Doubling time calculator
From two counts, a growth rate, or a growth curve. Counts, CFU/mL or OD600.
From two counts, a growth rate, or a growth curve. Counts, CFU/mL or OD600.
Doubling time is how long a growing population takes to double in size during exponential growth. For bacteria and yeast it is called the generation time; for cultured mammalian cells the population doubling time. It is the most direct way to compare growth between strains, cell lines, media or treatments, because it does not depend on how many cells you started with.
k is the specific growth rate, N₀ and Nₜ are the counts at the start and after time t. Because only the ratio Nₜ/N₀ matters, any measurement proportional to cell number works: hemocytometer counts, cells/mL, CFU/mL or OD600 within its linear range.
Worked example. 1,000 cells grow to 8,000 in 180 minutes. k = ln(8) / 180 min = 0.01155 per minute, so the doubling time is ln(2) / 0.01155 = 60 minutes. The population went through log₂(8) = 3 doublings.
With several readings over time, a straight line fitted to ln(count) against time gives a better estimate than any single pair of points. The slope of that line is k and ln(2)/slope is the doubling time. The Time series tab does this fit and reports R², which should be close to 1 if the points are really in exponential phase. Leave out lag-phase and stationary-phase readings, which bend the curve and lengthen the apparent doubling time.
These describe the same growth in different units. The specific growth rate k (per hour) is ln(2) divided by the doubling time. Generations per hour is 1 divided by the doubling time in hours. The number of doublings between two counts is log₂(Nₜ/N₀), which in cell culture is tracked as the cumulative population doubling level, PDL = 3.32 × log₁₀(Nₜ/N₀). For OD600-based bacterial growth predictions, see thebacterial growth calculator.
| Organism or cell line | Doubling time |
|---|---|
| E. coli, rich medium at 37 °C | about 20 min |
| Bacillus subtilis | 25–40 min |
| Saccharomyces cerevisiae | about 90 min |
| Mycobacterium tuberculosis | 15–20 h |
| HeLa, HEK293, CHO cells | 18–24 h |
| Primary human fibroblasts | 24–48 h |
| Human embryonic stem cells | 30–36 h |
Doubling time = ln(2) / k, where k is the growth rate constant. From two counts, k = ln(N_t / N_0) / t, so doubling time = t × ln(2) / ln(N_t / N_0). For 1,000 cells growing to 8,000 in 180 minutes, the doubling time is 60 minutes.
For microbes, yes: generation time is the time for the population to double during exponential growth. In cell culture the same number is called the population doubling time. Both are ln(2) divided by the specific growth rate.
Any measurement proportional to cell number: cell counts, cells/mL, CFU/mL or OD600. The formula only uses the ratio of final to initial, so the unit cancels out, as long as both readings are in the same unit and within the linear range.
The calculation assumes constant exponential growth between the two readings. Readings taken during lag phase or after the culture starts to plateau give a doubling time that is too long. Use readings from the straight part of a log-scale growth curve, or fit the time series here.
Paste time and count pairs into the Time series tab. The calculator fits a straight line to ln(count) against time; the slope is the growth rate and ln(2) divided by the slope is the doubling time. R² shows how well the points follow exponential growth.