Tread Depth Checker: Is Your Tyre Legal & Safe?

The 20p coin test is the simplest way to check whether your tyres are legal and safe — but knowing the exact depth in millimetres gives you far more useful information. Use the tool above to check one tyre or all four at once.

The UK legal limit

In the UK, the minimum legal tread depth for cars, light vans and light trailers is 1.6mm across the central three-quarters of the tyre, around its entire circumference. Going below this limit carries a fine of up to £2,500 and 3 penalty points per tyre — a potential £10,000 fine and 12 points if all four are illegal.

Why 1.6mm is not enough

The legal minimum is a floor, not a recommendation. Tyre manufacturers and motoring organisations consistently advise replacing tyres at 3mm for the following reason: wet-weather performance drops sharply below this threshold.

A tyre with 1.6mm of tread takes approximately 44% longer to stop in the wet than a new tyre from 80 km/h. On the A55, the A5 through Snowdonia, or any of North Wales’s coastal roads in a rain shower, that difference could be decisive.

How to check tread depth properly

The 20p test

Insert a 20p coin into the main tread groove. The outer band of the coin is approximately 2-3mm wide:

• If the outer band is fully hidden, your tread is above 3mm — you’re fine
• If you can see part of the outer band, your tread is below 3mm — check with a gauge
• If you can see the entire outer band, your tread may be below 1.6mm — replace immediately

Using a tread depth gauge

A proper gauge (available from any motor factor for a few pounds) gives you an exact reading in millimetres. Check in at least three positions across the tyre width and in multiple spots around the circumference — uneven wear tells you something is wrong with alignment, balancing or suspension.

Tread wear indicators

Modern tyres have small rubber blocks moulded into the grooves at 1.6mm height, known as tread wear indicators (TWI). When the tread surface is flush with these blocks, the tyre is at the legal limit. Look for a small triangle or ‘TWI’ marking on the tyre sidewall to locate them.

What causes uneven tread wear?

• Centre wear — over-inflation; the tyre is ballooning and only the centre contacts the road
• Edge wear (both sides) — under-inflation; the tyre is squashing out at the edges
• One-edge wear — wheel alignment is off (camber issue)
• Patchy/cupped wear — shock absorbers may be worn; the tyre is bouncing rather than rolling
• Flat spots — emergency stops or locked brakes

If you notice any of these patterns, it’s worth having the alignment checked before fitting new tyres, or they’ll wear the same way again.

How often should I check tread depth?

Once a month as part of your regular checks (pressure, lights, fluid levels). Also check before any long journey — if you’re heading to Holyhead for the ferry or driving to Manchester Airport, you want to know your tyres are safe before you set off, not when you’re stuck on the hard shoulder.