Log Reduction and Percent Reduction Calculator
What is log reduction
Log reduction is a base-10 way to describe how much a microbial count decreased after a process such as cleaning, disinfection, sterilization, filtration, or treatment. Each increase of 1 in log reduction means a 10× decrease in the count.
Core formula
Let \(N_0\) be the initial count and \(N\) be the final count. The log reduction is:
\[ LR = \log_{10}\left(\frac{N_0}{N}\right) \]
The remaining fraction and percent are:
\[ \text{Remaining fraction} = \frac{N}{N_0} \qquad \text{Remaining \%} = \frac{N}{N_0}\cdot 100 \]
The percent reduction is:
\[ \text{Reduced \%} = 100 - \text{Remaining \%} \]
How to use this calculator
Enter your initial count and final count. Optionally enter a detection limit if your measurement cannot reliably detect values below a threshold.
If the final count is at or below the detection limit, the calculator can display a conservative (lower-bound) estimate of log reduction based on that limit.
How to interpret common log reduction values
Because \(10^{-LR}\) is the survival fraction, each LR level corresponds to a remaining percentage:
- 1-log means \(10^{-1}=0.1\) remaining, or 10% remaining (90% reduction).
- 2-log means \(10^{-2}=0.01\) remaining, or 1% remaining (99% reduction).
- 3-log means \(10^{-3}=0.001\) remaining, or 0.1% remaining (99.9% reduction).
- 6-log means \(10^{-6}\) remaining, or 0.0001% remaining (99.9999% reduction).
Why a reference table helps
Real-world reporting often uses rounded LR levels (for example 3-log, 4-log, 6-log). The table shows what each LR implies for survival fraction, percent remaining, and the expected final count for your initial value. This makes it easier to sanity-check results and compare targets.
Notes about detection limits
If your final count is reported as “below detection,” your true log reduction could be higher than what is computed using the detection limit. A conservative estimate can be expressed as:
\[ LR \ge \log_{10}\left(\frac{N_0}{L}\right) \]
where \(L\) is the detection limit.
FAQ
Can log reduction be negative
Yes. If \(N > N_0\), then \(\frac{N_0}{N} < 1\) and \(LR\) becomes negative. In practice this usually indicates growth, a sampling mismatch, or inconsistent units.
Does the unit selection change the math
No. The unit selection is a labeling choice so the same calculator can be used for common reporting styles (for example CFU/mL or CFU/g).