How big is 1 zepto? 1 zepto is a prefix that is equal to 10 to the power of negative twenty-one .
1 zepto = 10-21
It is a 1 preceded by 20 zeros, a decimal point, and another zero.
1 zepto = 0.000000000000000000001
Below are a few examples of the word zepto being used as a prefix for meter and second.
1 zeptometer = 0.000000000000000000001 m
1 zeptosecond = 0.000000000000000000001 s