Calculate log reduction and percent reduction from microbial counts before and after treatment, such as CFU/mL, to see how many logs were removed.

Log Reduction Formula

The following formula is used to calculate a log reduction.

Log Reduction = log₁₀((A) / (B))

Percent Reduction = ((A - B) \times 100) / (A)

Where A is the number of viable microorganisms before treatment.
B is the number of viable microorganisms after treatment.

Log Reduction Definition

What is a log reduction? A log reduction is the base-10 logarithm of the ratio of positive, comparable counts or concentrations before and after treatment. Use matching units and measurement methods. A non-detect does not establish a true zero, and log reduction alone does not establish sterilization or safety.

Example Problem

How to perform log reduction?

First, determine the number of microorganisms before treatment.
For this example, this value is 2,000,000.
Next, determine the number of microorganisms after treatment.
After treatment, there are only 500 microorganisms left.
Finally, calculate the log reduction.
The log reduction is found to be: log10 ( A/B ) = 3.602.

About Log Reduction

Can a log reduction be negative? If the number of microorganisms increases after treatment, the resulting log reduction will be negative. This means that there was actually an increase as opposed to a reduction, hence the negative value.

What does 3 log reduction mean? A 3 log reduction is equivalent to a 99.9% reduction in quantity, which in this case is the quantity of microorganisms.

What is 6 log reduction? A 6 log reduction is equivalent to a 99.9999% reduction in a quantity.

What is 5 log reduction? A 5 log reduction is equal to a 99.999% reduction.

What is 4 log reduction? A 4 log reduction is equal to a 99.99% reduction.

What is 2 log reduction? A 2 log reduction is equal to a 99% reduction.

What is 1 log reduction? A 1 log reduction is equal to a 90% reduction.