How the calculator works
It uses daily proration: each plan's price is divided by the number of days in the billing period to get a daily rate, and the remaining days are counted from the change date to the next renewal date.
- Credit for the unused part of the old plan = old daily rate × remaining days.
- Charge for the rest of the period on the new plan = new daily rate × remaining days.
- Net amount = charge − credit. A negative result means the customer is owed a credit (a downgrade).
Watch out: Billing systems count days differently. Dividing by the actual days in the period and dividing by a fixed 30 give different daily rates, and some systems round each step or count the change day differently. Use the method your contract and billing system use, and state it in your terms.
The guide to the billing period walks through proration with a worked example.