A mid-month start
USD 3,100 for October 1–31, with service October 16–31, is 16 of 31 days: USD 1,600. October 16 is included.
Retainer billing
Free Retainer Proration Calculator
A client who starts or ends mid-period may need a partial retainer. Compare the agreed active dates with the full billing period and calculate one transparent calendar-day subtotal.
What you can do here
Calculate a partial-period retainer using inclusive calendar days within a billing period you define.
Built for real invoice workflows
Prepare the partial invoice separately, then schedule the regular retainer in InvoiceAgent.
Preparing your calculator with today’s date…
Calculation notes
How the result is calculated
Full-period fee × inclusive active calendar days ÷ inclusive period calendar days. Round only the final subtotal. The active interval must sit completely inside the billing period. A fixed onboarding fee, reserved-capacity agreement, or hourly retainer may require a different method; this tool does not choose the contract terms.
Formulas and examples reviewed October 2, 2026. Illustrative figures; no tax or contractual determination.
Leap-year February
USD 2,900 for February 1–29, 2028, with service February 20–29, is 10 of 29 days: USD 1,000. A fixed 30-day divisor would produce a different amount.
Questions about this calculation
Are weekends included?
Yes. This method counts calendar days. If your agreement uses working days or delivered hours, use that agreed method instead.
Does proration cancel the remaining service?
No. It calculates a subtotal only. Cancellation rights, minimum fees, notice periods, and any refund decision remain in the agreement.
How to use it
1
Enter the fee for the full billing period.
2
Choose the period dates and the active dates inside that period.
3
Confirm the calendar-day method with the client before using the amount.
Why it helps
Handle February and 31-day months without assuming a 30-day divisor.
Show which days are included in the charge.
Keep a partial period separate from the next full retainer.
Next step
From free tool to live invoice workflow.
Use the result when creating your invoice in InvoiceAgent. Planner setup links carry the settings they describe into a draft. Review your dates, amounts, PDF, and reminder settings before scheduling.
Related workflows
Move from the quick answer into the rest of the invoice system.
Retainer renewal email
Retainer renewal email
Confirm the next full period and any changed terms.
Close out a retainer
Close out a retainer
Check service dates, final charges, and scheduled invoices.
Invoice templates
Invoice templates
Use the result inside invoice formats that are ready for clients and easier to automate.
Knowledge base
Knowledge base
Review the billing concepts behind the numbers before they go onto a live invoice.
Profession pages
Profession pages
See how freelancers, consultants, and agencies apply the same calculator output in different workflows.