Stripe Billing: How Proration Actually Works

Proration confuses more billing implementations than any other single concept — mostly because teams reason about it in terms of "days used" rather than what Stripe actually does.

The Actual Model

When a subscription changes mid-cycle, Stripe credits the unused time on the old price and charges for the remaining time on the new price — both calculated to the second, added as line items on the next invoice by default.

The Common Mistake

Teams often try to calculate proration manually and pass a fixed amount, fighting against Stripe's built-in logic instead of configuring `proration_behavior` correctly for their use case.