Learning how to check tyre tread depth takes about 30 seconds and a coin you almost certainly have in your pocket. The UK legal minimum is just 1.6mm — but by the time you hit it, your tyres have already lost a lot of their grip in the wet. Here's the 20p test, what the tread depth numbers really mean, and when to replace.
The 30-second version
- The legal minimum tread depth in the UK is 1.6mm across the central three-quarters of the tyre.
- The 20p test is the quickest DIY check — if the coin's outer band shows, you're too low.
- Most experts suggest replacing at 3mm, well before the legal limit, for safer wet braking.
- Not sure where you stand? Book a free tyre check in St Helens and we'll measure every tyre for you.
How to check tyre tread depth with the 20p test
You don't need a garage or a gauge to get a rough answer. The 20p test works because the raised outer band of a 20 pence coin sits almost exactly 1.6mm from the edge — the same as the legal limit. It's the check the driving authorities themselves recommend.
Do it on each tyre, and in a few spots across the width, because tyres often wear unevenly.
The 20p test, step by step
- Grab a 20p coin. Any standard UK 20 pence piece will do.
- Slot it into the main tread grooves. Push it in as far as it goes, in the central part of the tyre.
- Look at the outer band. If the raised rim around the edge of the coin is hidden by the tread, you have plenty of depth — comfortably above 1.6mm.
- If you can see the band, act. When the outer band is visible, your tread may be at or below the legal limit and the tyre needs checking properly.
- Repeat everywhere. Test the inner, middle and outer grooves of all four tyres — and don't forget the spare.
Built-in tread wear indicators
Every modern tyre has small raised bars moulded into the bottom of the grooves, called tread wear indicators (TWIs). They sit at exactly 1.6mm.
When the surface of the tread wears down level with these bars, the tyre is at the legal limit and must be replaced. Look for small arrows or the letters "TWI" on the tyre's shoulder — they point to where the indicators are. If the tread is flush with the bars, don't wait: that tyre is no longer legal.
When to replace your tyres
Legally you can run to 1.6mm, but grip and wet braking fall off sharply well before that. Many tyre makers and safety bodies advise changing at 3mm, when stopping distances in the rain start to lengthen noticeably.
Also replace sooner if you spot cracking, bulges, exposed cords or uneven wear on one edge — those point to age or an alignment fault, not just mileage. If in doubt, get a professional set of tyres fitted before winter.
What each tread depth means
Tread channels water away so the rubber can keep contact with the road. The shallower the tread, the less water it can shift — and the greater the risk of aquaplaning. Here's a quick guide to the numbers.
Tread depth at a glance
- 8mm – 4mm: Healthy. Normal grip and water clearance.
- 3mm: Start planning replacements — wet braking is measurably worse.
- 2mm: Legal, but well past its best. Book a check.
- 1.6mm: The absolute legal minimum. Replace now.
- Below 1.6mm: Illegal and dangerous — do not drive on it.
The cost of ignoring it
Driving on a tyre below 1.6mm can mean a fine of up to £2,500 and 3 penalty points per tyre. With four illegal tyres, that's a possible ban — and it's an instant MOT failure. See the official rules on tyre safety and condition at GOV.UK.
🚗 Check Your Tyres
Plenty of tread ✅
You're comfortably above the limit. Keep checking every month and before long trips — and get a free measurement now and then to be sure.
Time to start planning
At roughly 3mm your wet grip is fading, even though you're still legal. This is the sweet spot to budget for replacements before performance drops further.
Get it measured now
A failed 20p test means you may be at or below 1.6mm. Have it measured properly before you risk a fine, points, or a wet-weather scare.
Illegal — replace it 🚨
Visible wear bars or metal mean the tyre is at or below the legal limit and unsafe. Don't drive on it any further than you must — pop in or call and we'll sort a replacement.
How often should you check tyre tread depth?
Get into the habit of checking at least once a month, and always before a long journey. It costs nothing and takes two minutes with a coin.
- Check all four tyres — and the spare if you carry one.
- Look across the width: inner, middle and outer grooves.
- Watch for uneven wear, which often means wheel alignment or fitting needs attention.
- Check pressures at the same time — under-inflation wears the edges fast.
Uneven wear is a clue
If only the inside or outside edge is low, it's usually alignment or worn suspension, not just mileage. Sorting the cause first stops your new tyres wearing out early too.
Tyres and your MOT
Worn, damaged or under-tread tyres are one of the most common MOT failures. A quick check beforehand saves a wasted retest — see our guide to the most common MOT failures.
Free tyre check in St Helens
Not sure how much tread you've got left? We'll measure every tyre, check for damage and give you an honest answer — free, with no obligation. RAC & AA approved, Motor Ombudsman accredited.
Frequently asked questions
Push a 20p coin into the main grooves of the tread. If the raised outer band of the coin disappears from view, your tread is above the 1.6mm legal minimum. If you can see that band clearly, the tyre is close to or below the limit and needs checking properly. Test three points across the width and three around the circumference. Our St Helens tyre centre will measure it accurately for free, or ring 01744 808586.
The raised border around the edge of a 20p coin measures very close to 3mm, which is the depth tyre manufacturers recommend as the change point rather than the 1.6mm legal floor. That built-in margin is why the test is useful: by the time the band is visible you have time to plan a replacement rather than facing an instant failure. It is a screening tool, not a measurement. See the legal tread depth rules for the exact figures.
A 10p works as a rough substitute because it also has a raised rim, but the band sits at a different height and gives a less reliable indication than a 20p. A one pound coin is no use at all. The most accurate home option is a tread depth gauge, which costs very little and gives a figure in millimetres. Bring the car to our tyre bay and we will measure all four with a proper gauge.
Test in the main circumferential grooves within the central three quarters of the tread, since that is the area the law and the MOT are measured against. Check the inner edge, the middle and the outer edge, then repeat at three points around the tyre. Wear is rarely even. If one edge is noticeably lower, that points at alignment. Book a free alignment check before fitting new tyres.
Check tread and pressures at least once a month and before any long journey, which takes about five minutes for all four. Tyres lose pressure naturally through the rubber, and an underinflated tyre wears its edges away quickly. Cold pressures give the accurate reading. Add it to the same routine as your oil and screenwash. Our pre-journey checklist covers the full list worth running through.
Most tyres have small raised bars moulded into the bottom of the main grooves at 1.6mm, marked by a triangle or the letters TWI on the sidewall. Once the tread surface is level with those bars the tyre is at the legal limit and must be replaced. They are a legal minimum marker rather than a recommendation. Use them alongside the coin test. Full detail is in our guide to UK tyre law.
Use the coin as a quick monthly reassurance and a professional measurement for decisions, because a gauge reads the actual depth and a technician also checks the sidewalls, the bead, the age code and the inner shoulder that you cannot see from outside. Inner edge wear hidden behind the wheel is the one drivers miss most often. We check all of that free of charge. Ring 01744 808586 or visit our tyre department.
Replacing at 3mm is the sensible target, because wet grip and aquaplaning resistance fall away sharply below that and the legal 1.6mm gives you almost no margin in heavy rain. Winter tyres should go earlier still, at around 4mm, as the sipes lose their bite. Nobody gains from running a tyre to the wire. Compare options at our St Helens tyre centre or read about all season tyres.
Take five minutes in daylight where you can see into the grooves, since a torch alone hides uneven wear and cracking. Winter is exactly when it matters most, as cold rain and standing water are what expose a tyre near the limit. Wet stopping distances at 1.6mm can be dramatically longer than on a new tyre. Our winter checks guide pairs the tyre check with lights and wipers.
Run your hand around the sidewall for bulges, cuts and cracking, look for stones and screws wedged in the grooves, and check the valve cap is present. The four digit code on the sidewall gives the manufacture week and year, and anything past six years deserves inspection whatever the tread. Uneven wear tells its own story. Our tyre advice page explains each wear pattern and what causes it.
The pressure monitoring light reports air pressure, not tread depth, so a tyre can be legal on pressure and still be down to the wire. The two checks are separate and both matter. A light that keeps returning after a reset usually means a slow puncture rather than a sensor fault. Get the tyre water-tested. See our page on puncture repairs in St Helens for what that involves.
A tyre measuring at or just above 1.6mm passes on the day, but it will often carry an advisory, and it can drop below the limit within a few thousand miles. Testers also fail cuts, bulges and exposed cord regardless of tread. Getting borderline tyres changed before the test avoids a retest fee. Book your MOT in St Helens with us and we will measure the tread first.
Yes, tyres can be paid for over monthly instalments with 0% Payment Assist, which helps when all four need doing at once. The application takes a few minutes and there is nothing extra to pay. We fit budget through to premium brands in the correct size and speed rating for your car. Details are on our tyre finance page, or ring 01744 808586 with your tyre size.