Use a tread depth gauge for the most accurate measurement
Insert the gauge probe into the deepest groove of the tire
Press the gauge base flat against the tread blocks
Read the measurement on the gauge
Measure in multiple spots across the tire
Check the inner, center, and outer tread areas
Compare readings on all four tires
Replace tires if tread depth is below the legal minimum
Use a coin test only as a quick check if no gauge is available
Measure when the tire is cool and parked on level ground