The specific rotation of the sample so obtained [] is then compared to the specific rotation of a sample of a pure enantiomer []_max determined under identical conditions. The quotient of these two quantities is multiplied by 100 to obtain the optical purity:

o.p.(%) = 100·[]/[]max

A pure enantiomer thus has 100% o.p., the racemate has 0% o.p..

Example:   [R] : [S]  =   70 : 30

where [R] and [S] are the relative concentrations or absolute amounts of the enantiomers.

The optical rotation of 30% S is compensated by 30% R. The measurement catches the optical rotation caused by the excess 40% R, thus o.p. = 40%.