CFU Calculator (Colony-Forming Units per mL)
Calculate colony-forming units per mL from the colonies counted on a plate, the dilution plated and the volume plated, with a check against the countable range.
Formula
- colonies counted on the plate
- fold dilution of the tube that was plated (10⁵ for a 10⁻⁵ dilution)
- volume spread or poured on the plate, in mL
How it works
Each colony on a plate is taken to have grown from one viable cell or clump, a colony-forming unit. Dividing the colony count by the volume plated gives the concentration in the diluted tube, and multiplying by the dilution factor scales that back to the original sample.
A count is only reliable within a countable range. The FDA's Bacteriological Analytical Manual uses 25 to 250 colonies per plate; below it the count is dominated by chance, and above it colonies crowd and merge.
Worked example
142 colonies grow from 0.1 mL of a 10⁻⁵ dilution.
- CFU/mL = 142 × 10⁵ / 0.1 mL = 1.42 × 10⁸.
The original sample contains 1.42 × 10⁸ CFU/mL (log₁₀ 8.15). The count is within the countable range.
These are the values the calculator opens with, so you can check its output against this example.
Assumptions
- Each colony arose from a single colony-forming unit, which may be one cell or a clump or chain of cells.
- The organism grows on the medium and under the conditions used. Cells that are viable but do not form colonies are not counted.
Common mistakes
- Entering the dilution as 10⁻⁵ rather than as the factor 10⁵, or the reverse.
- Forgetting the volume plated. Plating 0.1 mL adds a further factor of 10 compared with 1 mL.
- Using a plate outside the countable range when a better plate from the same series is available.
Related tools
Related equipment
Service documentation, failure modes and parts for the instruments this calculation is used with.