Free tool
Find the total dilution factor across a series of dilution steps, then turn a plate count into CFU/mL of the original sample. It also checks whether your plate is in the countable range.
Built for MicrobiologyThe factor is (sample + diluent) / sample, so 1 mL into 9 mL of diluent is a factor of 10.
Whole number. Factors multiply, so five 1:10 steps give 10^5.
Enter the known values.
Dilution math is a favorite exam problem because it is so easy to drop a factor of ten. Encodr drills the CFU/mL formula until it is automatic.
Get started freeThe dilution factor is the reciprocal of the dilution: a 1:10,000 dilution has a dilution factor of 10,000. In a serial dilution each tube is made from the one before, so the factors multiply. Five 1:10 steps give 10 x 10 x 10 x 10 x 10 = 10^5.
To get the concentration of the original sample, use CFU/mL = (colonies counted x dilution factor) / volume
plated (mL). Count only plates with 30 to 300 colonies. Fewer than 30 gives a high statistical counting error,
and more than 300 is too dense to count individually.
Worked example: you plate 0.1 mL from a 10^-5 dilution and count 145 colonies. CFU/mL = (145 x 100,000) / 0.1 = 1.45 x 10^8 CFU/mL of the original sample. The step-by-step version is in serial dilutions and CFU calculations, and the whole course is in the free Microbiology deck. For a single dilution of a stock solution, try the dilution calculator.
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