Customer lifetime value has intuitive appeal as a marketing concept, because in theory it represents exactly how much each customer is worth in monetary terms, and therefore exactly how much a marketing department should be willing to spend to acquire each customer. In reality, it is difficult to make accurate calculations of customer lifetime value.

The specific calculation depends on the nature of the customer relationship. Customer relationships are often divided into two categories. In contractual or retention situations, customers who do not renew are considered "lost for good". Magazine subscriptions and car insurance are examples of customer retention situations.

The other category is referred to as customer migrations situations. In customer migration situations, a customer who does not buy (in a given period or from a given catalog) is still considered a customer of the firm because she may very well buy at some point in the future. In customer retention situations, the firm knows when the relationship is over. One of the challenges for firms in customer migration situations is that the firm may not know when the relationship is over (as far as the customer is concerned).

Most models to calculate CLTV apply to the contractual or customer retention situation. These models make several simplifying assumptions and often involve the following inputs:

• Churn rate The percentage of customers who end their relationship with a company in a given period. One minus the churn rate is the retention rate. Most models can be written using either churn rate or retention rate. If the model uses only one churn rate, the assumption is that the churn rate is constant across the life of the customer relationship.
• Discount rate The cost of capital used to discount future revenue from a customer. Discounting is an advanced topic that is frequently ignored in customer lifetime value calculations. The current interest rate is sometimes used as a simple (but incorrect) proxy for discount rate.
• Retention cost The amount of money a company has to spend in a given period to retain an existing customer. Retention costs include customer support, billing, promotional incentives, etc.
• Period The unit of time into which a customer relationship is divided for analysis. A year is the most commonly used period. Customer lifetime value is a multi-period calculation, usually stretching 3-7 years into the future. In practice, analysis beyond this point is viewed as too speculative to be reliable. The number of periods used in the calculation is sometimes referred to as the model horizon.
• Periodic Revenue The amount of revenue collected from a customer in the period.
• Profit Margin Profit as a percentage of revenue. Depending on circumstances this may be reflected as a percentage of gross or net profit. For incremental marketing that does not incur any incremental overhead that would be allocated against profit, gross profit margins are acceptable.