Log Reduction Calculator
Compare microbial counts. Understand the reduction.
Microbial Count Inputs
Illustrative density · not one dot per CFU. Non-detects show a bound, not an empty sterile plate.
Reduction Summary
Base 10Each log means 10× fewer
Log reduction reference
Formula & Worked Example
% reduction = (1 − Nf / N₀) × 100
1,000,000 → 100 CFU = 4 log = 99.99%
Frequently Asked Questions
What does log reduction mean?
It is the base-10 logarithm of initial divided by final microbial count, when both measurements use the same basis. One log is a tenfold decrease.
How is percentage reduction calculated?
For measured counts, (1 − final / initial) × 100. For a specified log reduction L, (1 − 10 to the power −L) × 100.
When should I use CFU vs CFU/mL?
CFU denotes a count; CFU/mL denotes a concentration. Compare values on the same basis, accounting for sampling and dilution. The unit selector changes labels only, not the numerical measurement basis.
Does a 4-log reduction mean it is sterile?
No. This arithmetic alone cannot demonstrate sterility, safety, treatment suitability or compliance. Process validation and the relevant organism and conditions matter.
More questions
What if my final measurement is zero?
A non-detect is not an exact count of zero. Choose below detection limit and enter a positive, method-specific limit on the same basis. The result is a strict lower bound only when the measurement is reported as below that limit.
What if the final count is larger than the initial?
The signed log reduction is negative. This indicates an increase; it is shown with an amber result instead of being presented as successful reduction.
Are the petri dishes real colony counts?
No. The dots illustrate relative density and do not represent one colony or cell each. Images cannot validate an experiment, and a low-density drawing is not proof of no survivors.
Is the chart a measured kill-time curve?
No. It shows the mathematical relation between log reduction and remaining count for the entered initial value. It does not predict treatment time, dose or temperature.
Why does an illustrative final count fall below one?
An average concentration or model can be fractional. It should not be read as a literal fractional organism on a plate or a guarantee that no organisms remain.
Is 5 log a universal safety target?
No. FDA juice HACCP applies a specific standard to the pertinent microorganism and validated juice process. Requirements vary by application; a general total plate count is not interchangeable with that pathogen-specific validation.
How are very high log reductions displayed?
To avoid rounding an incomplete reduction to 100%, high values retain a mathematical residual expression. Scientific notation is used where appropriate; extremely small model counts are not rounded to zero.
What does selecting a reference chip do?
It switches to Log → Percentage mode and sets that log value. The illustrative initial count remains editable. Returning to count mode preserves your earlier count inputs.
Sources & References
Disinfection & SterilizationDefinitions and healthcare guidance
Juice HACCP: 5-log reductionA process-specific example, not a universal targetSource links are references, not institutional endorsement or process validation.
Compare initial and final microbial counts or convert a base-10 log reduction to percentage, with detection-limit bounds and an illustrative chart.