CFU Calculator
Calculate cfucalculator with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.
Formula
CFU/mL = Average Colony Count / (Dilution Factor x Volume Plated in mL)
Colony Forming Units per milliliter is calculated by dividing the average number of colonies counted on replicate plates by the product of the dilution factor (e.g., 10^-6) and the volume of diluted sample plated (typically 0.1 mL for spread plates or 1.0 mL for pour plates). The dilution factor accounts for the serial dilutions performed before plating.
Frequently Asked Questions
What does CFU stand for and what does it measure?
CFU stands for Colony Forming Unit, which is a measure of viable (living) bacterial or fungal cells in a sample. Unlike direct cell counting methods, CFU measurement only counts cells that are able to multiply and form visible colonies on agar plates. One CFU may originate from a single cell or from a clump of cells, which is why the term 'colony forming unit' is used rather than 'cell.' CFU/mL is the standard unit for expressing the concentration of microorganisms in liquid samples. It is the gold standard for quantifying viable microbes because only living organisms capable of reproduction will form colonies.
What is the countable range for CFU plates?
The generally accepted countable range for standard pour plates and spread plates is 25 to 250 colonies per plate (FDA BAM method). Some references use 30 to 300 colonies (ISO methods). Plates with fewer than 25 colonies have high statistical variability and results are reported as 'estimated' or 'TFTC' (too few to count). Plates with more than 250 colonies are reported as 'TNTC' (too numerous to count) because colonies may merge, compete for nutrients, or be obscured. For membrane filtration methods, the countable range is typically 20-80 colonies. Always plate multiple dilutions to ensure at least one falls within the countable range.
How do you perform serial dilutions for CFU counting?
Serial dilutions are performed by transferring a fixed volume of sample into a fixed volume of diluent in a stepwise fashion. The most common scheme is 1:10 (ten-fold) serial dilutions. Add 1 mL of sample to 9 mL of sterile diluent (or 0.1 mL to 0.9 mL), vortex thoroughly, then transfer 1 mL of this dilution to another 9 mL tube, and repeat. Each step reduces concentration by 10-fold. Label tubes clearly as 10^-1, 10^-2, 10^-3, etc. Use a fresh pipette tip for each transfer to prevent carryover. Plate 0.1 mL (spread plate) or 1.0 mL (pour plate) from each dilution. The dilution with 25-250 colonies is used for calculation.
How do you calculate CFU/mL from multiple plates?
When multiple plates are used (which is recommended for accuracy), average the colony counts from replicate plates at the same dilution before calculating CFU/mL. The formula is: CFU/mL = Average colony count / (Dilution factor x Volume plated in mL). For example, if duplicate plates at 10^-5 dilution show 142 and 158 colonies with 0.1 mL plated: CFU/mL = ((142+158)/2) / (10^-5 x 0.1) = 150 / 10^-6 = 1.5 x 10^8 CFU/mL. If plates at different dilutions are in the countable range, you can average the calculated CFU/mL values from each dilution. The coefficient of variation between replicates should ideally be less than 15%.
Is CFU Calculator free to use?
Yes, completely free with no sign-up required. All calculators on NovaCalculator are free to use without registration, subscription, or payment.
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