Log reduction calculator.
Measure how well a treatment kills microbes. Enter the count before and after and the calculator gives the log reduction, the percent killed and the surviving fraction.
Before and after
LiveLog reduction is the base-10 log of the initial count divided by the surviving count. Each log is a ten-fold drop, so 3 logs is 99.9%.
Log reduction
4.00
log10 drop in count
Assumes counts are accurate colony or cell numbers. For lab and process estimates. Calculations run in your browser; nothing you enter is stored.
How it works
Powers of ten
Log reduction measures how much a treatment cuts a microbial population, on a base-10 log scale. It is the log of the starting count divided by the surviving count, so each whole log is a ten-fold reduction: 1 log is 90%, 2 logs is 99%, 3 logs is 99.9%, and so on.
Going from a million cells to 100 is a drop of 10,000-fold, which is 4 logs, or 99.99% killed. The log scale keeps very large kills readable.
Reference
Log reduction to percent.
Each log reduction is a ten-fold drop in surviving microbes.
| Log reduction | Percent killed |
|---|---|
| 1 | 90% |
| 2 | 99% |
| 3 | 99.9% |
| 4 | 99.99% |
| 5 | 99.999% |
| 6 | 99.9999% |
The full guide
The complete guide to log reduction.
What log reduction means, how it is calculated, and where it is used.
What is log reduction?
Log reduction expresses how effectively a process kills or removes microbes, as a power of ten. A 1-log reduction means 90% are gone, leaving a tenth; 2 logs leaves a hundredth, and so on. It is the standard way to describe disinfection, sterilisation and filtration performance.
Because microbial counts span huge ranges, the log scale turns awkward percentages like 99.9999% into a simple number, 6.
How it is calculated
Take the starting count, divide by the surviving count, and take the base-10 logarithm. From 1,000,000 to 100 is a ratio of 10,000, and the log of 10,000 is 4, so a 4-log reduction. The percent killed is one minus the surviving fraction, times 100.
Whole logs map to familiar percentages, which is why the table above is worth memorising.
Where it is used
Log reduction is central to disinfectant testing, water and air filtration, food safety and sterilisation validation. Regulators often set targets such as a 5 or 6-log reduction for a process to be considered effective.
It also describes how well a HEPA filter, UV system or sanitiser performs against a challenge organism.
Reading the result
A higher log reduction is better. Note that even a 6-log reduction leaves a small surviving fraction, which matters when starting counts are enormous. Sterilisation aims for very high reductions, often 6 logs or more.
Measure counts carefully, since the result depends on accurate before and after numbers.
The formula
Tenfold
each log.
Log reduction is log10 of initial over surviving count. Each log is a ten-fold, or 90% further, kill.
Generation time ›# Log reduction
log_reduction = log10(initial / surviving)
percent = (1 − surviving/initial) × 100
# worked example
log10(1000000 / 100) = 4 (99.99%)Questions
Log reduction questions.
How do I calculate log reduction?
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Divide the initial count by the surviving count and take the base-10 log. From 1,000,000 to 100 is a ratio of 10,000, whose log is 4, so a 4-log reduction, or 99.99% killed.
What does a 3-log reduction mean?
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It means 99.9% of microbes are killed, leaving one in a thousand. Each whole log is a further ten-fold reduction.
How does log reduction relate to percent?
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One log is 90%, two is 99%, three is 99.9%, and so on, each adding a nine. The table on this page lists them.
What log reduction is needed for sterilisation?
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It depends on the standard, but high-level processes often target a 6-log reduction or more against a challenge organism.
Is this log reduction calculator free?
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Yes. It is completely free with no sign-up, and every calculation runs locally in your browser, so nothing you enter is stored or sent anywhere.
About the developer
Jean Borg
Jean builds and maintains every calculator on freecalculators.pro from Malta, with a focus on tools that are fast, free and show their working. The log reduction calculator uses the standard log10 method and is for lab and process estimates.