Understanding colony-forming units per milliliter
A plate count estimates the concentration of viable microorganisms able to form visible colonies under the selected culture conditions. The result is conventionally reported as colony-forming units per milliliter (CFU/mL).
This calculator handles a simple single-plate calculation. Validated laboratory methods may combine replicate plates, apply countable-range rules, and use weighted or statistical estimates.
How to use the CFU calculator
- Enter the colony count: Use the count from a plate accepted under your laboratory method.
- Enter the dilution factor: Enter the reciprocal factor, such as 1,000,000 for a 10⁻⁶ dilution.
- Enter plated volume: Provide the actual inoculum volume in milliliters.
- Calculate and document: Report the result with the method, dilution, volume, and appropriate significant figures.
Formula and variables
The dilution factor is the reciprocal of the plated dilution. For example, a 10⁻⁶ dilution has a dilution factor of 10⁶.
CFU/mL = colonies counted × dilution factor ÷ volume plated (mL)- N — Colonies counted
- Observed colonies on the selected plate (colonies)
- D — Dilution factor
- Reciprocal of the sample dilution
- V — Volume plated
- Diluted sample volume placed on the plate (mL)
Single-plate CFU/mL example
A plate from a 10⁻⁶ dilution contains 150 colonies after plating 0.1 mL.
- Colonies
- 150
- Dilution factor
- 1,000,000
- Volume
- 0.1 mL
- CFU/mL = 150 × 1,000,000 ÷ 0.1
- CFU/mL = 1.5 × 10⁹
Result: Estimated concentration: 1.5 × 10⁹ CFU/mL.
The estimate applies to organisms that formed colonies under the specified sampling, medium, incubation, and counting conditions.
Understanding your results
Interpret the estimate in method context
A CFU count is an operational measurement, not necessarily a direct count of individual cells.
- A colony may originate from one cell or a cell cluster.
- Too few colonies increase sampling uncertainty.
- Crowded plates can merge colonies and underestimate concentration.
Assumptions
- The dilution factor is entered as a positive reciprocal factor.
- Colonies are correctly counted and the plated volume is accurate.
- The selected plate meets the laboratory method’s acceptance criteria.
Limitations
- Calculates one plate only and does not pool replicates or confidence intervals.
- Does not account for viability loss, clumping, selective media, incubation conditions, or method-specific corrections.
- Not a substitute for an accredited laboratory procedure or regulatory method.
Common mistakes
- Entering 0.000001 instead of 1,000,000 for a 10⁻⁶ dilution.
- Entering microliters without converting to milliliters.
- Using a confluent or otherwise unacceptable plate.
- Reporting more precision than the count and dilution support.
Practical use cases
Teaching and calculation checks
Verify the arithmetic and units in a basic dilution-plating exercise.
Laboratory screening
Estimate concentration from a qualifying single plate before applying the full reporting method.
Frequently asked questions
What dilution factor should I enter?
Enter the reciprocal of the dilution plated: 10⁴ for 10⁻⁴, 10⁶ for 10⁻⁶, and so on.
What colony range is countable?
Use the range specified by your validated method. The FDA BAM aerobic plate-count method currently identifies 15–300 colonies per plate as suitable, while other methods can differ.
Does one colony always equal one cell?
No. A colony can arise from an individual viable cell or from a cluster, which is why results are expressed as colony-forming units.
Sources and review
- BAM Chapter 3: Aerobic Plate Count — U.S. Food and Drug Administration. Accessed 2026-07-13.
Reviewed 2026-07-13.