Checking tyre tread properly takes under a minute per tyre and needs nothing more than a 20p coin, yet only around a quarter of UK drivers actually know the legal minimum depth. Here's exactly how to check it correctly, why checking one spot isn't enough, and what genuinely happens if you don't.
Summary:
- The legal minimum tread depth is 1.6mm, measured across the central three-quarters of the tyre, around its entire circumference.
- The 20p test remains the quickest home check. Insert the coin into the main tread grooves. If the outer band stays hidden, you're legal.
- Check multiple points around each tyre, not just one spot, since wear is often uneven across the surface.
- Most safety experts recommend replacing at 3mm, not waiting until 1.6mm, since wet braking distances increase sharply below that point.
- Penalties apply per tyre, not per car. Four illegal tyres can mean up to £10,000 in fines and 12 penalty points.
What Is the Legal Minimum Tyre Tread Depth?
UK law sets the minimum at 1.6mm for cars and vans, measured across the central three-quarters of the tyre's width and continuing all the way around its full circumference, the actual legal threshold rather than a rough guideline. Driving below it is a genuine offence regardless of whether you've personally noticed the wear.
How Do You Actually Do the 20p Test?
Insert a 20p coin directly into one of the main grooves running around the tyre. If the outer band, the raised rim around the coin's edge, stays completely hidden once inserted, your tread depth sits comfortably above the legal minimum.
If you can still see part of that band, the tyre may be at or below 1.6mm and needs a proper check with a tread depth gauge or a professional inspection.
The coin test is quick and genuinely reliable for a rough check, though it isn't as precise as a purpose-made tread depth gauge, which measures the exact depth in millimetres rather than giving a simple pass or fail.
Where Exactly Should You Check on the Tyre?
This is the step most drivers skip, and it's the one that matters most. Tyre wear is frequently uneven, caused by things like incorrect pressure, wheel alignment issues, or simply how a car is driven, meaning one section of a tyre can be perfectly legal while another has already dropped below the limit.
Check at several points spaced around the tyre's circumference, not just wherever happens to be facing you, and check across the width too, inner, centre and outer sections. A tyre that passes the 20p test in one spot can still be illegal a few inches further round.
What About Tread Wear Indicator Bars?
Most modern tyres include small raised bars, typically around 5mm wide, built into the base of the main tread grooves, positioned at several points around the tyre. As the tread wears down, these bars become gradually more level with the surrounding surface.
Once the tread has worn flush with these indicators, the tyre has reached the legal minimum, giving you a genuine visual backup to the coin test without needing any tools at all.
Is Legal the Same as Genuinely Safe?
Not quite. While 1.6mm is the legal floor, most safety organisations, including the RAC and TyreSafe, recommend replacing tyres at closer to 3mm rather than waiting until they're legally worn out.
Wet braking performance drops off considerably once tread falls below 3mm, with stopping distances from 70mph extending by more than two car lengths compared to a tyre still above that threshold.
Treating 1.6mm as your genuine target, rather than the point at which you finally act, closes a real safety gap most drivers don't realise exists.
What Happens If You're Caught With Illegal Tyres?
Penalties apply per tyre, not per vehicle, a distinction that catches plenty of drivers out. Each illegal tyre carries a fine of up to £2,500 and 3 penalty points, meaning a car with all four tyres below the legal limit risks a genuine £10,000 fine and 12 points, easily enough to trigger a court summons and put your licence at real risk through totting-up.
Police and DVSA officers can also issue an immediate prohibition notice, preventing the car being driven at all until the tyres are replaced.
An illegal tyre also causes an automatic MOT failure outright, regardless of how well the rest of the car performs on the day.
Does This Affect Your Car Insurance?
Potentially, yes. Driving on illegal tyres can affect an insurance claim if you're involved in an accident, with insurers able to argue the worn tyres contributed to the incident, potentially reducing a payout or, in some circumstances, refusing a claim entirely, a risk existing on top of the fine and points, worth factoring into how seriously you treat a tyre that's genuinely due for replacement.
How Often Should You Actually Check?
Roughly every two weeks, plus before any longer journey, gives genuine peace of mind without becoming a chore, ideally checked when the tyres are cold rather than immediately after driving since a warm tyre can give a slightly inaccurate reading.
It's also worth checking tyre condition beyond tread depth at the same time, looking for cracks, bulges or cuts in the sidewall, since damage like this can make a tyre illegal and dangerous even with plenty of tread remaining.
Tyre age matters separately from tread depth too. Every tyre carries a DOT code on the sidewall showing its manufacture date, and many safety experts recommend considering replacement once a tyre passes around six years old, regardless of how much tread technically remains, since the rubber itself degrades gradually over time.
Frequently Asked Questions
Does the fine apply per tyre or per car?
Per tyre. Each illegal tyre carries its own separate fine and points, meaning the total penalty scales directly with how many tyres are actually below the legal limit.
Can I check tyre tread without a 20p coin?
Yes, using a purpose-made tread depth gauge for a precise millimetre reading, or checking the built-in tread wear indicator bars most modern tyres include as a visual backup.
Is 1.6mm actually safe to drive on?
Legally yes, though most safety experts recommend replacing closer to 3mm, since braking performance in wet conditions drops considerably once tread falls below that point.
Why does tyre wear differ across one tyre?
Uneven wear commonly stems from incorrect tyre pressure, wheel alignment issues, or driving style, exactly why checking multiple points around and across each tyre matters more than a single spot check.
Do illegal tyres cause an automatic MOT failure?
Yes. A tyre below the legal 1.6mm minimum results in an automatic MOT failure, regardless of how the rest of the car performs during the test.
Conclusion
The 20p test takes less time than reading this sentence, yet the gap between drivers who know the legal minimum and those who genuinely check for it stays surprisingly wide.
Checking properly means more than one quick press of a coin though, several points around each tyre, not just whichever section happens to be facing you, since that's exactly where an otherwise fine-looking tyre can quietly fail the law. Treat 3mm as your real target rather than 1.6mm as your deadline, and tyre safety stops being something you worry about and simply becomes routine.
Every car sold through Car Planet is checked thoroughly before it's ever listed, tyre tread included. You're never starting ownership with a hidden problem.
