The CFU/mL of a bacterial culture is calculated using the formula:
Colony forming units per milliliter provides standard measure of viable culturable bacteria assuming each colony derives from single viable unit capable of growth on agar. To achieve statistically reliable counts, serial tenfold dilutions are prepared in diluent and fixed aliquot such as 0.1 mL is spread onto nutrient agar via spread plate using sterile spreader or mixed into molten agar via pour plate. After appropriate incubation colonies are counted; ideal range 30 to 300 avoids overcrowding and sampling error. Because only diluted fraction was plated, conversion to original density requires normalization. Correct formula CFU per mL equals number colonies counted multiplied by total dilution factor reciprocal of dilution plated for example ten to six for ten to minus six dilution divided by volume plated in milliliters. Illustration 50 colonies on ten to minus five dilution plated 0.1 mL yields 50 times ten to five over 0.1 equals five times ten to seven CFU per mL. Omitting volume division underestimates by tenfold, dividing rather than multiplying by dilution factor inverts scaling leading to erroneous infectious dose, growth rate and regulatory compliance assessment in food and pharmaceutical microbiology quality control.
Ref: Prescott's Microbiology, 11th ed., Chapter 6: CFU/mL Calculation and Dilution Factor Method.