In short. In Great Britain a car tyre needs at least 1.6 mm of tread across the central three-quarters of its width, all the way round. In the United States there is no federal rule for private cars: each state sets its own, from 1/32 in (0.79 mm) in California to 2/32 in (1.59 mm) in New York and many others. Australia uses 1.5 mm in the main grooves (Victoria's rule), and Canada leaves it to the provinces: 1.5 mm in Ontario, 1.6 mm in Quebec. Those are legal floors, not safety targets: TyreSafe and Transport Canada both name a higher figure for wet or snowy roads.
The minimum tread depth in each country
| Where | Legal minimum for a car | Where it is measured | Text |
|---|---|---|---|
| Great Britain | 1.6 mm | "Throughout a continuous band" over the central three-quarters of the tread, round the entire circumference | Road Vehicles (Construction and Use) Regulations 1986, reg. 27(4)(f), in force from 1 January 1992 |
| US, California | 1/32 in (0.79 mm) | "In any two adjacent grooves at any location of the tire" | California Vehicle Code 27465(b) |
| US, New York | 2/32 in (1.59 mm) | "Two adjacent major grooves showing the most tread wear" | 15 NYCRR 79.21 (state inspection) |
| US, commercial vehicles | 4/32 in (3.18 mm) on front tyres of buses, trucks and truck tractors; 2/32 in elsewhere | Major tread groove, not at tie bars, humps or fillets | 49 CFR 393.75 (b) and (c) |
| Australia, Victoria | 1.5 mm | "In all principal grooves", except at the wear indicators; no wear indicator may touch the road | Vehicle Standards Information 16, October 2021 |
| Canada, Ontario | 1.5 mm | Any two adjacent major grooves at three equally spaced points round the tyre | R.R.O. 1990, Reg. 625, s. 3(1) |
| Canada, Quebec | 1.6 mm | "The grooves on your tires should not be less than 1.6 mm deep" | Government of Quebec, winter tyres page, last updated 5 October 2026 |
All texts were read on 16 October 2026. The way each rule is measured differs more than the numbers: Great Britain asks for a continuous band all the way round, California is satisfied by two adjacent grooves anywhere. And the US figure people quote, 2/32 in, is not a federal law for cars.
Great Britain: 1.6 mm, and what the MOT checks
Regulation 27 sets two levels. The general rule, for most vehicles, is 1 mm over three-quarters of the tread. For cars, light vans and light trailers, paragraph 4 replaces it "with effect from 1st January 1992" by a stricter one: grooves "of a depth of at least 1.6 mm throughout a continuous band comprising the central three-quarters of the breadth of tread and round the entire outer circumference of the tyre". It covers passenger vehicles with no more than eight passenger seats and goods vehicles up to 3,500 kg gross (4,250 kg for zero-emission vans).
Two definitions in paragraph 6 matter. The "breadth of tread" is the part "which can contact the road under normal conditions of use", measured at right angles to the direction of travel. The "tread pattern" excludes tie bars, tread wear indicators and "features which are designed to wear out substantially before the rest of the pattern". So you do not measure on the raised bars in the grooves.
The MOT inspection manual (section 5.2.3, updated 14 October 2026) applies the same rule and adds three practical points:
- only the primary grooves count; grooves "not cut as deep as the primary grooves are secondary grooves" and are ignored;
- "either side of the central three-quarters of the tyre can be devoid of tread": a bald shoulder alone is not a tread depth failure;
- a tread depth below the requirement is a dangerous defect, the most serious category, and a cord visible anywhere is a fail as well.
The Highway Code repeats the rule in Annex 6: "Cars, light vans and light trailers MUST have a tread depth of at least 1.6 mm". Northern Ireland has its own regulations, which we did not read.
Fines and penalty points
We traced the penalty through the texts, as nobody quotes them together:
- Using a vehicle whose tyres break a construction and use requirement is an offence under section 41A of the Road Traffic Act 1988.
- Schedule 2 of the Road Traffic Offenders Act 1988 sets the punishment: a level 4 fine (level 5 for a goods vehicle or a vehicle adapted for more than eight passengers), discretionary disqualification, obligatory endorsement and 3 penalty points.
- Section 122 of the Sentencing Act 2020 sets level 4 at £2,500 for offences committed on or after 1 October 1992 (England and Wales).
- On the licence, the offence appears as CU30, "Using a vehicle with defective tyre(s)", 3 points, kept on the record for four years (GOV.UK).
TyreSafe, the British tyre safety charity, says fines "can reach up to £2500 per tyre, and 3 penalty points per tyre". The texts we read speak of an offence, not of a tyre; the per-tyre reading is how the offence is commonly charged, and we did not find it written in the Acts themselves. A court decides the actual amount. TyreSafe also says insurance claims "may be invalidated" with illegal tyres: what your policy says is what counts.
United States: the state sets the rule
Federal law sets two things, neither of which is a tread limit for your car.
The wear indicator. Federal Motor Vehicle Safety Standard 139 (49 CFR 571.139, S5.4) requires each tyre above a 12-inch rim to have "not less than six treadwear indicators", spaced round the circumference, that let a person see "whether the tire has worn to a tread depth of one sixteenth of an inch". One sixteenth of an inch is 2/32 in, or 1.59 mm. When the tread is flush with those bars, you are at 2/32 in.
Commercial vehicles. 49 CFR 393.75 applies to commercial motor vehicles: front tyres of buses, trucks and truck tractors need 4/32 in, all others 2/32 in, never measured "where tie bars, humps, or fillets are located".
For private cars, the state decides, through its vehicle code or its inspection rules. We read two that show the spread:
- California, Vehicle Code 27465: no tyre may be used on a highway with less than "one thirty-second (1/32) of an inch tread depth in any two adjacent grooves at any location of the tire". The same section asks 6/32 in on snow tyres used instead of chains in posted control areas, and forbids dealers to sell or fit a tyre below the minimum.
- New York, 15 NYCRR 79.21: a tyre fails inspection if it "is worn to less than 2/32 of an inch of major tread design at the two worst adjacent points", with readings "taken in two adjacent major grooves showing the most tread wear".
Michelin US calls 2/32 in "the minimum tread depth", and it is the figure the federal wear bars mark. We did not read all fifty codes, and some states have no periodic inspection at all. If you need the figure for a state other than these two, read its vehicle code or the page of its department of motor vehicles.
Australia: 1.5 mm in the principal grooves
Victoria's Vehicle Standards Information 16 (October 2021), which covers vehicles up to 4.5 tonnes, puts it plainly: "Except at the tread wear indicators, tyres must have a minimum tread depth of 1.5mm in all principal grooves". Where wear indicators exist, "the tread must not be worn to the extent that any tread wear indicator contacts the road surface". The principal grooves are "the wide grooves that are usually positioned in the central zone of the tyre", and less than 1.5 mm in the shallower secondary grooves does not make the tyre unroadworthy. The same note adds that "a tyre must not have excessive uneven wear across its width".
Each state and territory publishes its own rules; we read Victoria's. Outside Victoria, check your own state's road authority.
Canada: each province writes its own
Canada has no single tread rule for cars. Two provinces show the range.
Ontario, Regulation 625 under the Highway Traffic Act, section 3(1): a tyre "shall not be worn to the extent that in any two adjacent major grooves at three equally spaced intervals around the circumference of the tire, (a) the tread wear indicators contact the road; or (b) less than 1.5 millimetres of tread depth remains". Siping does not count as tread, and section 2 bans exposed cords and any "abnormal visible bump, bulge or knot".
Quebec: the government's winter tyres page says "the grooves on your tires should not be less than 1.6 mm deep" and recommends "at least 4.8 mm" during the winter. We could not read the regulation itself on the day, so this figure comes from the government page, not from the text of the law.
Transport Canada, on its winter tyres page (modified 6 October 2025), also names a higher figure for snow: "Don't use tires that have less than 4mm tread depth", and it notes that some tyres "may come with tread-wear indicators that are 2mm". That is guidance, not a provincial law.
Inches and millimetres side by side
Our conversion table, with 1 in = 25.4 mm:
| Value | In mm | In 32nds of an inch | Where it is used |
|---|---|---|---|
| 1/32 in | 0.79 | 1 | California minimum |
| 1.5 mm | 1.5 | 1.89 | Victoria, Ontario |
| 2/32 in (1/16 in) | 1.59 | 2 | Most US states, US wear bars, US commercial rear tyres |
| 1.6 mm | 1.6 | 2.02 | Great Britain, Quebec |
| 3 mm | 3.0 | 3.78 | TyreSafe's replacement figure |
| 4/32 in | 3.18 | 4 | US commercial front tyres; Michelin's margin for heavy rain |
| 4 mm | 4.0 | 5.04 | Transport Canada, winter tyres on snow |
| 4.8 mm | 4.8 | 6.05 | Quebec government, recommended in winter |
| 6/32 in | 4.76 | 6 | California, snow tyres instead of chains |
The American wear bar (1/16 in, 1.59 mm) and the British limit (1.6 mm) differ by about 0.01 mm: for a driver, the same threshold. Continental UK writes the two together, "2/32 inches (1.6 millimetres)", when it gives the minimum.
How the depth is measured, and what the coin tests tell you
With a gauge. Continental UK describes the method: push the probe into a groove, bring the shoulders flush with the tread, read, and "be sure to check in various locations". It adds the rule that matters most: "The tyre's tread should be judged by the lowest depth you measured." Measure in the main grooves, not on the wear bars, and at several points round each tyre. The rules above all say where: three equally spaced points in Ontario, the two worst adjacent points in New York, a continuous band in Great Britain.
With the wear bars. Every tyre carries them in the main grooves. Continental and Michelin say the same thing: when the tread is flush with the bars, replace the tyre. On Michelin tyres a small Michelin Man identifies them.
The penny test (US). Michelin US: "place a penny into a tread groove with Lincoln's head facing down. If you can see the top of his head, tread is at or below 2/32 inch and the tire needs replacing." We did not find an official dimension for the gap between the coin's edge and Lincoln's head, so we cannot tell you how precise the test is. Read it as a pass or fail check against 2/32 in, not as a measurement.
The 20p test (UK). TyreSafe: insert a 20p coin into the main grooves; if "the outer band of the coin is visible", the tyres may be illegal and should be checked by a professional. Continental UK says a 20p or a one-pound coin shows that "the tread is likely to be less than 1.6mm deep" when the border shows. "Likely" is the right word: neither source gives the width of the border, and the test says nothing about a tyre above the limit but below 3 mm.
The quarter test (Quebec). The Quebec government describes it for winter: place a quarter in a groove "with the caribou's nose pointing down. If you can see the caribou's snout, this means the tire will not grip well in deep snow." It gives no depth in millimetres, so read it as a winter grip check, not a legal one.
Other coins. The US quarter and the Canadian two-dollar coin are also used this way, but no tyre maker's or authority's page we could read gives a depth for them, so we cannot vouch for the figures attached to them. A gauge settles the question.
A tyre worn on one edge. In Great Britain a bald shoulder outside the central three-quarters is not, by itself, an MOT failure on tread depth; Victoria forbids "excessive uneven wear across its width". Everywhere, a visible cord makes the tyre illegal. If you can see cords or wire on any part of a tyre, do not drive on it: fit the spare or have the car recovered, and have it checked before you go back on the road.
The legal minimum is not the safe minimum
On this point the sources do not agree, and the disagreement is useful to know.
- TyreSafe says that "tyre and safety experts recommend replacing tyres at 3mm", citing MIRA tests in which cars with 1.6 mm of tread "had up to 44% longer braking distances in the wet than those with 3mm". The page gives no speed or distance for those tests.
- Transport Canada says not to use tyres below 4 mm on snow-covered roads.
- Michelin US takes the opposite line: it "designs tires that perform well until the minimum tread depth is reached" and calls it "premature" to change them before. It still suggests that drivers who face heavy rain may replace at 4/32 in (3.18 mm) as a safety margin.
- Continental notes that "braking performance will progressively decline with lower tread depths" on wet or snowy roads, without a figure.
What it costs to replace early, in tread. Our example, with an assumed new depth of 8 mm (the middle of the 7 to 9 mm Continental gives for new winter tyres; summer tyres vary by model): from 8 mm down to 1.6 mm, a tyre has 6.4 mm of usable tread. Replacing at 3 mm leaves 5.0 mm used, so you give up 1.4 mm, about 22% of the usable tread. Replacing at 4 mm gives up 2.4 mm, or 37.5%. Whether that is worth it depends on how much rain and snow you drive in; we do not recommend a replacement depth, and the figures above are those of the sources named.
How much a worn tread lengthens a stop is in stopping distances in the wet, and how depth changes the speed at which a tyre aquaplanes in hydroplaning and tread depth. Tread is not the only limit: a tyre also ages, see how long tyres last and how to read the DOT date code.
Two things slow wear: the pressure from the car maker's placard and regular rotation.
When you buy replacement tyres
Fit the size, load index and speed rating given in the car maker's documents: how to read a tyre size, load and speed index explains the markings, and the tyre size calculator compares two sizes. Onyx Forged does not recommend a size or an offset. See also the MOT and TPMS with aftermarket wheels and winter tyres in the UK.
Frequently asked questions
What is the legal minimum tyre tread depth in the UK?
1.6 mm for cars, light vans and light trailers, in a continuous band across the central three-quarters of the tread and all the way round the tyre (regulation 27 of the Construction and Use Regulations 1986, since 1 January 1992). Most other vehicles need 1 mm.
Is 2/32 inch the law everywhere in the US?
No. There is no federal tread rule for private cars; states set it. California allows 1/32 in, New York fails a tyre below 2/32 in. Commercial vehicles need 4/32 in on the front tyres of buses and trucks, 2/32 in on the rest (49 CFR 393.75).
What is the minimum tread depth in Australia?
In Victoria, 1.5 mm in all principal grooves, and no wear indicator may touch the road (Vehicle Standards Information 16). Other states publish their own notes; check your state's road authority.
What is the legal tread depth in Canada?
It depends on the province: 1.5 mm in Ontario (Regulation 625, measured in two adjacent major grooves at three points), 1.6 mm in Quebec (government winter tyres page, which recommends at least 4.8 mm in winter). Transport Canada advises not to use tyres below 4 mm on snow.
Does the penny test work?
As a pass or fail check: Michelin US says that if you can see the top of Lincoln's head, the tread is at or below 2/32 in. A gauge is more precise.
Is the 20p test accurate?
TyreSafe and Continental UK both use it: if the outer band of the coin shows, the tread is likely below 1.6 mm. Neither gives the band's width, so treat a visible band as a reason to measure with a gauge or have the tyre checked.
What is the fine for bald tyres in the UK?
Up to £2,500 and 3 penalty points (code CU30) in England and Wales, from the Road Traffic Act 1988 s. 41A, Schedule 2 of the Road Traffic Offenders Act 1988 and the Sentencing Act 2020. TyreSafe says this applies per tyre. A court sets the actual amount.
Is a tyre at the legal limit safe to drive on?
It is legal. TyreSafe cites up to 44% longer wet braking at 1.6 mm than at 3 mm; Michelin says its tyres are designed to perform to the limit but suggests 4/32 in as a margin in heavy rain.
Do the wear bars show the legal limit?
In the US, yes: federal standard FMVSS 139 places them at 1/16 in (2/32 in). That is 1.59 mm, about 0.01 mm from the British 1.6 mm. Where the law is 1.5 mm (Victoria, Ontario), the bars sit slightly above it.
Onyx Forged makes made-to-order forged wheels, designed to JWL/VIA standards. Your tyre fitter mounts and balances the tyres on them. See how to customise a set.
Sources
- Great Britain: Construction and Use Regulations 1986, regulation 27; MOT inspection manual, section 5 (updated 14 October 2026); The Highway Code, Annex 6; Road Traffic Act 1988, s. 41A; Road Traffic Offenders Act 1988, Schedule 2; Sentencing Act 2020, s. 122; GOV.UK, endorsement codes.
- TyreSafe: tyres and the law, tread depth, the 20p test.
- United States: California Vehicle Code 27465; 15 NYCRR 79.21; 49 CFR 393.75; 49 CFR 571.139 (FMVSS 139).
- Australia: Victoria, Vehicle Standards Information 16 (October 2021).
- Canada: Ontario Regulation 625; Government of Quebec, requirements for winter tires (last update 5 October 2026); Transport Canada, using winter tires.
- Tyre makers: Michelin US, tire tread depth (updated 8 June 2026); Continental UK, tread depth and tyre tread.
All pages were read on 16 October 2026.
The Onyx Forged design office.