Proration is the calculation that happens when a customer changes their subscription plan in the middle of a billing cycle. Commet credits the unused portion of the current plan, charges the destination plan in full, and restarts the billing cycle on the change date.
How proration works
When a customer makes an immediate plan change, two amounts are combined: a time-based credit for the unused old plan and the full destination-plan charge.
Here is a concrete example. A customer is on the Starter plan at $29/month. Their billing cycle is 30 days. On day 15, they upgrade to the Pro plan at $99/month.
Step 1: Credit for unused time on Starter. The customer has 15 days remaining. The daily rate on Starter is $29 / 30 = $0.967/day. The credit is 15 days x $0.967 = $14.50.
Step 2: Charge Pro in full. The new cycle starts on the change date, so the charge is $99.
Step 3: Net amount. The customer pays $99 - $14.50 = $84.50. Their next renewal is one full interval after the change date.
The credit prevents the unused Starter time from being lost while the restarted cycle keeps future billing anchored to the plan change.
When proration applies
Not every plan change triggers proration. The rules depend on the direction of the change and the starting point.
With the same interval, higher-sort-order plan changes are immediate. Inside a plan group, sortOrder defines the product progression. Price does not determine whether the move is an upgrade.
With the same interval, lower-sort-order plan changes are scheduled. The customer keeps the current plan through the existing period and moves at renewal.
Interval direction takes precedence. A shorter interval is scheduled and a longer interval is immediate, regardless of the destination plan's order. Paid-to-free changes are always scheduled.
Free to paid has no old-plan credit. The customer pays the full destination-plan price and starts a new paid period immediately.
For a detailed walkthrough of every plan change scenario, see the guide on what happens when a customer changes plans.
Why proration matters
Without proration, you have two bad options when a customer wants to upgrade.
Option one: charge the full new plan price immediately and let the customer lose the unused time on their old plan. This feels unfair and generates support tickets.
Option two: schedule the upgrade for the next renewal date. This means the customer has to wait days or weeks to access the destination plan's features.
Proration eliminates both problems. The customer gets immediate access, pays a fair price, and the billing system handles the math automatically.
Proration and addons
Addons follow the same proration logic as plan upgrades. When a customer activates an addon mid-cycle, they pay a prorated amount for the remaining days in the current period. At the next renewal, the addon's full price is added to the invoice.
For example, a customer with 10 days remaining in their billing cycle activates a $30/month addon. They pay $10 immediately ($30 / 30 days x 10 remaining days). On their next renewal invoice, the full $30 addon charge appears alongside the plan's base price.
Edge cases in proration
Proration calculations have a few details that matter at scale.
Day calculation. Proration uses the actual number of days in the billing period, not a fixed 30-day assumption. A February billing cycle has 28 or 29 days. A billing period from January 15 to February 15 has 31 days. The daily rate adjusts accordingly.
Multiple immediate changes. Each successful change starts a new period. A later change credits the unused time from the currently active plan and charges the next destination plan in full.
Rounding. Proration amounts are calculated to the cent. Because of rounding, the sum of prorated charges across a full billing period might differ from the plan price by a cent. This is standard and expected.
Commet applies these proration rules automatically on every plan change — start with the Commet docs.
Related
- Billing Cycle: the period between two invoice dates, the window within which proration is calculated
- Subscription Billing: recurring charging model where proration enables mid-cycle changes
- Recurring Billing: automated collection at regular intervals
- What Happens When a Customer Changes Plans: detailed guide covering every plan change scenario