Free · Laboratory calculations

Log Reduction, Surviving Count & Percent Reduction

Convert positive before-and-after counts into decimal-log and percentage reductions, including inverse count calculations.

Calculator inputs

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Enter the known values in the units shown. Results update as you type.
Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
Use the worked example to check the method. Reset restores the starting fields.

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

A decrease from 10,000,000 to 100,000 on the same measurement basis is a 2-log reduction and a 99% reduction.

Log reduction = log₁₀(initial / final); percent reduction = 100 × (1 − final / initial)

Enter these known values and leave the other values blank.

Initial and final values must use the same sampling or concentration basis. The ×10 selectors scale each entered mantissa independently.
This finite-log tool requires positive counts and a final count no greater than the initial count. No change gives zero log reduction.
A reported zero or nondetect does not establish an infinite reduction. Use a justified positive detection limit to calculate a lower bound, and identify it as a bound.
Arithmetic reduction alone does not demonstrate sterility, treatment efficacy or compliance with a required test protocol.

The defaults are ×10⁷ and ×10⁵. Entering 1 in each therefore means 10,000,000 initially and 100,000 finally.

No. A 1-log reduction leaves one tenth and removes 90%; a 2-log reduction leaves one hundredth and removes 99%.

Bookify requires a positive final count for a finite log reduction and restricts the percentage reduction to [0, 100%).