators
Every road tyre has small raised bars set into the bottom of the main grooves at exactly 1.6mm. Look into the grooves and you will find them, usually marked on the shoulder with a small triangle, TWI, or the manufacturer’s logo.
When the surrounding tread has worn down level with those bars, the tyre is at the legal limit. It is a yes/no check with no ambiguity.
The 20p test
Insert a 20p coin into the main tread groove. If the outer band of the coin is hidden by the tread, you have more than 1.6mm and the tyre is legal. If you can see the band, the tyre is at or near the limit.
Do it in several places across the width and at three or four points around the circumference. A single check at one spot is how people miss uneven wear.
Using a gauge properly
A tread depth gauge costs a couple of pounds. Push the probe into the main groove until the base sits flat on the tread blocks, then read the scale.
Measure the inner, centre and outer main grooves separately, and repeat at three points around the tyre. Note the lowest reading — that is the one that counts legally.
Recording the numbers each time means you can see how fast a tyre is wearing and plan replacement rather than being caught out.
What to look for while you are there
Uneven wear across the width tells you something is wrong: both edges worn suggests under-inflation, a worn centre strip suggests over-inflation, and one edge worn suggests alignment.
Also check for cuts, bulges, embedded objects and cracking in the sidewall. Any bulge means the tyre needs replacing immediately regardless of tread.
Frequently asked questions
How do I check my tyre tread depth?
Use the wear indicator bars in the grooves, the 20p test, or a tread depth gauge. Measure in several places across and around the tyre.
What is the legal tread depth in the UK?
1.6mm across the central three-quarters of the tread, around the entire circumference.
How accurate is the 20p test?
Good enough as a pass/fail check at 1.6mm. It will not catch uneven wear unless you test several points.
Where are the tread wear indicators?
Raised bars in the bottom of the main grooves, set at 1.6mm. The shoulder is usually marked with a triangle or TWI.
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