Calculate CFU/g from colony count, reciprocal dilution factor, sample preparation and plated volume, or explicitly solve for colonies, dilution or preparation values with log10 results.
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CFU/g Formula
For a homogenate, N = C × D × H / (m × v). N is CFU/g, C is the count or a mean for plates at identical dilution and plated volume, D is the reciprocal of subsequent dilution, H is final/effective homogenate volume in mL, m is original sample mass in g, and v is plated volume in mL. Exclude initial homogenization from D because H/m already accounts for that preparation. For a further 10-6 dilution, D is 1,000,000. With known original mass concentration ρ (g/mL), use N = C × D / (ρ × v).
To solve for the other fields, the same equation is rearranged:
- CFU/g = colony-forming units per gram
- CFU/kg = colony-forming units per kilogram
- C = number of colonies counted
- DF = dilution factor entered as the reciprocal of the dilution
- Mg = sample mass in grams
- Mkg = sample mass in kilograms
Choose the value to solve for. Use either sample mass plus final/effective homogenate volume, or known original mass concentration in g/mL. All calculations include plated volume. Inverse mass and dilution require positive concentration and count; zero/zero does not identify a unique solution. The calculator converts kilograms to grams and CFU/kg to CFU/g consistently.
Common Dilution Factors and Count Ranges
| Dilution | Enter as dilution factor | Scientific notation |
|---|---|---|
| 10-1 | 10 | 1e1 |
| 10-2 | 100 | 1e2 |
| 10-3 | 1,000 | 1e3 |
| 10-6 | 1,000,000 | 1e6 |
| Plate count | Typical interpretation |
|---|---|
| 0 colonies | Report as below the detection limit for the method, not necessarily sterile. |
| 1 to 29 colonies | Low counts may be less statistically reliable; interpret using your laboratory method. |
| 30 to 300 colonies | A USDA/AOAC pour-plate guide, not universal. Current FDA BAM uses 15–300; APHA uses 25–250. Follow your method. |
| More than 300 colonies | Check your method’s upper countability limit; do not enter TNTC as a numeric count. |
CFU/g Examples
Example 1: You count 100 colonies after a further 1:100 dilution of a 25 g sample in a final/effective 250 mL homogenate, plating 0.1 mL.
N = 100 × 100 × 250 / (25 × 0.1) = 1,000,000 CFU/g.
The result is 1 × 106 CFU/g.
Example 2: You know the concentration is 1 × 106 CFU/g, the subsequent reciprocal dilution is 100, the sample mass is 25 g, final/effective homogenate volume is 250 mL, and plated volume is 0.1 mL. To solve for expected colonies:
The expected colony count is 100 colonies.
CFU/g Calculator FAQ
What dilution factor should you enter?
Enter the reciprocal of the dilution. If the plate came from a 10-4 dilution, enter 10,000 or 1e4. Do not enter 0.0001, because the calculator expects the dilution factor, not the dilution fraction.
What is the difference between CFU/g and CFU/kg?
CFU/g expresses colony-forming units per gram. CFU/kg expresses colony-forming units per kilogram. Since 1 kg equals 1,000 g, a value in CFU/kg is 1,000 times the same concentration expressed in CFU/g.
Does this calculation include plated volume?
Yes. Enter the actual plated volume in mL or µL. Do not apply a separate plated-volume correction. In both preparation modes, D describes further dilution relative to the specified original suspension. Zero colonies does not establish sterility; use your method’s reporting limit. The one-colony equivalent is the reciprocal of the original sample mass represented on the plate, not a confidence bound.
