Log reduction calculator
Enter the microbial count before and after a treatment to get the log reduction and the percent killed. Or convert a claim such as “4-log” or “99.99 %” into the other form and see how many organisms out of a starting population would survive.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose what you know: counts before and after, a log reduction, or a percentage.
- Enter the values. With a log or percentage, the count before treatment shows how many organisms would survive (enter 0 to leave it out).
- Read the log reduction, the percent reduction, the survivors and the surviving fraction.
- Log reduction = log₁₀(N₀ ÷ N)
- Percent reduction = (1 − N ÷ N₀) × 100 = (1 − 10^(−log)) × 100
- Log from percent = −log₁₀(1 − percent ÷ 100)
| Log reduction |
Percent |
Survivors of 1,000,000 |
| 1 |
90 % |
100,000 |
| 2 |
99 % |
10,000 |
| 3 |
99.9 % |
1,000 |
| 4 |
99.99 % |
100 |
| 5 |
99.999 % |
10 |
| 6 |
99.9999 % |
1 |
Worked example
A disinfectant test starts with 1,000,000 CFU and recovers 10 CFU: log₁₀(1,000,000 ÷ 10) = log₁₀(100,000) = 5-log, which is 99.999 %. One organism in 100,000 survived.
From 5 × 10⁷ to 2,500: log₁₀(20,000) = 4.301, a 99.995 % reduction.
Tips
- Use the same units before and after. CFU/mL with CFU/mL, CFU per surface with CFU per surface.
- Logs add up. Two treatments of 2 log and 3 log in sequence give 5 log — percentages do not add that simply.
- Watch rounding in claims. “99.9 %” is only 3 log; the difference between 99.9 % and 99.999 % is a hundredfold.
Frequently asked questions
What is a log reduction?
The base-10 logarithm of how many times the number of live organisms went down: log reduction = log₁₀(N₀ ÷ N). A 1-log reduction leaves a tenth, 2-log a hundredth, 5-log one in 100,000.
How do I convert log reduction to percent?
Percent reduction = (1 − 10^(−log)) × 100. So 1 log = 90 %, 2 log = 99 %, 3 log = 99.9 %, 4 log = 99.99 % and 5 log = 99.999 %. Back the other way: log = −log₁₀(1 − percent ÷ 100).
Is 99.9 % killed good enough?
It depends on how many there were. 99.9 % of a million bacteria still leaves 1,000 alive. That is why disinfection and sterilisation are specified in logs: sterilisation processes are commonly designed for far higher reductions than disinfectants.
What if no organisms are recovered after treatment?
A count of 0 gives an infinite logarithm, so it is reported as “greater than” the log of the starting count divided by the detection limit. Enter the detection limit (for example 1 or 10) as the count after treatment and read the result as a minimum.