Vehicles under 4.5 tonne gross vehicle mass are legally required to have at least 1.5mm of tread remaining in all the principal grooves (i.e. the wide main grooves in the centre part of the tread).
These grooves have tread wear indicators in them and the tyre should not be worn so much that the tread wear indicators contact the road.
Our minimum allowed tread depth is 3.0mm, double the legal limit.
Other countries have very similar minimum tread depth requirements. 1.5mm or 1.6mm is well accepted globally as the minimum allowable tread depth for passenger and van tyres.