The formula to calculate the retention rate is: Retention Rate (%) = [(Number of Customers at End of Period - Number of New Customers Acquired During Period) / Number of Customers at Start of Period] x 100 For example, if you had 100 customers at the start of the month, acquired 20 new customers, and ended the month with 110 customers, the retention rate would be: [(110 - 20) / 100] x 100 = 90%