Log Reduction Calculator
Each log removes 90 percent of what is left — no number of logs ever reaches zero.
The formula
log reduction = log10(before / after)
A proportional scale, because killing is proportional
Disinfection removes a fraction of what is present, not a fixed number of organisms, so the natural scale is logarithmic. One log reduction removes 90 % of what was there, two removes 99 %, three 99.9 %. Each further log takes away nine tenths of whatever survived the last one.
Nothing ever reaches zero
No number of logs gives sterility in the absolute sense — the curve approaches zero without arriving. This is why sterilisation standards are written as a sterility assurance level instead: a 12-log reduction leaves a one-in-a-million chance that a single organism survives in a given unit, and that is what "sterile" means in practice.
The numbers that matter in practice
Hand washing achieves roughly 2 to 3 logs. Chlorination of drinking water is specified at 4 logs for viruses. Terminal sterilisation of medical devices targets 12. The gap between 3 and 12 sounds modest and is a factor of a thousand million.
Ratios are expectations, not promises
Every genetic ratio on these pages is a probability distribution, not a guarantee. A 3:1 cross does not produce three dominant offspring for every recessive one in a litter of four — it produces each offspring independently with a three-quarters chance. Small families depart from the expected ratio routinely, and that is the reason Mendel needed thousands of pea plants rather than dozens.