Reduction is often expressed as a percentage. The closer it is to 100%, the better.
Letting  and  be as before, a reduction by  % is achieved, where
;Example:
Let, as in the earlier example, the concentration of some contaminant be 580 ppm before and 0.725 ppm after treatment. Then
So this is (better than) a 99% reduction, but not yet quite a 99.9% reduction.
The following table summarizes the most common cases.
In general, if  is a whole number, an -log reduction corresponds to a percentage reduction with  leading digits "9" in the percentage (provided that it is at least 10%).