CFU calculator
Colony forming units per mL from plate counts, dilution and volume plated.
Colony forming units per mL from plate counts, dilution and volume plated.
A colony forming unit is one viable cell, or clump of cells, that grows into a visible colony on an agar plate. Counting colonies after plating a known volume of a known dilution gives the concentration of living, culturable microbes in the original sample, reported as CFU/mL for liquids or CFU/g for solids. Unlike optical density or a microscope count, CFU only counts cells that can still divide.
The dilution factor is the reciprocal of the dilution. A 10⁻⁵ plate has a dilution factor of 100,000. The volume plated is usually 0.1 mL (100 µL) for spread plates or 1 mL for pour plates.
Worked example. 156 colonies on the 10⁻⁵ plate, 100 µL spread: 156 × 100,000 ÷ 0.1 = 1.56 × 10⁸ CFU/mL, or 8.19 log CFU/mL. With replicate plates the calculator averages the CFU/mL of every countable plate and reports the standard deviation.
Only plates with 30 to 300 colonies give a reliable count. Fewer than 30 colonies is too small a sample to be statistically sound, and more than 300 colonies crowd and merge, so the count is too low. Plates outside the range are flagged here and left out of the mean when at least one countable plate exists. Some laboratories use 25–250; the principle is the same. When every plate is overgrown, report the result as "too numerous to count" and plate a higher dilution next time.
Most samples need a 10-fold serial dilution before plating. Each step transfers 1 mL into 9 mL of diluent, and the factors multiply: three steps give 10⁻³, five give 10⁻⁵. Enter the exponent of the plate you counted, not the single-step dilution. If a solid sample was first homogenised, for example 1 g in 9 mL, include that 1:10 step in the exponent to get CFU/g.
Multiplying CFU/mL by the original sample volume gives the total number of viable cells, which is what you need for inoculum sizes, transformation efficiency and survival curves. Enter the volume above the result to see it.
Multiply the number of colonies by the dilution factor and divide by the volume plated in mL. For 156 colonies on a 10^-5 plate with 0.1 mL plated: 156 × 100,000 / 0.1 = 1.56 × 10^8 CFU/mL.
Plates with 30 to 300 colonies. Below 30 the count is statistically unreliable; above 300 colonies overlap and are undercounted. Some labs use 25 to 250. The calculator flags plates outside the range and averages the countable ones.
Report the result as greater than 300 × dilution factor / volume plated, and repeat the plating with further 10-fold dilutions.
Include the initial homogenisation step in the dilution factor. If 1 g of sample was suspended in 9 mL of diluent, that is a 1:10 dilution, so multiply the CFU/mL of the suspension by 10 to get CFU/g.
The base-10 logarithm of the CFU/mL value. 1.56 × 10^8 CFU/mL is 8.19 log CFU/mL. Microbial counts are usually compared and averaged on the log scale because they span many orders of magnitude.