Bacterial Doubling Time Calculator
Doubling time, or generation time, is how long a bacterial population takes to double during exponential growth. It is a defining characteristic of a culture and a standard microbiology measurement. This calculator finds the number of generations from two timed cell counts and divides the elapsed time by that number to give the doubling time. Enter the initial and final cell counts and the elapsed time to see the result.
Doubling time formula
n = log2(final count / initial count)
doubling time = elapsed time / n
growth rate = n / (elapsed time in hours)
The number of generations is the base-2 logarithm of the fold increase. Dividing the elapsed time by the generation count gives the doubling time, valid during exponential growth.
Worked example
A culture grows from 1,000 to 64,000 cells in 120 minutes. The fold increase is 64,000 / 1,000 = 64, and log2(64) = 6 generations. The doubling time is 120 / 6 = 20.00 minutes, the textbook value for fast-growing E. coli. The growth rate is 6 generations in 2 hours = 3.00 generations per hour.
Bacterial doubling time: frequently asked questions
What is bacterial doubling time?
Doubling time, also called generation time, is the period required for a bacterial population to double in number during exponential (logarithmic) growth. It is a key characteristic of a microbial culture: Escherichia coli under ideal laboratory conditions can double in around 20 minutes, while slow-growing species such as Mycobacterium tuberculosis may take many hours. Doubling time depends on the species, the medium, temperature and other conditions.
How do you calculate the number of generations?
The number of generations n is the base-2 logarithm of the ratio of final to initial cell count: n = log2(final / initial). This counts how many times the population doubled. For example, growing from 1,000 to 64,000 cells is a 64-fold increase, and log2(64) = 6, so six generations have occurred.
How is doubling time found from the number of generations?
Divide the total elapsed time by the number of generations: doubling time = time / n. If a culture undergoes 6 generations in 2 hours (120 minutes), the doubling time is 120 / 6 = 20 minutes. This is the standard method used in microbiology to characterise growth rate from two timed cell counts.
Does this method apply outside exponential growth?
The doubling-time formula assumes the culture is in the exponential (log) phase, where growth is balanced and the rate is constant. During lag phase (before growth starts) and stationary phase (when nutrients are exhausted), the population is not doubling at a steady rate, so the calculated doubling time would not be meaningful. Take both cell counts during exponential growth for a valid result.
Official sources
- National Center for Biotechnology Information: NCBI microbiology resources.
- Centers for Disease Control and Prevention: CDC laboratory and microbiology.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.