The formula for serial dilution is: 

Final concentration = Initial volume / (1/dilution factor)^ number of dilutions

For example, if an experiment began with a concentration of 500 CFU/mL and a 1:10 dilution was carried out 4 times, the calculation would be as such: 

500 * (1/10)^4 = 0.05 CFU/mL 

To calculate the dilution factor, the equation is as follows: 

Dilution factor = initial volume / final volume