> For the complete documentation index, see [llms.txt](https://guidelines.nhmrc.gov.au/australian-drinking-water-guidelines/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://guidelines.nhmrc.gov.au/australian-drinking-water-guidelines/appendices/appendix-3/a3.12-understanding-log10-reductions.md).

# A3.12 Understanding log₁₀ reductions

To keep large numbers manageable, these are often expressed using scientific notation. When calculating the magnitude of change in cell numbers, a logarithmic scale is often used. As the series demonstrates, if the log reduction is a whole integer, the numerical value equals the number of nines in the percent reduction figure.

* 1 log reduction = 90%
* 2 log reduction = 99%
* 3 log reduction = 99.9%
* 4 log reduction = 99.99%
* 5 log reduction = 99.999%

Similarly, the relationship can be expressed in terms of showing the number of organisms eliminated during the treatment process:

* 1 log reduction = 10 times smaller
* 2 log reduction = 100 times smaller
* 3 log reduction = 1 000 times smaller
* 4 log reduction = 10 000 times smaller
* 5 log reduction = 100 000 times smaller