Colony Forming Units Per mL Calculator
Calculate colony forming units per ml with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
CFU/mL = (Colony Count x Dilution Ratio^Steps) / Volume Plated (mL)
The CFU per mL is calculated by multiplying the colony count by the total dilution factor (dilution ratio raised to the power of the number of serial dilution steps), then dividing by the volume of diluted sample that was plated. The total dilution factor converts the count back to the original sample concentration.
Worked Examples
Example 1: Standard Serial Dilution CFU Count
Problem:A bacterial sample undergoes 5 serial 1:10 dilutions. 0.1 mL is plated from the 10^-5 tube. 180 colonies are counted. What is the CFU/mL?
Solution:Total dilution = 10^5 = 100,000\nVolume plated = 0.1 mL\nColonies = 180\nCFU/mL = (180 x 100,000) / 0.1\nCFU/mL = 18,000,000 / 0.1 = 1.80 x 10^8\nLog10 CFU/mL = 8.26
Result:CFU/mL: 1.80 x 10^8 | Log: 8.26 | Colony count in ideal range
Example 2: Two-Fold Dilution for MIC Testing
Problem:Using 1:2 serial dilutions with 8 steps, 95 colonies are counted from 0.1 mL plated. Calculate CFU/mL.
Solution:Total dilution = 2^8 = 256\nCFU/mL = (95 x 256) / 0.1\nCFU/mL = 24,320 / 0.1 = 243,200\nCFU/mL = 2.43 x 10^5\nLog10 = 5.39
Result:CFU/mL: 2.43 x 10^5 | Log: 5.39
Frequently Asked Questions
How do you calculate CFU per mL from serial dilutions?
To calculate CFU/mL from serial dilutions, use the formula: CFU/mL = (Number of colonies x Total dilution factor) / Volume plated (mL). The total dilution factor is the product of all individual dilution steps. For ten-fold serial dilutions, the total dilution at step N is 10^N. For example, after 5 serial 1:10 dilutions, the total dilution factor is 10^5 = 100,000. If you counted 180 colonies from 0.1 mL plated at this dilution: CFU/mL = (180 x 100,000) / 0.1 = 1.8 x 10^8. Always ensure your colony counts fall within the countable range of 25-250 colonies per plate for accurate results.
What is dilution plating and how do I calculate colony counts?
Dilution plating estimates bacterial concentration by serially diluting samples and plating on agar. Count colonies on plates with 30-300 colonies. Calculate: CFU/mL = colonies / (dilution factor * volume plated). For example, 150 colonies on a 10^-5 dilution with 0.1 mL plated = 1.5 x 10^8 CFU/mL.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy