Microbiology
Log Reduction Calculator
Converts microbial counts before and after a disinfection or sterilisation step into a log reduction and a percent kill.
Log reduction
4.00log₁₀
Moderate – acceptable for water treatment.
Percent reduction
99.99%
4.00-log = 99.99% of organisms killed
Microbial counts
Colony-forming units (CFU) before and after treatment.
Viable count before treatment, e.g. 10⁶ = 1000000.
Survivors after treatment, same unit as N₀.
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Working
Count comparison
Log scale, so both bars stay visible.
Log to percent conversion
Final count is what would remain from your initial count at each log reduction.
| Log reduction | % reduction | Survivor ratio | Final count (N) |
|---|---|---|---|
| 1-log | 90% | 1 in 10 | 100,000 |
| 2-log | 99% | 1 in 100 | 10,000 |
| 3-log | 99.9% | 1 in 1,000 | 1,000 |
| 4-log | 99.99% | 1 in 10,000 | 100 |
| 5-log | 99.999% | 1 in 100,000 | 10 |
| 6-log | 99.9999% | 1 in 1,000,000 | 1 |
Log reduction = log₁₀(N₀ ÷ Nₜ)
Percent reduction = (1 − Nₜ ÷ N₀) × 100%
N₀ = viable count before treatment, Nₜ = viable count after treatment. The two are linked by percent reduction = (1 − 10^−log reduction) × 100%, which is where the table comes from.
Bands used for the reading: below 3 log is low (insufficient for disinfection), 3 to below 5 is moderate (acceptable for water treatment), 5 and above is high.
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References
- Aulton ME, Taylor KMG, editors. Aulton's Pharmaceutics: The Design and Manufacture of Medicines. 6th ed. Edinburgh: Elsevier; 2022.
Common questions
Because the differences that matter are hidden in the decimals. 99.9% and 99.9999% look almost the same, but the second leaves a thousand times fewer survivors: 3-log versus 6-log.
A count of zero cannot be logged. Report the reduction as “greater than” the value calculated with the limit of detection as Nₜ — for example, 1 CFU if you plated the whole sample.
It is rounded to two decimals. Anything above 99.995% displays as 100.00%, which is exactly why the log reduction is the more useful figure for high kills.
Yes — convert each plate count back to the original sample (count × dilution factor) before entering it, and use the same volume basis for both counts.
Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy