Most recent answer. Use the formula: [Number of colonies counted] × 10 × [how many times the sample must be multiplied to get to the original concentration: for example, 105] = Number of colony forming units (CFU) per milliliter of starting culture. This is the bacterial growth in your petri dishes.Click to see full answer. Besides, how do you count colonies in a petri dish?The primary trick in counting colonies is to count each colony dot once. One approach is to set the Petri dish on a grid background and count the colonies in each grid cell, moving in a methodical pattern through all of the cells. Marking counted colonies on the back of the Petri dish can also be a helpful approach.Subsequently, question is, how do you calculate CFU in serial dilution? To find out the number of CFU/ ml in the original sample, the number of colony forming units on the countable plate is multiplied by 1/FDF. This takes into account all of the dilution of the original sample. 200 CFU x 1/1/4000 = 200 CFU x 4000 = 800000 CFU/ml = 8 x 10. CFU/ml in the original sample. Similarly one may ask, how is viable count calculated? To calculate the number of viable cells/mL: The final value is the number of viable cells/mL in the original cell suspension. Example: If the cell counts for each of the 16 squares were 50, 40, 45, 52, the average cell count would be: (50 + 40 + 45 +52) ÷ 4 = 46.75.How do you calculate heterotrophic count?To compute the heterotrophic plate count for pour plate, spread plate, and membrane filter methods (CFU/mL), divide either the total or average number of colonies per plate by the sample volume. (Use the average number of colonies if duplicate plates of the same dilution.)