A sextillion (10 21 ) bytes. In computing and storage systems, a ZB is exactly 1,180,591,620,717,411,303,424 (2 70 ) bytes, since the measurement is based on a base 2, or binary, number system. The term ZB comes from the fact that 1,180,591,620,717,411,303,424 is nominally, or approximately, 1,000,000,000,000,000,000,000, which suggests that mathematicians consider a difference of 180,591,620,717,411,303,424 to be a rounding error. See also byte and Z.