Tread depth is measured in 32nds of an inch. New tires start with 10/32 or 11/32 inch. For safety and road handling confidence, your tires should have a tread depth of at least 4/32 of an inch. That’s just to the top of George Washington’s head on a quarter placed upside down in the tread grooves. Be sure to check the depth at several spots.
Autotire will be happy to measure your tires for free. Just stop by any store!