General haircut clipper sizes range from number one through number eight. One is the smallest at 1/8 of an inch and eight is the largest size at 1 inch. These measurements refer to the length of the guard on the clippers.
When someone is cutting hair, they use the different guard lengths to achieve different looks. Those who want a short haircut that is very close to the scalp should use a number one or a number two guard length. The number five and number six guard lengths generally work to taper the hair during styling because the two ends of the guard are different lengths.