How many zeptometres are inside one femtometre?

The story behind the answer

Femto- means 10^-15 and zepto- means 10^-21. To find how many zeptometres are in a femtometre, divide 10^-15 m by 10^-21 m: 10^-15 / 10^-21 = 10^6 = 1,000,000. Another way: moving from femto to atto (10^-18) to zepto (10^-21) is two steps of 10^3, giving a millionfold difference.

Context tip: These ultra-small prefixes appear in nuclear and particle physics; a proton’s radius is about 0.84 femtometres (also called a fermi). To convert between SI prefixes, subtract exponents (−15 − (−21) = 6) or remember the sequence in groups of three zeros—femto, atto, zepto—each step adds three zeros.