Let  and  be the numerical values of the concentrations of a given contaminant, respectively before and after treatment, following a defined process.
It is irrelevant in what units these concentrations are given, provided that both use the same units.
Then an and -log reduction is achieved, where
For the purpose of presentation, the value of  is rounded down to a desired precision, usually to a whole number.
;Example:
Let the concentration of some contaminant be 580 ppm before and 0.725 ppm after treatment. Then
Rounded down,  is 2, so a 2-log reduction is achieved.
Conversely, an -log reduction means that a reduction by a factor of  has been achieved.