The 20p test is the check almost anyone can do, because the only tool needed is loose change. It is the quickest way to know whether a tyre is roughly safe or needs a closer look, and it costs nothing.
How to do it
The method is simple:
- Take an ordinary 20p coin
- Slot it into one of the main grooves of the tread, edge first
- Look at the raised outer band that runs around the rim of the coin
- Check whether the tread hides that band or leaves it showing
If the tread covers the outer band, there is a healthy amount of rubber left. If the band stays visible, the tread is getting low and the tyre needs a proper check.
What the result means
The clever part is the coin itself. The outer band of a 20p sits a few millimetres in from the edge, a little above the 1.6mm legal minimum. That is deliberate: the test is built to warn early, so a visible band points to a tyre worth checking rather than one that is automatically illegal.
- Band hidden: tread is above the limit at that spot, with some margin in hand
- Band visible: tread is low; measure it with a gauge or have it looked at, because it may be near or below 1.6mm
Because it errs on the safe side, the 20p test is best treated as a "check this now" signal, not a way to run a tyre down to the wire.
Test in more than one place
A single dip of the coin is not enough. Tyres wear unevenly, so the test should be repeated:
- At three points across the tread, inner, middle and outer grooves
- At several points around the tyre as the wheel is turned
- On all four tyres, plus the spare if it is a full-size one
The spot that shows the most coin band is the one that decides it. A tyre can look fine in the centre and be worn into the band on the inner edge, which is exactly where alignment-related wear tends to show first.
Its limits
The 20p test answers "roughly safe or not?", it does not give a number. For an exact depth, for tracking how fast a tyre is wearing, or for judging a borderline case, a tread depth gauge is the better tool. The coin is the monthly habit; the gauge is for when the coin says to look closer.
Understand what a hidden band proves
If the band is hidden at one point, it says nothing about an unmeasured area elsewhere around the tyre or on its inner edge. Nor does it establish that the sidewall and casing are sound. Use the coin to prompt a closer check, not to certify legality. A tread gauge provides a more useful reading, especially when replacement is approaching. Keep fingers away from exposed cords and arrange professional inspection if the inner tread cannot be checked safely.
Common questions
How does the 20p tyre test work?+
Slot a 20p coin into a main tread groove. If the raised outer band around the edge of the coin is hidden by the tread, there is a safe amount left. If the band is visible, the tread is low and the tyre should be checked properly.
Is the 20p test accurate?+
It is a quick screen, not a precise measurement. The coin's outer band sits a little above the 1.6mm legal minimum, so the test deliberately errs on the safe side, a visible band flags a tyre to check, not necessarily an illegal one.
Where do I put the 20p on the tyre?+
In the main circumferential grooves, at several points across the width and around the tyre. The lowest spot is the one that matters, so test the inner and outer grooves too, not just the middle.
Does a hidden coin band mean my tyre is legal?+
It means the tread is above the legal minimum at that spot, with a margin. It is still worth confirming the worn areas, particularly the inner edge, and using a gauge if any point looks borderline.