Microbiology · Growth
Bacterial doubling time calculator.
Doubling time, specific growth rate and generations from two measurements in exponential phase.
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How doubling time is calculated
In exponential growth, N = N0 ert, so the specific growth rate is r = ln(N / N0) / t and the doubling (generation) time = ln 2 / r. The number of generations is log2(N / N0). On a plot of ln N against time, r is the slope of the straight line, which is how the specific growth rate is defined.
Worked example. OD600 rises from 0.05 to 0.8 in 3 hours: ln(16) / 3 = 0.924 per hour, doubling time 0.75 h = 45 minutes, 4 generations. To predict: with a 30-minute doubling time, 0.05 grows to 0.05 x 26 = 3.2 in 3 hours if growth stayed exponential (real cultures leave log phase well before that).
Use two points taken inside the exponential phase, ideally from a log plot. Use OD or a count, as long as both are in the same unit and in the instrument's linear range. See also the OD600 calculator and the CFU per mL calculator.
Frequently asked questions
What is the formula for doubling time?
Doubling time = t x ln 2 / ln(N / N0), where N0 and N are the counts at the start and end of an interval of length t.
Can I use OD600 instead of CFU?
Yes, if both readings are in the linear range of the instrument and growth is exponential. The ratio is what matters, not the unit.
What is a typical E. coli doubling time?
It depends heavily on medium, temperature and strain, so measure your own rather than assuming a value.
Why do I get a negative or no doubling time?
If the final count is equal to or below the start there is no growth. The calculator reports the decline instead.
Sources
Checked
- Wikipedia, Doubling time (r = ln(N/N0)/t; doubling time = ln 2 / r)
- Wikipedia, Bacterial growth (slope of ln N against time is the specific growth rate)
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Research use and scope
This is arithmetic for research-bench planning, not clinical, diagnostic or safety advice. Conversion factors are standard approximations; strain, instrument, reagent and protocol differences matter. Check results against your own protocol, kit manual and lab supervisor, and follow your institution's biosafety rules.