Tyre (Tire) Tread Depth: Legal Limits & Wear Bars 2026

Tread depth at a glance

Tread depth (tire tread depth in US usage) is measured in the main grooves of the tyre. Each country sets its own legal minimum. The wear bars moulded into the grooves show when a car tyre has reached the usual limit. Some countries set a higher minimum for winter tyres, so “above the bars” does not always mean legal abroad.

Tyre (Tire) tread depth: what the law actually measures

Tread depth laws cover the main circumferential grooves, the deep channels that run around the tyre. The fine slits cut into the tread blocks (sipes) and any shallow decorative cuts don’t count. A tyre that looks fine in the middle but is worn down on one shoulder can still fail. Many national rules require the minimum across a band of the tread width and all the way round the tyre, not just at one spot.

A tread depth law gives you a floor, not a target. A tyre at the legal limit is legal, but on wet roads it clears less water and takes longer to stop than a tyre with more tread left. That gap between “legal” and “still performing well” is where the debate about replacing early versus replacing at the minimum comes from.

Wear bars: the built-in limit indicator

Wear bars are small raised blocks across the bottom of the main grooves. When the tread around them wears down to the same height, you are at the limit they mark, not still approaching it.

To find them:

• Look on the tyre shoulder for a small triangle, the letters TWI (tread wear indicator) or a manufacturer logo. The bar sits in the groove directly in line with that mark.
• There are several bars around the tyre. Check more than one, because tyres often wear unevenly.
• If any bar is flush with the tread next to it, treat the whole tyre as worn out.

The bars only show one fixed height. They can’t tell you how much tread is left above it, and they don’t show a country’s higher winter-tyre minimum. For that you need a gauge.

Measuring tread depth: gauges, metric and imperial

A tread depth gauge has a probe that you push into a main groove until the base sits flat on the tread blocks. Metric gauges read in millimetres. Most US gauges read in fractions of an inch, and many gauges show both scales. If you are used to one scale and driving under rules written in the other, use a dual-scale gauge rather than converting in your head at the roadside.

Measure in this order:

1. The inner groove, which is the hardest to see and often the most worn.
2. The centre grooves.
3. The outer groove.
4. Repeat at several points around the tyre, rolling the car forward between readings.

Write down the lowest reading on each tyre. Coin tests (the 20p test in the UK, the penny test in the US) are a rough check for whether a tyre is near the limit. They don’t give a figure you can compare with a foreign country’s rule.

To estimate how long your tyres have left from your readings and mileage, use the tyre wear calculator.

Winter tyres and countries with a higher minimum

Winter tyres depend on deep tread and sipes to grip snow and slush, and they lose that ability well before they wear down to the general legal minimum. Some countries reflect this with a higher minimum for tyres used to meet a winter-equipment rule. A winter tyre that is perfectly legal at home can count as non-compliant once you cross a border in the cold months.

Before a winter trip, look up the destination in the winter tyre laws by country guide or the tyre requirement checker. For choosing and timing winter tyres, see the winter tyre guide.

Why tyres wear unevenly

Uneven wear means one part of the tyre hits the legal limit while the rest still looks healthy:

• Both shoulders worn: usually long-term underinflation.
• Centre worn: usually overinflation.
• One edge worn: wheel alignment out of specification.
• Front tyres worn faster than rear (or the other way round): normal for the driven axle, and the reason to rotate.

Regular rotation and an alignment check stop one tyre from setting the replacement date for the whole set. See the tyre rotation and alignment guide.

Drive, repair or replace?

• Tread above the bars, even wear, no damage: drive on, and check again before any long trip.
• Tread above the bars but uneven: drive on to a garage and get pressures and alignment corrected. Measure again at the lowest point.
• Tread approaching the bars before a winter or wet-weather trip: replace before you leave rather than abroad, where you may not be able to choose tyres or compare prices.
• Tread level with any wear bar: replace. Tread wear can’t be repaired.
• Plenty of tread but cracked sidewalls or an old date code: depth isn’t the only limit. See when to replace tyres by age.
• Winter tyre above the general minimum but below the destination’s winter rule: fit new winter tyres before entering that country.

Replace tyres in axle pairs, so both tyres on an axle have similar tread depth and grip.