Free tool
Proration Calculator
A subscriber changes plans mid-cycle — what do they owe today? Enter both prices and the two dates to get the credit, the prorated charge, and the next invoice, with the arithmetic shown line by line.
How proration is calculated
Day-based proration values each day of the billing cycle equally. When a plan changes mid-cycle, the subscriber is credited the unused share of the old plan and charged the same share of the new one:
unused credit = old price × days remaining ÷ days in cycle new charge = new price × days remaining ÷ days in cycle net due today = new charge − unused credit
When the net is negative — a downgrade — nothing is collected today; the difference becomes a credit balance applied to the next invoice. That's the behavior subscribers expect and the one billing platforms implement, and this proration calculator shows every line of it so the result can be audited against an invoice.
How to use the proration calculator
- 1
Pick the billing cycle
Monthly or annual — the calculator derives the cycle end date automatically, including month-length and leap-year quirks.
- 2
Enter both plan prices
The full per-cycle price of the plan being left and the plan being moved to. Upgrades, downgrades, and same-price swaps all work.
- 3
Set the cycle start and change dates
The day the current period began and the day the switch takes effect. Day-based proration is computed from these two dates.
- 4
Read the result — and the arithmetic
Unused credit, new-plan charge, net due today, and the next invoice, each shown as an auditable line so finance can verify it.
Frequently asked questions
- What is proration?
- Proration splits a billing period's price by how much of it was actually used. When a subscriber changes plans mid-cycle, they get a credit for the unused part of the old plan and are charged for the remaining part of the new plan — so nobody pays twice for the same days.
- How is the prorated amount calculated?
- Day-based proration uses three numbers: the days in the billing cycle, the days remaining at the change, and the two plan prices. Unused credit = old price × days remaining ÷ cycle days. New charge = new price × days remaining ÷ cycle days. Net due today is the difference. This calculator shows each of those lines so you can audit the result.
- What happens on a downgrade?
- The unused credit from the more expensive plan usually exceeds the prorated charge for the cheaper one, so nothing is due today — the difference becomes a credit balance applied to the next invoice. The calculator never returns a negative charge; it shows the credit and what the next invoice looks like after it's applied.
- Will this match Stripe's proration to the penny?
- Very close, but not always exactly. Stripe prorates by the second between the change timestamp and the period end, while this calculator uses whole days — the convention most invoices and finance teams reconcile with. The difference is at most one day's fraction of the price. Chargebee, Recurly, and most billing systems document day-based proration like this.
- What if the plan change happens on the cycle start date?
- Then the full old price is credited and the full new price is charged — the net is simply the difference between the two plans, which is what you'd expect for a swap at renewal time.
- Is my data stored anywhere?
- Your inputs are saved only in your own browser's localStorage so they survive a refresh. Nothing is sent to a server unless you choose to enter your email.
- How do subscription businesses handle proration automatically?
- Billing platforms compute the credit and charge at the moment of the plan change, apply credit balances across invoices, and handle edge cases like downgrades, cancellations, and quantity changes. That's what Pelcro does for publishers and membership businesses — plan changes, prorated invoices, and dunning without spreadsheet math.