DIY Frequently Asked Questions / How does the number on the shank of a drill bit relate to its size?
Bit diameters are usually marked by a single number—the numerator of a fraction. For example, an auger bit, which is marked by 16ths of an inch, with a number 8 would stand for 8/16″ or 1/2″. Twist bits are usually marked in the same manner by 64ths of an inch. Thus a No. 8 bit would stand for 8/64″ or 1/8″.