Enter Laboratory Inputs
Example Data Table
| Sample | Plate Counts | Dilution Plated | Volume | Average Count | Estimated CFU/mL |
|---|---|---|---|---|---|
| Yogurt Batch A | 145, 152, 149 | 1/100000 | 0.1 mL | 148.67 | 1.4867E+08 |
| Water Sample B | 42, 39, 44 | 1/1000 | 1.0 mL | 41.67 | 4.1667E+04 |
| Soil Slurry C | 310, 295, 287 | 1/1000000 | 0.1 mL | 291.00 | 2.9100E+09 |
Formula Used
CFU/mL = Average colony count ÷ (Dilution plated × Volume plated in mL)
Where:
- Average colony count is the mean of usable replicate plates.
- Dilution plated is the decimal fraction of the original sample on the plate.
- Volume plated in mL converts the inoculated amount into milliliters.
Example: If the average count is 148.67, the plated dilution is 1/100000, and the plated volume is 0.1 mL, then CFU/mL = 148.67 ÷ (0.00001 × 0.1) = 1.4867 × 108.
How to Use This Calculator
- Enter a sample name, analyst, and plating method.
- Fill in one to four colony counts from replicate plates.
- Enter the plated dilution as a numerator and denominator.
- Enter plated volume, then choose mL or uL.
- Set the acceptable colony range for countable plates.
- Submit the form to view CFU/mL, log count, precision, and the graph.
- Download the result summary in CSV or PDF format for reporting.
Frequently Asked Questions
1. What does CFU per mL measure?
It estimates the concentration of viable microorganisms in one milliliter of the original sample. Each colony is assumed to arise from one viable cell or a clumped group of cells.
2. Why are some plates excluded from the calculation?
Plates outside the selected countable range may be too sparse or too crowded for reliable counting. The calculator prefers countable plates first and uses all plates only when no countable plate exists.
3. What countable range should I use?
Many laboratories use 30 to 300 colonies for standard plate counts, but the correct range depends on the method, medium, and SOP. Adjust the limits to match your protocol.
4. Should I enter the total dilution or the dilution factor?
Enter the plated dilution as a fraction of the original sample. For a 10-5 plated dilution, enter numerator 1 and denominator 100000.
5. Can I use microliters instead of milliliters?
Yes. Choose uL in the volume unit field. The calculator automatically converts the plated volume into milliliters before computing the microbial concentration.
6. What does the CV percentage mean?
CV is the coefficient of variation. It shows how consistent the replicate plate counts are. Lower CV values generally indicate better plating precision and repeatability.
7. Why is log10(CFU/mL) reported?
Log values are easier to compare across large microbial ranges. They are commonly used in microbiology reports, validation work, and trend analysis.
8. Can this calculator replace laboratory SOPs?
No. It is a calculation and reporting aid. Final interpretation should still follow your validated method, incubation conditions, colony morphology review, and laboratory documentation requirements.