Prorated Billing for Subscription Upgrades – Quick Guide

Got a customer who upgrades mid‑cycle and you’re not sure how to bill them without losing money? In the next few minutes you’ll learn a clear, repeatable formula that captures the exact amount owed, creates the right credit, and keeps your revenue intact. Follow the step‑by‑step checklist below and you’ll never have to guess or issue manual refunds again.

Why Common Prorating Mistakes Drain Revenue

Most SaaS founders make two simple errors when a user switches plans halfway through a billing period:

Charging the full new price immediately, which creates instant revenue leakage.
Leaving the old plan active, which confuses the customer and spikes support tickets.

If you’ve seen support emails like “Why was I billed $35 instead of $25?” you’re already experiencing these pitfalls. The fix is a single math routine that can be automated or run in a spreadsheet.

Step‑by‑Step Prorated Billing Process

Below is the exact workflow I use for every mid‑cycle upgrade. Replace the example numbers with your own plan prices and dates.

Identify the current billing window – e.g., the 1st to 30th of the month.
Calculate the old plan’s daily rate:

daily_rate_old = old_monthly_price ÷ days_in_period

For a $10 plan over 30 days → $0.33 per day.
Count the remaining days after the upgrade.

Upgrade on the 15th → 15 days left.
Compute the credit for unused time:

credit = daily_rate_old × remaining_days

$0.33 × 15 = $5.00.
Calculate the new plan’s daily rate:

daily_rate_new = new_monthly_price ÷ days_in_period

$25 ÷ 30 = $0.83 per day.
Determine the prorated charge for the new plan:

charge_new = daily_rate_new × remaining_days

$0.83 × 15 = $12.50.
Derive the net amount due:

net_due = charge_new – credit

$12.50 – $5.00 = $7.50.
Apply the amounts in your billing system – create a credit note for $5.00 (if required) and generate an invoice for $7.50. Send a short explanation to the customer so they understand the adjustment.

Quick Spreadsheet Shortcut (Optional)

Old Price	New Price	Days in Period	Upgrade Day	Remaining Days	Daily Rate Old	Daily Rate New	Credit	Charge New	Net Due
$10	$25	30	15	15	=B2/C2	=B3/C2	=E2*F2	=E2*G2	=I2-H2

Copy the table into Google Sheets or Excel, replace the values, and let the formulas do the work.

Final Checklist Before You Click “Send”

[ ] Billing period dates are correct.
[ ] Daily rates use the same number of days for both plans.
[ ] Remaining days are calculated from the upgrade date up to the period end.
[ ] Credit note (if required) matches the unused portion of the old plan.
[ ] Net amount due is added to the next invoice or charged immediately per your policy.
[ ] Customer receives a concise email outlining the credit and the new charge.

Following this list guarantees no surprise leaks in your revenue and a smooth experience for your users.