Calculator inputs
Formula used
The calculator first averages the available replicate colony counts. It then subtracts the blank correction, converts the chosen dilution from 10^-n into its reciprocal factor, adjusts for plated volume, and optionally corrects for recovery efficiency and any extra preparation scaling.
The result is reported as CFU/mL or CFU/g. Replicate standard deviation, coefficient of variation, and an approximate 95% confidence interval help you judge plate consistency.
How to use this calculator
- Enter up to three replicate colony counts from the same dilution level.
- Add any blank correction if your control plate produced colonies.
- Enter the dilution exponent for the plated sample, such as 5 for 10^-5.
- Enter plated volume and choose the proper unit.
- Set recovery efficiency below 100% only when you need correction.
- Use preparation factor for upstream concentration or resuspension adjustments.
- Review the concentration result, variability metrics, quality notes, and graph.
- Export the current result to CSV or PDF for records.
Example data table
| Sample | Replicates | Blank | Dilution | Plated volume | Estimated concentration |
|---|---|---|---|---|---|
| Wash water A | 124, 118, 121 | 2 | 10^-5 | 0.1 mL | 1.19e+08 CFU/mL |
| Broth culture B | 48, 52, 50 | 1 | 10^-4 | 0.1 mL | 4.90e+06 CFU/mL |
| Filter rinse C | 286, 279, 291 | 3 | 10^-6 | 0.2 mL | 1.41e+09 CFU/mL |
Frequently asked questions
1. What does this calculator estimate?
It estimates bacterial concentration from plate counts, dilution, and plated volume. The output is expressed as CFU per milliliter or CFU per gram, depending on your selected reporting basis.
2. Why are replicate counts helpful?
Replicates improve reliability. They reduce the impact of random plating variation and let you inspect standard deviation, coefficient of variation, and approximate confidence limits before reporting a final concentration.
3. What is the blank correction field for?
Use blank correction when a control or blank plate shows background colonies. The calculator subtracts that value from the average plate count before computing the final concentration.
4. How should I enter dilution?
Enter the exponent from your plated dilution. For a 10^-5 dilution, enter 5. The calculator converts that notation into the reciprocal dilution factor needed for back-calculation.
5. When should recovery efficiency be less than 100%?
Use a lower recovery value only when validated recovery losses are known. This correction compensates for incomplete capture, extraction, or transfer during the test workflow.
6. What does the preferred countable range mean?
It helps flag whether your average plate count falls inside the range you trust most. Counts far below or above that range may reduce precision or indicate overcrowded plates.
7. Why can the result become zero?
A zero result appears when the blank correction equals or exceeds the average colony count. In that case, the adjusted count is clipped at zero to avoid negative concentrations.
8. What do the CSV and PDF exports include?
The exports include the sample name, calculation settings, summary metrics, replicate values, and quality notes. They are designed for lab records, review, and quick sharing.