Retention metrics are a tool to measure how long the customers stick around and engage with the product. It can be measured by comparing the number of customers at the beginning with the number at the end. To get the value from the formula, there is a need to understand two key phrases,   "Continue to use" means customers have been using the product. "Period " means what period a retention rate refers to, i.e., weekly, monthly, or yearly. Formula: Retention rate = All customers at the end of the period – New customers acquired during the period / All customers at the beginning of the period.