Log Reduction Calculator (log kill ⇄ percent reduction ⇄ survivors)

Understanding the log reduction calculator — concept and principle

This calculator converts between log reduction, percent reduction and surviving fraction for disinfection, sterilisation, filtration or cooking, and can work from counts before and after treatment. It is used in microbiology labs, food safety, water treatment and when reading disinfectant claims.

Each log step removes 90 % of what is left; 4.3 logs leaves 50 of every million.

How it is calculated

Log reduction LR = log₁₀(N₀ ÷ N)

Percent reduction = (1 − 10^(−LR)) × 100

LR from a percentage = −log₁₀(1 − percent ÷ 100)

Surviving fraction = 10^(−LR); survivors per million = 10⁶ × 10^(−LR)

N₀ and N must be in the same unit, such as CFU/mL.

The principle

Microbial populations span many orders of magnitude, so treatments are compared by how many factors of ten they remove. Each log is a tenfold reduction: 1-log removes 90%, 2-log 99%, 3-log 99.9%, and 6-log 99.9999%. Percentages crowd together near 100% and hide big differences, while logs keep them visible: 99.9% and 99.9999% look similar but differ by a factor of 1,000 in survivors.

Worked example

Default inputs: counts, 1,000,000 before and 50 after.

LR = log₁₀(1,000,000 ÷ 50) = log₁₀(20,000) = 4.3.
Percent reduction = (1 − 50/1,000,000) × 100 = 99.995%.
Surviving fraction = 50 ÷ 1,000,000 = 0.00005.
Survivors per million organisms = 50; count after = 50.

Each point is the calculator re-run with only “Count after treatment (same unit)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Count after treatment (same unit)	Log reduction
12.5	4.9
25	4.6
37.5	4.43
50	4.3
62.5	4.2
75	4.12
100	4

How to read the result

A higher log reduction means a more effective treatment. Required levels depend on the application and regulations, so compare with the standard that applies to you. When nothing grows after treatment, the log reduction can only be stated as "at least" the value calculated from the detection limit.

Common mistakes

Entering zero survivors, which gives an infinite result; use the detection limit instead.
Comparing log reductions measured with different test methods.
Reading 99.9% as "almost sterile"; one in a thousand still survives.
Mixing units between the before and after counts.

Keep in mind: Disinfection and sterilisation claims must be validated with approved test methods. This calculator converts numbers; it does not confirm that a process is safe.

Frequently asked questions

What is a 6-log reduction?

A millionfold reduction, 99.9999% removed.

How do I convert 99.99% to logs?

−log₁₀(1 − 0.9999) = 4-log.

What if no organisms are recovered?

Report "at least" the log reduction calculated with the detection limit as the after count.

Is a 4.3-log reduction good?

It depends on the target; compare it with the requirement for your application.

Sources: Standard definition of log reduction, log₁₀(N₀/N), as used in disinfection and sterilisation guidance (US EPA, CDC)

Checked 9 Nov 2026. Standard definition of log reduction, log₁₀(N₀/N), as used in disinfection and sterilisation validation (e.g. US EPA and CDC disinfection guidance; 6-log = 99.9999%).

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