Enter your Colony Count, Dilution Factor, Volume Plated, and Original Sample Volume into this Colony Forming Units (CFU) Calculator to find the CFU/mL concentration of your sample, plus the Log10 CFU/mL and — if you set a Confidence Level — the Lower and Upper confidence intervals around your count.
CFU/mL
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Log10 CFU/mL
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Lower CI
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Upper CI
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A Colony Forming Units (CFU) calculator is an essential microbiology tool that determines the concentration of viable bacterial cells in a culture. It calculates CFU/mL from colony counts on agar plates, accounting for dilution factors and plated volumes to provide accurate bacterial concentration measurements.
CFU counting is a method used in microbiology to estimate the number of viable microorganisms in a sample. It involves plating diluted samples on agar plates, counting the resulting colonies, and using the count to calculate the original concentration of cells in the sample.
One CFU represents one viable cell or cell cluster capable of forming a visible colony under specific growth conditions. While ideally 1 CFU equals 1 cell, in practice it may represent a small cluster of cells that were attached together during plating.
Common dilution factors range from 10^-1 to 10^-8 depending on expected cell density. Start with 10^-4 to 10^-6 for most bacterial cultures, aiming for 30-300 colonies per plate for accurate counting.
Confidence intervals provide a statistical range that likely contains the true CFU value, accounting for natural variation in colony distribution. This helps assess the reliability and precision of your measurements.
Typically 100 µL (0.1 mL) is plated for standard CFU counting. Smaller volumes like 10 µL can be used for highly concentrated samples, while larger volumes up to 1 mL may be used for very dilute samples.
The dilution factor is the inverse of the dilution ratio. For a 10^-5 dilution (1:100,000), the dilution factor is 100,000. For serial dilutions, multiply each step's dilution factor together.