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 require
Ref: Prescott's Microbiology, 11th ed., Chapter 6: CFU/mL Calculation and Dilution Factor Method.