Determine the starting number of bacteria, b, in your sample. The starting number can be found by multiplying the starting concentration of bacteria by the volume of the sample.
Incubate the bacteria with sufficient nutrients and media volume to allow it to multiply every 20 minutes (the faster end of the doubling time spectrum for bacteria) for the duration of the incubation period.
Measure the concentration of the bacterial sample after the incubation. The concentration can be found by measuring turbidity or by plating the sample in a series of dilutions.
Determine the final number of bacteria, B, in your sample. This number can be found by multiplying the final concentration of bacteria by the volume of the sample.
Solve for the generation time, g, by using the following equation: g = (log B - log b)/(log 2).
Divide the incubation period by g. This quotient is equal to the generation time of the bacteria.