GIFSQ Tools · Module 3 — Food Microbiology & Enumeration · Tool 044 of 500
Enter the colony counts at up to three dilutions. The tool picks the countable plate (30 to 300 colonies, FDA BAM) and calculates the result from it.
Try sample values — fills the form with the worked example below.
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Selected plate
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Colonies on selected plate
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CFU/g
Equation: CFU/g = C ÷ (V × 10⁻ᵉ) · C = colonies on the selected plate, V = volume plated (mL), e = dilution exponent of the selected plate.
Method: Plate selection by the FDA BAM Chapter 3 countable range, 30 to 300 colonies. The least dilute plate inside the range is used. ISO 7218 uses a 10 to 150 range instead — this tool applies the FDA BAM range, stated here so the choice is explicit.
Assumptions: plates at different dilutions come from the same homogenate; each colony grew from one viable unit; the dilution series was prepared correctly.
Limitations: if no plate is countable the tool reports no result rather than guessing from an uncountable plate; two plates in range are not averaged — the least dilute countable plate is used, per FDA BAM practice.
Rounding: CFU/g shown as a whole number; intermediates unrounded. In a laboratory report, give CFU/g to 2 significant figures.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: Plate selection by the FDA BAM Chapter 3 countable range, 30 to 300 colonies. The least dilute plate inside the range is used. ISO 7218 uses a 10 to 150 range instead — this tool applies the FDA BAM range, stated here so the choice is explicit.
Assumptions: plates at different dilutions come from the same homogenate; each colony grew from one viable unit; the dilution series was prepared correctly.
Limitations: if no plate is countable the tool reports no result rather than guessing from an uncountable plate; two plates in range are not averaged — the least dilute countable plate is used, per FDA BAM practice.
Rounding: CFU/g shown as a whole number; intermediates unrounded. In a laboratory report, give CFU/g to 2 significant figures.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: Plates at 10⁻², 10⁻³ and 10⁻⁴ give 420, 68 and 7 colonies with 1 mL plated. 420 is too many and 7 is too few, so the 10⁻³ plate is selected: CFU/g = 68 ÷ (1 × 0.001) = 68,000 CFU/g.
Enter values above to see results.
FDA BAM countable range 30 to 300 colonies; ISO 7218 uses 10 to 150. This tool applies the FDA BAM range. A plate with fewer than 30 colonies or more than 300 is an estimate, not a reportable count. Tool 044 of 500 — GIFSQ
