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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".