Tyre tread depth chart UK: the 1.6 mm legal limit

UK legal tyre tread depth is 1.6 mm across the central three-quarters. Chart in mm, fines, MOT, part-worn 2 mm rule and the 20p test.

The legal limit is 1.6 mm

The legal minimum tread depth for a car tyre in the UK is 1.6 mm. It has to be there in a continuous band across the central three-quarters of the tread and right round the whole circumference, not just in one good patch.

That wording comes from regulation 27 of the Road Vehicles (Construction and Use) Regulations 1986. The 1.6 mm rule covers passenger vehicles with no more than 8 passenger seats, goods vehicles up to 3,500 kg gross weight and light trailers. Everything else that the regulation covers, from coaches to lorries to motorcycles over 50 cc, falls back to 1 mm. Mopeds only need the original pattern to be visible.

The outer eighth on each shoulder isn't part of the legal measurement. That doesn't make a bald shoulder fine. The same regulation also bans tyres with cuts over 25 mm or 10% of the section width that reach the cords, any lump, bulge or tear, and any exposed ply or cord, whatever the tread depth says.

Tread depth chart in mm

UK fitters work in millimetres. If your gauge reads in 32nds of an inch because it came from the States, 1.6 mm is 2/32, and that's about the only conversion you'll ever need over here.

The 3 mm line isn't law. West Yorkshire Police's guidance on tyre law cites tests by the Motor Industry Research Association showing stopping distances rise sharply once tread drops below 3 mm. That's the reason so many garages tell customers to change at 3 mm rather than wait for 1.6 mm.

Fines, points and the MOT

Get stopped with an illegal tyre and you're looking at a fine of up to £2,500 and three penalty points per tyre. That's how Torbay Council's road safety team put it when it joined police tyre checks. The endorsement code is CU30, ‘using a vehicle with defective tyre(s)’, carrying 3 points according to GOV.UK's list of endorsement codes. On a car with four bald tyres, that could add up to twelve points. The same GOV.UK page says 12 or more points within 3 years means the driver can be disqualified.

GOV.UK's Check your vehicle is safe to drive page gives the same figures in plain terms: 1.6 mm for cars, light vans and light trailers, tread across the middle three-quarters and round the whole tyre, and up to £2,500 plus 3 points for driving a vehicle in a dangerous condition.

At the MOT, the tester measures the primary grooves. The MOT inspection manual defines a primary groove as one that contains the tread wear indicators, or one cut as deep as those were when new. Shallower sipes and secondary grooves don't count. Tread below the requirement is a dangerous defect, the most serious category in the manual. The same section lists a cut down to the cords as a major defect, and a bulge or exposed cords as dangerous.

Part-worn tyres: 2 mm, not 1.6 mm

This is where used-tyre shops get caught. A part-worn tyre can sit on a car at 1.7 mm and be legal, yet you can't sell that same tyre. Regulation 7 of the Motor Vehicle Tyres (Safety) Regulations 1994 says the original grooves of a part-worn must be at least 2 mm deep across the full breadth of the tread, all the way round. Full breadth. Not the central three-quarters.

Regulation 7 also requires that:

• the base of every original groove is clearly visible;
• there are no cuts over 25 mm or 10% of the section width (whichever is greater) deep enough to reach the ply or cord;
• there are no lumps, bulges or tears and no exposed ply or cord, checked again with the tyre inflated to its maximum design pressure, because some faults only show under pressure;
• any repair has been done properly, to the British Standard for tyre repairs, BS AU 159 (Business Companion notes the regulations name an older edition and advises working to the current one);
• a non-retreaded car tyre carries its E approval mark, plus a load index and speed symbol moulded in at the factory;
• the words PART-WORN appear next to each approval mark, in capitals at least 4 mm high, applied permanently but not by hot branding or cutting into the rubber.

Possessing a tyre for supply counts as supply under the regulations. So a 1.8 mm tyre sitting on the for-sale rack is already a problem, even if nobody has bought it yet. Trading Standards enforce this, and the government-backed Business Companion guide to part-worn tyres is the version written for traders. That guide covers England, Scotland and Wales; in Northern Ireland it points traders to NIBusinessInfo instead. Keep a separate, clearly marked scrap pile.

How to measure it properly

Use a tread depth gauge. They're cheap, and they settle arguments at the counter.

1. Find the tread wear indicators. These are small raised bars across the bottom of the main grooves; many makers flag where they sit with a small mark on the shoulder (Michelin uses a little Michelin Man). Under UN ECE Regulation 30 they show when the groove is down to 1.6 mm.
2. Measure in the primary grooves, at the inner, centre and outer part of the central three-quarters.
3. Repeat at three or four points round the tyre. Uneven wear hides.
4. Write down the lowest reading. Not the average.

If the tread is flush with the wear bars anywhere, the tyre is at the limit. Done.

The 20p test is the one every driver knows. Push a 20p into the main grooves: if the outer band of the coin is hidden, there's probably enough tread; if you can see it, get it gauged. Treat it as a screen and nothing more. The Royal Mint publishes the 20p's diameter (21.4 mm) and thickness (1.7 mm), but we couldn't find any official figure for the width of that outer band, so nobody can honestly tell you the 20p measures exactly 1.6 mm. A gauge can.

Winter tyres, and the rules in Ireland

There's no winter tyre law in the UK, and none in Ireland either, according to Continental's country-by-country winter tyre list. The 1.6 mm limit applies to winter and all-season tyres exactly as it does to summer ones. Snow grip comes from deep, open grooves and sipes, though, so a winter tyre worn close to the limit won't give much of it. Say so when a customer asks about running last year's set again.

Ireland uses the same 1.6 mm limit for cars and 1 mm for motorcycles. An Garda Síochána warned in its Tyre Safety Day 2021 release that defective tyres could mean up to 4 penalty points and a fixed charge of up to €120. Tyres are checked at the NCT rather than the MOT; the same release reported that 7.68% of full NCT tests between September 2020 and August 2021 failed on tyres. Part-worn rules in the Republic are not the ones above, and we haven't verified an Irish equivalent, so check locally before selling part-worns there.

Where to draw your own line

The law gives you two numbers: 2 mm to sell a part-worn, 1.6 mm to drive on one. Neither is a good stocking policy. A part-worn at 2.1 mm is legal to sell and won't stay legal for long, and the customer will remember who sold it.

Many careful part-worn sellers set their own floor at 3 mm or 4 mm and write the depth on every invoice. Check the date code before you bother measuring, too; there's no point gauging a tyre you'd refuse on age. Our guide to reading a tyre date code covers that, and the tyre size guide covers matching sizes on an axle. For a full intake routine, the used tyre grading checklist is in US English but the steps are the same.

DuroBay is a free app for tyre shops that keeps track of stock and reads sizes and dates straight off the sidewall, with the tread depth saved against each tyre.