Proration

Period-based pricing treats a month as one unit. Customers don't change on the first of the month. Three people join on the 12th; the team outgrows its plan on the 15th. When the conditions change in the middle of a period, there was no settled answer for what that month's charge should say.

Early on, companies picked one of two blunt options. Either the month was billed at the old price and the new price started next month, or the new price was charged in full from the day of the change. The first leaves the company unpaid for half a month of the higher plan. The second makes the customer pay for half a month they never had. Both produce a wrong number; they only differ in who absorbs it. And because neither rule was written down anywhere, the same change could be handled one way in January and the other way in February.

The discomfort surfaced after the invoice went out. A customer who upgraded on the 15th asks why a full month is on the bill, and a support rep works out the difference on a calculator and sends part of it back. Once, that's good service. Several dozen times a month, it's a hole where every person calculates a little differently. Proration is what that work turned into once it became a rule instead of a favor.

Proration cuts the billing period in two at the moment of the change, prices each piece by its share of the days, and adds them together.

1. Fix the effective date. Not the moment the customer asked, but the date the change took effect — usually midnight on the day it was approved, with that boundary written into the subscription terms. One day off is one wrong number.
2. Count the days in the billing period. Months run from 28 to 31 days, so the value of a day changes from month to month. The denominator is the period being billed, not a generic month, which matters on annual and quarterly plans where a period can be 365 or 92 days long.
3. Price each piece and add them. A team on an $80 plan that moves to a $160 plan on the 15th of a 30-day month pays half of the old price, $40, plus half of the new one, $80 — so $120 for that month.
4. Reconcile against what was already collected. If $80 was prepaid, only the $40 difference gets charged, either as its own charge or as a line on the next invoice. When the card is charged immediately, only that difference goes back through the payment gateway.
5. Put both pieces on the invoice as separate lines. Days 1 through 15 on the old plan, days 16 through 30 on the new one. A single total invites the question all over again, because the customer can't check it.

Per-seat pricing applies the same arithmetic to seats instead of plans. A 20-person team paying $8 per seat per month that adds five people on the 12th of a 30-day month owes five seats for the remaining 18 days, which comes to $24 on that month's bill.

Proration earns its keep in subscription businesses with period-based pricing and frequent changes — team plans where seat counts rise and fall every quarter, customers who upgrade at any point in the year. A one-time sale, or a contract whose terms are fixed for its whole length, never needs it. The clearer signal is how often a support rep is asked to explain a bill: when that question arrives weekly, the rule is overdue.

Once you use it, the limits have to be set too.

1. Below a certain amount, processing costs more than the money involved. Reissuing an invoice and correcting a tax invoice to settle a $2 difference costs more effort than the $2. So companies set a floor, something like $5 or $10, and roll anything smaller into the next invoice.
2. Many companies don't apply it downward. Prorating a downgrade means handing money back, which drags in the refund process. Letting the customer keep the current plan for the rest of the period and applying the lower price from the next cycle is simpler.
3. On annual contracts the contract wins. The remaining term is long enough that the difference runs into thousands of dollars, so the contract clause governs rather than the billing rule.
4. Don't feed prorated amounts into metrics. Monthly recurring revenue (MRR) counts what repeats every month, so adding that $120 as it stands records a customer worth $160 a month as a $120 one. Bill by the day; count by the standard monthly price.

"This month is prorated, so it's $120, and it goes to $160 from next month."

The new plan only applied to half of the current period.

"The difference was $2, so I didn't bill it separately — it's on the next invoice."

The amount fell under the floor, so it wasn't worth its own transaction.

"We don't prorate downgrades. You keep this plan for the rest of the period and the lower price starts next month."

Explaining that immediate settlement only runs in one direction.

At Pagedeck this calculation comes up most often on team plans. Sofia's accounts add ten seats when a project starts and drop them when it ends; the additions get priced for the remaining days that same day, while the removals land in the next cycle.

Grace added a $5 floor to the rule. Anything smaller rolls into the next invoice, and Theo built that condition into the system along with the rest. Several dozen cases a month that used to be handled by hand now print as two lines on the invoice.

The first assumption is that it applies automatically to every change. The word suggests dividing things evenly, which makes upgrades and downgrades sound symmetrical. In practice most companies settle immediately on the way up and wait for the next cycle on the way down. Leave that asymmetry out of the terms and the in-product copy and customers feel shortchanged, and some of those conversations end in a cancellation.

The second is that a day can safely be priced at one thirtieth of the monthly fee. The arithmetic gets easier and in most months the gap is small. But February has 28 days and March has 31. On a $230 plan a day is worth $8.21 in February and $7.42 in March, while the fixed rate calls both $7.67. Over half a month that's about $8 undercollected in February and $4 overcollected in March — and the month that overcollects is the invoice a customer catches first.