How It Works
This calculator works in 4 steps.
Enter the colony count
Type the count per plate, or replicate counts to average.
Enter the plated dilution
Type the dilution as a fraction or exponent, such as 10⁻⁶.
Enter the plated volume
Type the spread or pour volume, such as 0.1 mL (100 µL).
Read CFU/mL
The calculator returns CFU/mL in scientific notation and flags counts outside 30 to 300.
The Formula
The formula divides the colony count by the plated dilution and the plated volume.
CFU/mL = colonies / (d × V)CFU/mL = colonies × DF / VCFU/g = CFU/mL × (V_diluent + m) / m- d:
- Plated dilution as a fraction (e.g., 10⁻⁶)
- DF:
- Reciprocal of d
- V:
- Volume plated (mL)
- m:
- Sample mass (g)
152 colonies from 0.1 mL of the 10⁻⁶ dilution give 152 / (10⁻⁶ × 0.1) = 1.52 × 10⁹ CFU/mL.
Conversion Table
The conversion table back-calculates CFU/mL for 6 plate counts.
| Colonies | Dilution plated | Volume plated | Original sample |
|---|---|---|---|
| 152 | 10⁻⁶ | 0.1 mL (100 µL) | 1.52 × 10⁹ CFU/mL |
| 48 | 10⁻⁵ | 0.1 mL (100 µL) | 4.8 × 10⁷ CFU/mL |
| 230 | 10⁻⁴ | 1 mL (1,000 µL) | 2.3 × 10⁶ CFU/mL |
| 35 | 10⁻⁷ | 0.1 mL (100 µL) | 3.5 × 10⁹ CFU/mL |
| 87 | 10⁻³ | 0.05 mL (50 µL) | 1.74 × 10⁶ CFU/mL |
| 300 | 10⁻⁶ | 0.1 mL (100 µL) | 3 × 10⁹ CFU/mL |
What Is the CFU Dilution Correction Calculator?
The CFU Dilution Correction Calculator is a tool that back-calculates colony-forming units per mL (CFU/mL) in an original sample from a plate count, the plated dilution, and the plated volume. CFU/mL equals colonies divided by the product of the dilution and the volume plated.
Plate counts are reliable in the 30 to 300 colony range for a 90 mm plate. The calculator warns when a count falls outside the user-set range and averages replicate plates. For solids, the calculator reports CFU per gram (CFU/g) based on the initial sample weight and diluent.
Common Uses
There are 5 main uses of this calculator.
Culture titers
Microbiologists find the viable count of a culture.
Food aerobic plate counts
Food labs report CFU/g for products.
Water testing
Utilities report heterotrophic plate counts.
Probiotic QC
Producers verify CFU per dose on labels.
Disinfectant efficacy
Labs compute log reduction from before and after counts.