Cell cultures grow through a process called binary fission, meaning each cell divides into two identical cells at a constant rate. Population sizes are easily predictable when the generational time, or length of time per cell divisions, is known. You can calculate the mean generational time (the time it takes for cell doubling to occur) from population sizes at given times.
Use a calculator to find the log of the population size at two different times. Find the difference between the log of the later population size and the log of the initial population size. For example, if a population starts with 256 members, and two hours later it stands at 4,096 members, then the log of the initial population equals 2.408, while the log of the final population equals 3.612. The difference between the two is 1.204.
Divide the difference in the two logs of population sizes by the log of two, or 0.301. The converts the difference in population into the number of generations that have grown. In the example, 1.204 divided by 0.301 equals 4.
Find the difference between the times at which the population sizes were measured. Subtract the second time by the initial time. In the example, two hours pass between the population measurements. Therefore, four generations passed in two hours.
Divide the elapsed time in hours by the number of generations that passed during that time. For example, two hours divided by four generations equals 0.5 hours per generation. Multiply the result by 60 to convert to minutes per generation. In the example, the doubling time is 0.5 * 60, or 30 minutes.