What tyre tread depth fails an MOT?

For cars, the legal minimum tyre tread depth is 1.6mm, measured across the central three-quarters of the tyre's width, and it has to meet that depth all the way around the tyre's circumference, not just at one point.

The legal measurement specifically covers the central three-quarters of the tyre, but that doesn't mean the outer edges don't matter. Uneven wear towards the edges can be a sign of incorrect tyre pressure or wheel alignment, and a tyre that's worn unevenly, or looks damaged in any way, is worth getting checked properly rather than judged on a single measurement.

It's worth remembering 1.6mm is a legal minimum, not necessarily a safe target: grip, especially in the wet, drops off well before tyres reach that point, which is why many people choose to replace tyres before they're legally required to.

Source: gov.uk: MOT inspection manual, axles, wheels, tyres and suspension