Log Reduction Calculator
Calculate log reduction (LR), percent reduction, and final microbial count for disinfection / sterilization. Works with CFU, PFU, cells, or any count-based unit. Includes quick picks, step-by-step, and a mini kill-level visual.
Background
In microbiology, a log reduction tells you how much the microbial population dropped on a base-10 scale. A 1-log reduction = 90% reduction, 2-log = 99%, 6-log = 99.9999%. The core idea is: LR = log₁₀(N₀ / N).
How to use this calculator
- Pick a mode (LR, percent reduction, final count, or D-value).
- Enter values (counts can be typed like 1e6).
- Click Calculate to see the kill level, percent reduction, and optional steps.
How this calculator works
- Log reduction: LR = log₁₀(N₀/N)
- Percent reduction: % = (1 − 10^(−LR)) × 100
- Final count: N = N₀ × 10^(−LR)
- D-value: LR = time / D (so D = time/LR, time = D×LR)
Formula & Equation Used
Log reduction: LR = log₁₀(N₀/N)
Percent reduction: % = (1 − 10^(−LR)) × 100
Final count: N = N₀·10^(−LR)
D-value relation: LR = time/D
Example Problems & Step-by-Step Solutions
Example 1 — Log reduction from counts
If N₀ = 1×10⁶ CFU/mL and N = 1×10³ CFU/mL, find LR.
- LR = log₁₀(N₀/N) = log₁₀(10⁶ / 10³)
- = log₁₀(10³) = 3 → 3-log reduction.
Example 2 — Percent reduction from LR
What percent reduction is a 2-log reduction?
- % = (1 − 10^(−LR)) × 100
- = (1 − 10^(−2)) × 100 = (1 − 0.01)×100 = 99%.
Example 3 — Final count after LR
If N₀ = 1×10⁸ and LR = 6, what is the final count?
- N = N₀ × 10^(−LR) = 10⁸ × 10^(−6)
- = 10² = 100.
Frequently Asked Questions
Q: What does “1-log reduction” mean?
A 1-log reduction means the count dropped by a factor of 10 (about 90% reduction).
Q: What is a “6-log reduction”?
A 6-log reduction is a 10⁶-fold decrease (99.9999% reduction), often used as a sterilization benchmark.
Q: Can I use this for viruses (PFU) or cells?
Yes. Any count unit works because the math depends only on ratios.
Q: What is the D-value?
The D-value is the time needed for a 1-log (90%) reduction under specific conditions (temperature, disinfectant, etc.).
