What formula is used to determine a company's customer retention rate?

1. (The number of new customers during the period + the number of customers at the end of that
period)/ the number of customers at the start of the period x 100
2 . (The number of new customers during the period - the number of customers at the end of that
period)/ the number of customers at the start of the period / 100
3. (The number of customers at the end of the period - the number of new customers acquired
during the period)/ the number of customers at the start of the period x 100
4. (The number of new customers during the period - the number of customers at the end of that
period) x the number of customers at the start of the period x 100