Cell Doubling Time Calculator
Two counts and the time between them is all it takes — but only within the log phase.
The formula
doubling time = t ln2 / ln(N1/N0) ; log reduction = log10(before / after)
Doubling time from any two counts
Exponential growth needs only two measurements and the time between them. The logarithm converts the ratio into a number of doublings, and dividing the elapsed time by that gives the doubling time. It does not matter what the absolute counts are, only their ratio.
Only the log phase is exponential
A real culture lags first, grows exponentially for a while, then plateaus as nutrients run out and waste accumulates. The doubling time is only meaningful within that middle phase. Fitting an exponential across the plateau produces a figure that describes nothing.
Log reduction is a disinfection scale
Each log reduction removes 90 % of what remains: one log leaves a tenth, three logs leave a thousandth, six logs leave one millionth. Sterilisation standards are written this way because killing is proportional rather than absolute — there is no number of logs that reaches zero, only one that makes survival improbable enough.
Ratios are expectations, not promises
Every genetic ratio on these pages is a probability distribution, not a guarantee. A 3:1 cross does not produce three dominant offspring for every recessive one in a litter of four — it produces each offspring independently with a three-quarters chance. Small families depart from the expected ratio routinely, and that is the reason Mendel needed thousands of pea plants rather than dozens.