Enter the number of colonies, dilution factor, and plated volume into the calculator to determine the colony-forming units per milliliter (CFU/g) of a sample.

CFU/g Calculator

Enter any three values to solve for the fourth

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CFU/g Formula

The following formula is used to calculate the CFU/mL.

CFU/mL = (Number of Colonies × Dilution Factor (reciprocal)) / Volume Plated (mL)

Variables:

  • CFU/mL is the colony-forming units per milliliter
  • Number of Colonies is the count of colonies observed on a plate
  • Dilution Factor (reciprocal) is the inverse of the plated dilution (e.g., for a 10-6 dilution, enter 106)
  • Volume Plated is the volume of the diluted sample plated (in milliliters)

To calculate CFU/mL, multiply the number of colonies by the reciprocal dilution factor, then divide by the volume plated. This estimates the concentration of viable bacteria or fungi in the original liquid sample or suspension. (For solid samples reported as CFU/g, you must also account for how many grams of sample are represented per mL of the initial suspension/homogenate.)

What is CFU/mL?

CFU/mL stands for colony-forming units per milliliter and is a measure of the viable bacterial or fungal concentration in a liquid sample or suspension. It is commonly used in microbiology to estimate the number of viable microorganisms in a sample and is important for assessing the safety and quality of liquids and suspensions (and, after appropriate preparation, some homogenized samples).

How to Calculate CFU/mL?

The following steps outline how to calculate the CFU/mL.

  1. First, count the number of colonies that grew on the agar plate.
  2. Next, determine the dilution factor used (use the reciprocal of the plated dilution, e.g., 106 for a 10-6 dilution).
  3. Next, measure the volume that was plated (in milliliters).
  4. Use the formula CFU/mL = (Number of Colonies × Dilution Factor) / Volume Plated (mL) to calculate the CFU/mL.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Number of Colonies = 250

Dilution Factor (reciprocal) = 1 × 103

Volume Plated (mL) = 1

CFU/mL = (250 × 1 × 103) / 1 mL = 2.5 × 105 CFU/mL