The 20p test is the fastest free way to check whether your tyres are road-legal. Place a 20p coin into the main tread grooves of your tyre. If the outer band of the coin is hidden, your tread is likely above the legal minimum. If you can see that outer band, your tyre may be illegal and needs checking. It takes under a minute per tyre and needs nothing but a coin.
Why the 20p coin works
The raised outer rim of a UK 20p coin sits roughly 3mm from the edge. Because the legal minimum is 1.6mm, a coin that disappears into the groove gives you a comfortable safety margin. The test was popularised by tyre-safety charity TyreSafe for exactly this reason. It's an indicator, not a legal measurement, but it's a reliable early warning that catches a worn tyre while there's still time to plan a replacement.
How to do the 20p test
It's genuinely a 30-second job per tyre:
- Park safely and make sure the tyre is cool and clean.
- Insert a 20p coin into a main groove around the centre of the tyre.
- Check whether the outer band of the coin is visible. If it is, get the tyre inspected.
- Repeat at the inner edge, centre and outer edge, and at three points around the tyre.
Test every tyre, in several places
Tyres rarely wear evenly. Worn shoulders often point to under-inflation or alignment problems, while a worn centre strip suggests over-inflation. Checking multiple points catches a tyre that's legal in the middle but bald at the edge, which is one of the most common MOT surprises. The inner shoulder is the one to watch, because it's the hardest to see and the first to go on a car with slight misalignment. While you're there, look for cracks, bulges and embedded nails too.
When the 20p test isn't enough
The 20p test is brilliant as a quick screen, but it has limits. It tells you roughly whether you're above a safe margin, not your exact tread depth, and it won't reveal a tyre that's legal on tread but unsafe for another reason, such as age, a sidewall bulge or internal damage. If the coin's outer band shows even at one point, treat that as a prompt to have the tyre measured properly with a gauge by a fitter, who can also check condition and age. Think of the coin as the smoke alarm, not the full inspection. For anything borderline, a proper measurement settles it.
Failed the test? Don't risk it
If any tyre is borderline, replace it rather than gambling on the next MOT or a wet motorway. Our mobile fitters bring budget and premium tyres to you anywhere across the UK, so you can swap a worn tyre on your own driveway. Book online or call us for same-day help.
