About this tool
The CFU/mL Colony Count Calculator is a free, in-browser microbiology tool that turns a plate colony count into a concentration of colony-forming units per millilitre. Enter the number of colonies you counted, the dilution that was plated, and the volume spread, and it reports the CFU/mL of the original sample plus its log₁₀ value. All arithmetic runs locally in your browser; nothing you type leaves your device.
The formula is CFU/mL = colonies ÷ (dilution factor × volume plated). The dilution can be entered as a fraction such as 1e-5 or in exponent form like 10^-5, or as the fold value 10^5 — the tool takes the reciprocal of the fraction (1/dilution) automatically. For example, 150 colonies at a 10⁻⁵ dilution on 0.1 mL gives 150 × 10⁵ ÷ 0.1 = 1.5 × 10⁸ CFU/mL.
Accurate plate counts rely on landing in the statistically valid window, so the tool flags whether your colony count is inside the standard countable range of 30–300. You can also enter a target colony count and it back-calculates the dilution fold that would put roughly that many colonies on the next plate.