How to use the subscription upgrade proration calculator
- Enter the price of your current plan and the price of the new plan, each for one full billing cycle.
- Choose the billing cycle: weekly, monthly, every 3 months or yearly, and enter the day the current cycle started.
- Enter the day you change plans. It must fall inside the cycle.
- Read the days left, the credit, the charge for the new plan, and the net amount.
Formula
days in cycle = days from the cycle start to the next cycle start
credit = old price x days left / days in cycle
charge = new price x days left / days in cycle
net = charge - credit, with each amount rounded to the cent last
The new plan is charged only for the days left, and the old plan is credited for the same days. If the net amount is negative, it is a credit toward the next bill.
Upgrading from $29 to $79 on March 11
A monthly cycle starts March 1 and the next one starts April 1, so the cycle has 31 days. Changing on March 11 leaves 21 days. The credit is $29.00 x 21 / 31 = $19.65 and the charge is $79.00 x 21 / 31 = $53.52. The net amount is $53.52 - $19.65 = $33.87.
A yearly plan going from $480 to $240, started January 15, 2026 and changed on July 1, has a 365 day cycle with 198 days left. The credit is $260.38 and the charge $130.19, so the net is a credit of $130.19.
Why results differ between providers
Providers prorate in different ways. Some bill by the second, some by whole days, some count the day of the change as used and some do not, and some start a new cycle on the day of an upgrade instead of keeping the old one.
This page counts the day of the change as the first day of the new plan, and every amount is figured from exact days. Use it as an estimate and compare it with the invoice.
Upgrades, downgrades and annual plans
A downgrade gives a negative net amount, which many providers keep as a credit toward the next bill instead of refunding it. An annual plan has a long cycle, so a small difference in days can change the amount by a few dollars.
For a rent that must be prorated by days, use the Proration Calculator. To compare a yearly plan with a monthly one, use the Annual vs Monthly Plan Calculator.
Assumptions and limits
- Each plan price is for one full cycle and has at most two decimal places.
- The cycle length is counted in calendar days from the start date to the same date in the next cycle.
- The change takes effect at the start of the change date.
- Taxes, fees, discounts and free trials are not modeled.
- Credits and charges are rounded to the cent half up, after the fraction is applied.
Frequently asked questions
Why is the net amount not just the price difference?
Because only the days left are affected. In the example, the price difference is $50 but only 21 of 31 days remain, so the net amount is about two thirds of it.
Does the provider have to use this method?
No. Billing terms vary, and some providers round to whole days, charge the full new price or reset the cycle. Check your provider’s terms before you rely on a number.
What if I change on the first day of the cycle?
Then all of the days are left, the credit is the full old price and the charge is the full new price, so the net amount equals the price difference.
Why does the page reject my change date?
The date must fall between the cycle start and the day before the next cycle starts. A date outside the cycle belongs to a different billing period.