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Proration is the adjustment Alguna makes when an advance charge changes part-way through a billing period: the customer is credited for the unused share of what was already billed and charged for the remainder of the period at the new rate. It happens automatically whenever an amendment or a version takes effect mid-cycle. This page covers the rule, the arithmetic, and how to schedule changes so no proration occurs; it does not repeat how to make the change itself.

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What gets prorated

Proration applies only to charges that are billed in advance for a fixed-class price: Metered and arrears charges are never prorated because their measurement window already bounds them: a change on the 16th simply means the old rate applies to usage before the 16th and the new rate after. Only an advance charge that has already been paid for the full period needs unwinding.

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The calculation

Proration is day-based. The full-period amount is divided by the number of days in the full billing cycle to get a daily rate, which is multiplied by the days in the partial period. The result is rounded to two decimal places at the line-total level (not per unit), so multi-unit lines do not accumulate rounding error.
Both lines are computed over the same remaining days, so an upgrade nets the price difference for the rest of the period and a downgrade nets a credit. On the invoice they appear as a prorated charge line and a prorated discount line, each with its date range; when the two are equal and opposite (for example a re-issued version with no price change) they cancel out and neither line is shown.

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Upgrade

100/month→100/month → 200/month on day 15 of a 30-day cycle:

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Downgrade

200/month→200/month → 100/month on day 20 of 30:

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Added product

A $50/month advance add-on added on day 10 of 30:

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Where it shows up

A mid-cycle change generates an invoice for the net difference at the effective date; the dashboard save dialog calls this out when you choose Mid-cycle. From the next period onward the new price bills at its full amount. Prorated lines carry their own date range on the invoice so the customer can see which days each covers.
Preview the change first. POST /subscriptions/{id}/changes/preview resolves the effective date and the item delta without writing anything; in the dashboard, the save dialog shows the same before you confirm.

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Avoiding proration

Proration is a consequence of timing, not a setting you switch off. To make a change with no mid-cycle adjustment, land it on a cycle boundary: This is the usual choice for downgrades and removals (the customer keeps what they paid for until the period ends) and for anything touching metered products, which are not prorated anyway.

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Proration on cancellation

Cancelling at the end of the current billing period needs no proration: the customer has paid for the period and keeps access until it ends. Cancelling immediately ends billing today; review the subscription’s invoices afterwards and issue a credit note if the customer is owed money for an unused advance charge. See Canceling Subscriptions.

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Next steps

Amendments

Make the change that triggers proration.

Versioning

Effective dates and the draft/published model.

Credit notes

Manual credits for anything proration does not cover.

How Alguna Works

Where invoices come from.