Free Tool

Proration Calculator

Work out the credit, the new charge, and the net amount to invoice when a customer upgrades, downgrades, changes seats, or cancels in the middle of a billing period.

No signup. Everything is calculated in your browser and nothing is stored.

Billing period

Plans (price per period)

Result

Credit for unused time

-$26.67

8 of 30 days on the current plan

Charge for new plan

$66.67

Same 8 days on the new plan

Net due now

$40.00

Invoice this amount now

Next renewal

Thu, Oct 1, 2026

$250.00 per period from then

Invoice explanation
Plan change on Wed, Sep 23, 2026 (8 of 30 days left in the period)
Credit for unused time on current plan: -$26.67
Charge for remaining time on new plan: $66.67
Net amount due now: $40.00
From Thu, Oct 1, 2026: $250.00 per month

Why proration mistakes cost more than the difference

A proration error is rarely large, but it lands on the customer’s desk at the worst moment: right after they agreed to pay you more. A mid-cycle upgrade invoice that does not match what the customer expected is one of the most common reasons subscription invoices get disputed or paid late.

The fix is to calculate it the same way every time, show the credit and the new charge as separate lines on the invoice, and state the proration method in your terms. Use the summary below as the explanation line on the invoice.

Frequently asked questions

How do you calculate proration?

Divide the plan price by the number of days in the billing period to get a daily rate, then multiply by the days left after the change. The customer gets a credit for the unused days on the old plan and is charged for the same days on the new plan. The difference is the prorated amount.

What happens when a customer downgrades mid-cycle?

The credit for the unused time on the more expensive plan is larger than the charge for the cheaper plan, so the net result is a credit. Most billing systems carry it forward against the next invoice rather than refunding it.

Should proration use days or months?

Daily proration is the most accurate and the default in Stripe, Chargebee, and most billing platforms. Some contracts prorate by whole months for simplicity. Whatever you choose, state it in the contract so the invoice is never a surprise.

Does the change day count as used on the old plan?

In this calculator the change takes effect at the start of the change date, so the old plan covers the days before it and the new plan covers the change date onward. Billing systems differ slightly, so small differences of one day are normal.

Can I prorate seat or license quantity changes?

Yes. Enter the per-seat price and the old and new quantities. The calculator prorates the difference in total plan value the same way as a plan change.

Doing this by hand for every plan change? We automate subscription changes, proration, and invoicing between your CRM and billing system.

See invoice automation

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