420,000,000
CFU/mL
8.62
1,000,000
x
42,000,000
CFU
420,000,000
CFU/mL
8.62
1,000,000
x
42,000,000
CFU
The CFU per mL Calculator determines the concentration of viable bacteria or fungi in a sample based on colony counts from plate culture. Colony-forming units per milliliter (CFU/mL) is the gold standard measurement for quantifying viable microorganisms in clinical, food, environmental, and research microbiology.
Enter the number of colonies counted on your plate, the dilution factor used, and the volume plated to get the original sample concentration.
The formula is straightforward:
CFU/mL = Number of Colonies / (Dilution Factor × Volume Plated in mL)
For example, if you plated 0.1 mL of a 10⁻⁶ dilution and counted 42 colonies: CFU/mL = 42 / (10⁻⁶ × 0.1) = 4.2 × 10⁸. The dilution factor should be expressed as a decimal (e.g., 10⁻⁶ = 0.000001). The log₁₀ value is useful for plotting growth curves and comparing concentrations.
Inputs
Results
42 colonies on a 10⁻⁶ dilution plate (0.1 mL plated) indicates an original concentration of 4.2 × 10⁸ CFU/mL, typical of an overnight E. coli culture.
Inputs
Results
25 colonies from a 10⁻² dilution with 1 mL plated gives 2,500 CFU/mL, which may be relevant for water quality assessments.
The ideal countable range is 25 to 250 colonies per plate (some guidelines use 30-300). Fewer than 25 colonies introduces high statistical error, while more than 250 leads to overcrowding where colonies merge and individual counts become unreliable. Choose the dilution plate that falls in this range.
Express it as a decimal fraction. For a 1:1,000,000 dilution (10⁻⁶), enter 0.000001. For a 1:100 dilution (10⁻²), enter 0.01. If you performed serial 1:10 dilutions, multiply them together: three 1:10 dilutions = 10⁻³ = 0.001.
CFU measures only viable cells capable of forming colonies. A total cell count (e.g., by microscopy or flow cytometry) includes both live and dead cells. CFU/mL will always be equal to or less than total cell count because dead cells and cells that cannot grow under the plating conditions are not counted.
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