CFU/mL Plating Calculator
Back-calculate colony-forming units per millilitre from a plate count, the volume plated and the dilution factor.
How it works
CFU/mL = colonies ÷ (volume plated in mL × dilution factor)
A viable count works backward from the countable plate to the original culture. Divide the colony number by the volume you spread to get colonies per mL on that dilution, then divide by the dilution factor to undo the serial dilution. For statistical reliability, only plates with about 30–300 colonies are counted — fewer is too noisy and more is too crowded to resolve individual colonies.
Worked example
Counting 150 colonies from 0.1 mL of a 10⁻⁶ dilution gives 150 ÷ (0.1 × 10⁻⁶) = 150 ÷ 10⁻⁷ = 1.5 × 10⁹ CFU/mL in the undiluted culture.
Sources: Colony-forming unit — Wikipedia
Frequently asked questions
Why only count plates with 30–300 colonies?
Below about 30, random variation makes the count unreliable; above about 300, colonies merge and are undercounted. The 30–300 window balances statistical precision against crowding.
What is the difference between CFU/mL and cells/mL?
CFU counts only viable cells that can grow into a colony, and clumped cells may form one colony. An OD600 or chamber count reports total cells including dead ones, so it is usually higher.
How do I enter the dilution factor?
Use the fraction of the original that reached the plate — 1 × 10⁻⁶ for a 10⁻⁶ dilution. The calculator divides by it to scale the count back to the neat culture.
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This calculator is for educational purposes. Double-check important results before acting on them.