Log Reduction Calculator

Background.

A kitchen spray that kills 99.9% of bacteria and a juice process that must deliver a 5-log reduction are making the same kind of claim, written two ways. Microbiologists count kill in powers of ten: a 1-log reduction leaves a tenth of the population, 2 logs a hundredth, 3 logs a thousandth. So 99.9% is exactly 3 logs, and the juice standard asks for a hundred times more kill than the spray label promises.

The log scale is used because killing is multiplicative. A dose that removes 90% of the organisms present does so whether there are a thousand or a billion, so each further log takes roughly the same extra treatment, and the starting load decides how many are left at the end. On a surface carrying 10⁸ bacteria per millilitre, a 99.99% kill still leaves 10,000 per millilitre.

Three modes cover the usual questions: the log reduction a treatment achieved, from plate counts taken before and after; the survivors a target will leave, from a starting count; and the logs hidden in a percentage claim. Each result is set against three levels written into US federal rules: 3 logs for Giardia cysts and 4 logs for viruses under EPA's surface-water treatment rule, and 5 logs in the pertinent microorganism under FDA's juice HACCP rule. Reaching one of those numbers is not the same as running a validated process, and the reading under the result says so.

What is log reduction calculator?

A log reduction is the base-10 logarithm of the ratio between a microbial population before and after a treatment, LR = log₁₀(N₀ ⁄ N). It has no unit, so CFU per millilitre, plaque-forming units, cells per swab or whole-sample totals all give the same answer as long as both counts use the same one. Each whole log is a tenfold drop, which is why the percentages pile up nines: 1 log is 90%, 2 logs 99%, 3 logs 99.9%, 6 logs 99.9999%. Values in between are fractional, so 99.95% is 3.3 logs. A negative log reduction means the count went up, and it is reported as such rather than rounded to zero.

How to use this calculator.

1. 01Pick the question. 'From counts' uses N₀ and N; 'survivors remaining' uses N₀ and a target in logs; 'from a percent kill' uses N₀ and the percentage. Fields that do not belong to the chosen mode are ignored.
2. 02Enter both counts in the same unit, measured by the same method on comparable samples. Make sure the treated sample was neutralised, or the disinfectant keeps working in the dilution tube and inflates the kill.
3. 03If the treated plate came back blank, enter your method's detection limit instead of zero: one colony divided by the volume of original sample on the plate. Then report the answer as 'greater than'.
4. 04For a label or specification claim, type the percentage as written, such as 99.9 or 99.999. Anything below 100 is accepted, and a negative percentage describes growth.
5. 05Read the surviving count and fraction as well as the log figure. The percentage boxes show two decimals, so for kills beyond 99.99% use the reading line, which spells the percentage out in full.

The formula.

The three modes are one relation solved three ways. From two counts, the reduction factor is N₀ ⁄ N and LR = log₁₀(N₀ ⁄ N). From a target, the survivors are N = N₀ ⁄ 10^LR. From a percent claim p, the surviving fraction is 1 − p ⁄ 100, so N = N₀ × (1 − p ⁄ 100) and LR = −log₁₀(1 − p ⁄ 100). Every mode then reports the same set: percent reduction = (1 − N ⁄ N₀) × 100, surviving percent = (N ⁄ N₀) × 100, which equals 100 × 10^−LR, and the fold reduction N₀ ⁄ N.

Only the ratio enters, so the starting count never changes the log figure; it only scales how many organisms are left. Taking the logarithm also tames counting error. A plate count that is out by a factor of two shifts the log reduction by log₁₀ 2 ≈ 0.3, which is why results are usually quoted to one or two decimals. What the arithmetic cannot do is describe how killing proceeds over time. A D-value, the exposure needed for one log, equals contact time ÷ LR only if survivors fall in a straight line on a log plot, and two counts cannot show that. Real survivor curves often have a shoulder before killing starts or a tail of resistant organisms at the end.

A worked example.

Frequently asked questions.

References& sources.

1. [1]US Environmental Protection Agency, 40 CFR 141.70(a) — National Primary Drinking Water Regulations, Subpart H (Filtration and Disinfection), General requirements. Verbatim: 'at least 99.9 percent (3-log) removal and/or inactivation of Giardia lamblia cysts' and 'at least 99.99 percent (4-log) removal and/or inactivation of viruses between a point where the raw water is not subject to recontamination by surface water runoff and a point downstream before or at the first customer.' This is the regulation that states the percent↔log mapping explicitly in both forms. Retrieved 2026-07-29.
2. [2]US Food and Drug Administration, 21 CFR 120.24(a) — Hazard Analysis and Critical Control Point (HACCP) Systems, Subpart B (Pathogen Reduction), Process controls. Verbatim: control measures achieving 'at a minimum, a 5 log (i.e., 10⁵) reduction, for a period at least as long as the shelf life of the product when stored under normal and moderate abuse conditions, in the pertinent microorganism', with 'pertinent microorganism' defined as 'the most resistant microorganism of public health significance that is likely to occur in the juice'. Independent second authority: a different agency, a different medium, and its log↔fold phrasing composes exactly with EPA's percent↔log phrasing. Retrieved 2026-07-29.
3. [3]US Food and Drug Administration, Bacteriological Analytical Manual, Chapter 3 'Aerobic Plate Count', January 2001 Edition. §C.4: when plates from all dilutions have no colonies, 'report APC as less than 1 times the corresponding lowest dilution used' — the rule that governs the denominator of a log reduction when the post-treatment plate comes back blank. §D: the two-significant-figure reporting rule for the counts a log reduction is built from. Free, read directly 2026-07-29.
4. [4]ISO/TC 34/SC 9, 'Excel tool to implement the calculations of the colony-count technique according to ISO 7218 — Verification Report', S. Grosz, 28 August 2020. Documents the '<1,0E+01' detection-limit convention for a plate with no colonies, from a standards body independent of the FDA — the same convention this page applies when refusing a survivor count of zero. Free ISO committee document, retrieved 2026-07-29.