Hair grows at a rate of 4.6 yoctometres per 1 femtosecond. The yoctometre is the smallest derivation of the meter within the standard SI unit of measurement. 1 meter is equal to 1,000,000,000,000,000,000,000,000 (1 septillion or 10²⁴) yoctometers. 1 femtosecond is nearly the smallest derivation of the second within the standard SI unit of measurement (the smallest being an attosecond which is 10¹⁷). 1 second is equal to 1,000,000,000,000,000 (1 quadrillion or 10¹⁵) femtoseconds