Tread depth is the measure of the tire's tread from the top of the tread down to the bottom of the tire's deepest groove. In order for a tire to be considered legal, it must have a minimum tread depth of 4/32. This means that the tire's tread must be at least 4/32 deep in order for it to be considered legal. Anything less than 4/32 is not legal and needs to be replaced.