How this calculator works
A one-log reduction divides the count by ten, a two-log reduction by one hundred, and so on. Both measurements must have the same units, sampled volume basis and recovery method. Below-detection observations need a stated detection limit; do not replace a nondetect with zero in a logarithm. The calculation alone does not validate a disinfection process.
Formula
L = log10(N0/N). Reduction fraction = 1 - 10^(-L). N = N0 × 10^(-L). L = -log10(1 - reduction fraction).
Worked example
From 1,000,000 CFU to 1,000 CFU, the reduction is 3 log or 99.9%. From 10,000 CFU to 100 CFU, the reduction is 2 log or 99%.
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Frequently asked questions
What if no colonies are detected?
Use the test's detection limit to report a lower bound on reduction. Zero cannot be used as the denominator of a log ratio.
Can a count increase have a log reduction?
It would have a negative log change. This tool is for reductions and rejects an increased final count.
Does 99.9% reduction mean a sample is sterile?
No. Remaining organisms depend on the starting load, and detection, sampling and organism type also matter.