The decimal period of a repeating decimal is the number of digits that repeat. For example, has decimal period one, has decimal period two, and has decimal period three.

Any nonregular fraction is periodic and has decimal period independent of , which is at most digits long. If is relatively prime to 10, then the period of is a divisor of and has at most digits, where is the totient function. It turns out that is the multiplicative order of 10 (mod ) (Glaisher 1878, Lehmer 1941). The number of digits in the repeating portion of the decimal expansion of a rational number can also be found directly from the multiplicative order of its denominator.

A prime such that is a repeating decimal with decimal period shared with no other prime is called a unique prime.