Log Reduction Calculator: Log Kill, Percent Reduction and Survivors
Convert microbial counts before and after treatment into log reduction and percent kill, turn 99.99% into log values, or get the log kill from a D-value and contact time.
How it works
Log reduction = log₁₀(N₀ ÷ N) · Percent reduction = (1 − 10^−log) × 100 = (N₀ − N) ÷ N₀ × 100 · Log reduction from a percent = −log₁₀(1 − % ÷ 100) · Survivors N = N₀ × 10^−log · Log reduction from a D-value = exposure time ÷ D
Disinfection, cooking and sterilization kill microbes in fixed proportions, so the count falls by a factor of ten again and again. A log reduction counts those factors of ten: one log is a 90% kill, two logs 99%, three logs 99.9%, and so on. To get it from lab counts, divide the count before treatment by the count after and take the base-10 logarithm. Both counts must be in the same units, such as CFU per mL or per surface area. The calculator also works the other way. Enter a log value to see the percent kill and the survivors left from your starting load, or enter a percent claim such as 99.999% to get its log value (5). In the D-value mode it uses first-order kinetics: each D-value of exposure at a fixed temperature or dose gives one log of kill, so 10 minutes at a D-value of 2 minutes is a 5-log reduction. Set a target under More options to check a process against a requirement. For example, US juice HACCP rules (21 CFR 120.24) require at least a 5-log reduction of the most resistant pathogen. Results assume uniform exposure and straight-line (log-linear) kill. Real survival curves can have shoulders and tails, and a final count of zero only means “below the detection limit”. This is a microbiology converter. To size a chlorine or bleach dose, use the Chlorine Dose or Bleach Disinfecting Ratio calculators.
Worked example
A surface swab shows 1,000,000 CFU before cleaning and 50 CFU after. The ratio is 1,000,000 ÷ 50 = 20,000, and log₁₀(20,000) = 4.30, so this is a 4.30-log reduction. The percent reduction is (1 − 10^−4.30) × 100 = 99.995%, or about 1 in 20,000 organisms surviving, and 999,950 CFU were killed. Rounded down, it supports a 4-log (99.99%) claim. It misses a 5-log target by 0.70 log.
Sources: Omni Calculator: Log reduction (CFU before and after, worked examples) · Wikipedia: Log reduction (definition, 580 ppm to 0.725 ppm example) · Wikipedia: Decimal reduction time (D-value) · eCFR 21 CFR 120.24: 5-log pathogen reduction for juice
Frequently asked questions
What does a 6-log reduction mean?
It means the number of live microbes fell by a factor of 1,000,000 (10⁶), which is a 99.9999% kill. If a surface started with 1,000,000 bacteria, about one would survive.
How do I convert log reduction to percent?
Percent reduction = (1 − 10^−log) × 100. One log is 90%, 2 logs 99%, 3 logs 99.9%, 4 logs 99.99% and 5 logs 99.999%. Each extra log adds another 9.
How do I calculate log reduction from CFU counts?
Divide the count before treatment by the count after and take log₁₀ of the result. For 10,000,000 CFU before and 100,000 after, the ratio is 100 and the log reduction is 2 (99%).
What if no colonies grew after treatment?
A zero count has no logarithm. Use the detection limit of the method (for example 1 CFU per plate, scaled by the dilution) as the final count and report the result as “greater than or equal to” that log value.
Is 99.9% the same as a 3-log reduction?
Yes. 99.9% leaves one survivor in 1,000, which is 10³, so it is a 3-log reduction. 99.99% is 4-log and 99.999% is 5-log.
How is log reduction related to the D-value?
The D-value is the time (or dose) that gives a 1-log kill at set conditions. The log reduction is the exposure time divided by D, so a 12-minute hold at D = 3 minutes gives 4 logs, and a 5-log target needs 5 × D.
Related calculators
For education and planning. Disinfection and food-safety processes must be validated with approved methods and the product label or regulator’s requirements.