CFU Calculator
🔢 CFU Results
CFU Concentration
—
Average Colony Count
—
colonies on plate
Dilution Factor
—
applied
Scientific Notation
—
CFU/mL
Log₁₀ CFU
—
log CFU/mL
How to Use the CFU Calculator
Enter the number of colonies counted on your agar plate(s), the dilution factor used, and the volume plated. If you counted two replicate plates, enter both counts for an averaged result.
- Colony Count: Number of visible colonies on the plate. Countable range is 30–300 CFU per plate.
- Dilution Factor: The total dilution applied before plating (e.g. 10⁻⁴ = 0.0001). Use the preset dropdown to select quickly.
- Volume Plated: Volume spread or poured onto the plate in mL (typically 0.1 mL or 1.0 mL).
CFU Calculation Formula
The standard formula for calculating CFU/mL from plate counts is:
CFU/mL = Average Colony Count / (Dilution Factor × Volume Plated)
Example:
Colonies = 148, Dilution = 10⁻⁴, Volume = 0.1 mL
CFU/mL = 148 / (0.0001 × 0.1) = 1.48 × 10⁷ CFU/mL
Example:
Colonies = 148, Dilution = 10⁻⁴, Volume = 0.1 mL
CFU/mL = 148 / (0.0001 × 0.1) = 1.48 × 10⁷ CFU/mL
Plates with 30–300 colonies are considered statistically reliable (TNTC = Too Numerous To Count; TFTC = Too Few To Count).
Countable Colony Range
- 30–300 colonies: Ideal countable range — statistically valid result.
- < 30 colonies (TFTC): Too few to count — use a less diluted sample.
- > 300 colonies (TNTC): Too numerous to count — use a more diluted sample.
- Duplicate plates: Average both counts before calculating for better accuracy.