Formula used
Known concentration: CFU/g = CFU/mL × effective mL/g × additional dilution factor.
Direct plate count: CFU/g = mean colonies ÷ (plated mL × decimal dilution) × effective mL/g.
Reverse conversion: CFU/mL = CFU/g ÷ (effective mL/g × additional dilution factor).
The effective ratio links liquid concentration with solid sample mass. It must match the laboratory preparation method. Never apply the same dilution correction twice.
How to use this calculator
- Select the required calculation mode.
- Enter the concentration or replicate plate counts.
- Choose the sample-preparation entry method.
- Enter the sample mass and suspension information.
- Review dilution values and plated volume carefully.
- Select precision and result formatting.
- Calculate and inspect every displayed step.
- Export the result for your laboratory record.
Example data
| CFU/mL | Effective ratio | Additional factor | CFU/g |
|---|---|---|---|
| 25,000 | 10 mL/g | 1 | 250,000 |
| 4.2 × 10⁵ | 10 mL/g | 1 | 4.2 × 10⁶ |
| 8,500 | 5 mL/g | 100 | 4.25 × 10⁶ |
| 1.3 × 10⁴ | 20 mL/g | 10 | 2.6 × 10⁶ |
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Frequently asked questions
Can CFU/mL convert directly to CFU/g?
No fixed conversion exists without sample-preparation information. The effective suspension ratio connects liquid volume with sample mass.
What does mL/g mean?
It describes effective suspension milliliters per original sample gram. A 1:10 preparation commonly represents ten effective milliliters per gram.
Should dilution be entered twice?
No. Enter an additional factor only when the concentration remains uncorrected. Duplicate corrections produce an inflated final result.
Which dilution formats are accepted?
The plate mode accepts decimal, factor, ratio, and power notation. Select the matching format before calculation.
How are replicate plates combined?
The calculator averages all accepted replicate counts. It also reports spread, standard deviation, and coefficient of variation.
Can invalid plates be excluded?
Yes. Enable countable-range filtering to remove outside values. Always follow the applicable laboratory method.
What happens when no colonies appear?
The result should use a detection-limit statement. Zero observed colonies do not prove complete microbial absence.
Why is log10 CFU/g useful?
Logarithmic values simplify comparisons across large concentration ranges. The logarithm is unavailable for an exact zero.
Can microliters be entered?
Yes. The calculator converts microliters into milliliters automatically. Confirm the selected unit before submitting.
Does this calculator replace a laboratory method?
No. It performs arithmetic using entered assumptions. Method-specific acceptance rules still control final reporting.
Why might my result look extreme?
Incorrect units or duplicated dilution factors often cause extremes. Review every displayed conversion and calculation step.
Laboratory-use notice
This tool supports educational and routine calculation work. It does not validate sampling quality or laboratory compliance. Confirm every result against your approved method.