To calculate phenol coefficient, the concentration of phenol at which the compound kills the test organism in 10 minutes, but not in 5 minutes, is divided by the concentration of the test compound that kills the organism under the same conditions (or, probably more common, dividing the dilution factor at which the tested substance shows activity by the dilution factor at which phenol shows comparable activity).
Distinguished bacteriologist Sir Ashley Miles, reviewing the subject, described the test as "...at best a grossly over-simplified answer to a difficult problem and, at worst little short of bacteriological prostitution".
[1] Calculations by Harriette Chick showed that the killing of bacteria by disinfectants followed first order kinetics.
[5] So, the bactericidal activity of a particular disinfectant at a given concentration can be expressed as a constant (k) calculated by employing the formula k = N/C·T where N is the number of surviving cells, C is the concentration of agent applied and T is the time for which the agent is applied, so k is inversely proportional to dose (C·T is collectively called dose).
The disinfectant to be tested is compared with phenol on a standard microbe (usually Salmonella typhi or Staphylococcus aureus).