The first invoice from a SaaS product often shows two line items: a prorated charge for the rest of the current billing cycle and the full price for the next cycle. The prorated line is not an extra fee. It is the daily rate applied to the days between your signup date and the start of your first full cycle. This guide shows the formula, three worked examples, and the traps that make invoices hard to reconcile.
To apply this to your own numbers, try the pro rata calculator on the homepage; it handles mid-month starts and leap years.
The formula
- Daily rate = plan price ÷ days in the billing cycle
- Prorated charge = daily rate × days remaining in the cycle
- Next invoice: the full plan price
Billing cycles are usually 30 or 31 days, but annual plans exist too. The formula is the same; only the denominator changes.
New signup example
- Plan: $30 per month, 30-day billing cycle
- Signup on day 10, so 21 days remain
- Prorated charge: $30 ÷ 30 × 21 = $21
- First invoice: $21, next invoice: $30
If the vendor anchors the cycle to your signup date instead of the calendar, you get a full 30 days for $30 and no prorated line appears at all. The prorated charge only shows up when your signup date and the cycle start date differ.
Upgrading mid-cycle
A plan change mid-cycle is usually two prorated lines: a charge for the days on the new plan and a credit for the unused days of the old one.
- Old plan: $20 per month, new plan: $60 per month, change on day 15 of a 30-day cycle
- First 15 days on the old plan: 15 ÷ 30 × $20 = $10
- Last 15 days on the new plan: 15 ÷ 30 × $60 = $30
- Cycle total: $40, minus the $20 already paid, so the mid-cycle invoice is $20
Downgrades run the same calculation in reverse. Most vendors apply the credit to future invoices rather than refunding the card, so read the refund policy before you downgrade.
The four traps
- Calendar vs signup anchoring. Two vendors with the same price can produce different first invoices, because one anchors cycles to the calendar and the other to your signup date.
- Taxes and rounding. Sales tax is usually applied to the prorated amount, and rounding can put your total a cent or two away from your own math. That difference is normal.
- Annual plans. Prorating an annual plan usually converts to a monthly equivalent first. A $120 a year plan becomes $10 a month, then the daily rate applies to the remaining days.
- Free trials. If a 7-day trial ends on day 12 of the cycle, the prorated charge starts from day 12, not from your signup date.
Cancelling mid-cycle produces the same math in reverse, covered step by step in our guide on pro rata subscription refunds.
Check your invoice: enter the plan price and the days used. The calculator returns the prorated amount, free and no signup.
Related guides
How pro rata subscription refunds work How to calculate pro rata: formula and worked examples What does "pro rata" mean?Frequently asked questions
What does the prorated charge on my first SaaS invoice mean?
It covers the days between your signup date and the start of your first full billing cycle, billed at the daily rate. It is not an extra fee on top of your plan.
How is an upgrade billed in the middle of a cycle?
You pay for the remaining days at the new plan's daily rate and get credit for the unused days of the old plan. The difference lands on your next invoice.
Do downgrades get refunded pro rata?
Usually not in cash. Most vendors credit the unused portion of the old plan toward future invoices. The refund policy varies by vendor, so check before you downgrade.
Why is my second invoice higher than my first?
Because the first invoice only covered the partial cycle. The second invoice is a full period at the plan price, plus any plan changes made in between.
Is proration calculated by days or months?
Days. The vendor divides the plan price by the number of days in the cycle and multiplies by the days you will use in that cycle.