Checking tread depth takes about two minutes and needs no tools you do not already own. Doing it monthly is the single easiest habit for keeping a car safe and avoiding a nasty MOT surprise.
The legal limit
In the UK the minimum is 1.6mm, measured across the central three quarters of the tread width and around the entire circumference. Below that, each illegal tyre is worth three penalty points and a fine, so all four could cost you your licence.
Insurers can also treat illegal tyres as grounds to reduce or refuse a claim.
Three ways to check
The 20p test
Push a 20p coin into one of the main grooves. If the outer band of the coin is hidden, you have more than about 3mm. If you can see the band, you are at or below 3mm and should plan replacement.
The 20p test flags 3mm, not the 1.6mm legal limit. That is deliberate. It gives you warning while you still have safe tyres rather than telling you once they are illegal.
Tread wear indicators
Every tyre has small raised bars sitting in the bottom of the main grooves, set at exactly 1.6mm. Look for a small triangle, the letters TWI, or a brand mark on the shoulder pointing to where they are.
When the tread surface is flush with those bars, the tyre is at the legal limit and needs replacing now.
A depth gauge
A few pounds, and the only method that gives you an actual number. Push the probe into the groove until the shoulder sits flat on the tread and read off the measurement.
Where to measure
This is the part most people get wrong. A single reading tells you almost nothing, because tyres rarely wear evenly.
- InnerTake a reading near the inside shoulder
- CentreAnd one in the middle of the tread
- OuterAnd one near the outside shoulder
Do that in three places around the tyre, and on all four tyres. The lowest number anywhere is the one that counts legally.
The inner shoulder is where wear hides. It faces away from you and is where alignment problems show first, so a tyre that looks fine from outside the car can be illegal on the inside edge.
What the differences tell you
If the readings vary a lot across one tyre, something is wrong beyond normal wear. Both edges worn means under inflation. Centre worn means over inflation. One edge only means alignment. Fix the cause before fitting new tyres, or you will wear the new set the same way.
How often
Monthly is plenty for most drivers, and it pairs naturally with checking pressures. Check more often before a long trip, after a kerb strike or pothole, and at the start of winter when you want maximum wet grip.