Tread wear indicators, usually shortened to TWI, are the small raised bars set into the bottom of a tyre's main grooves. They are a built-in gauge: when the tread wears down level with them, the tyre has reached the legal minimum and needs replacing.
What they are and where they sit
Look into the main circumferential grooves of any modern tyre and there are short rubber bridges crossing the groove at intervals. These are the tread wear indicators, and they are moulded to a height of exactly 1.6mm, the UK legal minimum tread depth.
To find them quickly, look at the tyre's shoulder for a small triangle or the letters TWI. These markers point inward to the nearest indicator bar in the groove alongside, so there is no need to search the whole tyre.
How to read them
The indicators work by comparison. When a tyre is new, the tread sits well above the bars and they are clearly recessed. As the tyre wears, the gap closes. Once the surrounding tread is flush with the top of the bars, the tread has worn to 1.6mm and the tyre is at the legal limit. At that point the tyre is illegal to drive on and unsafe, and replacement is overdue rather than optional.
The legal minimum, and why earlier is safer
In the UK the legal minimum tread depth is 1.6mm across the central three-quarters of the tread, around the entire circumference. That is the line the indicators mark.
However, reaching the indicators is the last warning, not the ideal moment to act. Wet-weather grip and braking fall away well before 1.6mm, which is why much of the trade advises replacing tyres at around 3mm, and a fresh set well before the limit is easy to order online from Tyres.co.uk. Waiting until the tread is level with the indicators means accepting the worst wet performance the tyre will ever give, right before it becomes illegal.
A quick check: the 20p test
For a fast check without a gauge, the 20p test is reliable enough to flag a problem. Place a 20p coin into the main tread grooves at several points around the tyre. If the outer band of the coin is hidden by the tread, there is a reasonable amount left. If that band is visible, the tread is getting low and the tyre should be measured properly with a depth gauge and inspected.
Checking the whole tyre
Tread rarely wears evenly, so the indicators and any check should be read at several points across the width and around the circumference. Wear concentrated on one edge or in the centre points to a separate issue, such as incorrect pressures or alignment, which is worth addressing so the next set of tyres lasts as it should. Alongside tread, a tyre's age is worth checking too, since rubber hardens over the years and grips less even when plenty of tread remains.
Check between the indicators as well
Uneven wear can leave one part of a tyre below the required depth while another part still looks healthy. Measure across the relevant band and at several positions around the circumference, keeping the gauge clear of the raised bars themselves. Inspect the shoulders and sidewalls separately for damage. A tyre with plenty of depth beside one indicator can still need replacement because of another worn area, exposed reinforcement or a structural fault.
Common questions
What are tread wear indicators?+
Tread wear indicators, or TWIs, are small raised bars moulded into the bottom of a tyre's main grooves at a height of 1.6mm. When the surrounding tread wears down level with them, the tyre has reached the UK legal minimum tread depth.
Where are the tread wear indicators on a tyre?+
They sit inside the main grooves, spaced around the tyre. Their position is marked on the tyre shoulder by a small triangle or the letters TWI, which point to the nearest bar in the groove.
At what tread depth should I replace tyres?+
The legal minimum in the UK is 1.6mm, which is where the indicators are set. Many in the trade recommend replacing earlier, around 3mm, because wet braking and grip fall away noticeably below that point well before the legal limit is reached.
How can I check tread depth without a gauge?+
The 20p test is a quick check: place a 20p coin in the main grooves, and if the outer band of the coin is hidden, there is likely enough tread. If the band is visible, the tyre is getting low and should be measured properly.