Formula for computing a prorated upgrade charge amount in Stripe

proration_formula.md

If upgrading (say from a $10/month to a $20/month plan 3 days into the subscription):

1. Calculate the fraction of the period used to this point under the previous plan, x.

3 days /30 days = 0.1 = x

Note: This is approximate, since months can have different numbers of days, there are leap years, etc.

2. Multiply x by the previous plan's billing amount, amount_prev to get how much to credit the customer back, credit.

credit = x * amount_prev  = 0.1 * 10 = 1

3. Calculate the % of the period not used to this point under the previous plan, y = 1 - x.

1 - x = y 0.10 = 0.90 = y

4. Multiply y by the new plan's billing amount, amount to get how much to charge the customer, charge.

y * amount = charge = 0.90 * 20 = 18

5. Subtract credit from charge for the prorated total:

charge - credit = total = 18 - 1 = 17

So, in this example, you would expect to be charged an additional $17 (approximate) for this first billing cycle under the new plan.