GIFSQ Tools · Module 3 — Food Microbiology & Enumeration · Tool 042 of 500
Plate-count method for liquids. Enter the colonies counted, the volume plated and the total dilution factor to get the count in CFU per millilitre.
Try sample values — fills the form with the worked example below.
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CFU/mL
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Log₁₀ CFU/mL
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Scientific notation
Equation: CFU/mL = C ÷ (V × d) · C = colonies counted, V = volume plated (mL), d = total dilution factor (for example 0.0001 for 10⁻⁴). Log₁₀ CFU/mL = log₁₀(CFU/mL). Detection limit = 1 ÷ (V × d).
Method: Aerobic plate count, FDA BAM Chapter 3 / ISO 4833-1. Countable range 30 to 300 colonies per plate (FDA BAM). For a liquid the sample is plated directly, so there is no initial 1:10 homogenate step — the total dilution is the dilution of the liquid itself.
Assumptions: each colony grew from one viable unit; the liquid was mixed evenly; the dilution series was prepared correctly.
Limitations: counts below 30 or above 300 colonies are estimates, not reportable counts; clumps and chains count as one colony; injured cells may not grow on the medium used.
Rounding: CFU/mL shown as a whole number; log₁₀ to 2 decimal places; intermediates unrounded. In a laboratory report, give CFU/mL to 2 significant figures.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: Aerobic plate count, FDA BAM Chapter 3 / ISO 4833-1. Countable range 30 to 300 colonies per plate (FDA BAM). For a liquid the sample is plated directly, so there is no initial 1:10 homogenate step — the total dilution is the dilution of the liquid itself.
Assumptions: each colony grew from one viable unit; the liquid was mixed evenly; the dilution series was prepared correctly.
Limitations: counts below 30 or above 300 colonies are estimates, not reportable counts; clumps and chains count as one colony; injured cells may not grow on the medium used.
Rounding: CFU/mL shown as a whole number; log₁₀ to 2 decimal places; intermediates unrounded. In a laboratory report, give CFU/mL to 2 significant figures.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: 180 colonies counted on a plate with 0.1 mL plated at a total dilution of 10⁻⁴. CFU/mL = 180 ÷ (0.1 × 0.0001) = 18,000,000 CFU/mL (1.80 × 10⁷ CFU/mL), log₁₀ = 7.26. 180 colonies is inside the countable range.
Enter values above to see results.
Pasteurised milk is commonly specified under 20,000 CFU/mL. A 0.1 mL spread plate at 10⁻⁴ has a detection limit of 100,000 CFU/mL. Tool 042 of 500 — GIFSQ
