What are the different men haircut numbers and how do they correspond to specific hair lengths?

Men's haircut numbers refer to the clipper guard sizes used to achieve specific hair lengths. The numbers range from 0 to 8, with 0 being the shortest and 8 being the longest. For example, a 1 guard will leave hair about 1/8 inch long, while a 4 guard will leave hair about 1/2 inch long. The higher the number, the longer the hair will be left.