Invoice Late Fee and Interest Calculator
A client paid late, or hasn't paid yet? Enter the invoice amount, the due date and the payment date (or today's date), then the interest rate or fee from your contract or invoice terms. The formulas are the ones the U.S. Treasury's Bureau of the Fiscal Service publishes for Prompt Payment interest.
Use the terms you agreed to. This calculator doesn't supply a rate for your invoices. Whether you can charge interest or a late fee, and how much, depends on your contract and your state's law, which may cap interest or late fees. The default rate shown is the federal Prompt Payment interest rate for July 1 – December 31, 2026 (4.75%), which applies when a federal agency pays a vendor late; replace it with your own. This is general information, not legal advice.
This calculator needs JavaScript turned on. It runs entirely in your browser: nothing you type is sent anywhere or stored, and the page sets no cookies.
Amount due
| Days late (payment date − due date) | — |
|---|---|
| Annual interest rate used | — |
| Interest | — |
| Flat late fee | — |
| Interest plus fee | — |
| Total now due (invoice + interest + fee) | — |
Formulas used
- Days late = payment date − due date (paid on or before the due date = 0, so no interest or fee).
- Simple daily interest = P × (r ÷ 360) × d, where P is the invoice amount, r the annual rate and d the days late (Fiscal Service formula; choose 365 if your terms say so). A monthly rate is converted to a yearly rate by multiplying by 12.
- Monthly compounding = P × (1 + r/12)n × (1 + (r ÷ 360) × d) − P, where n is the number of whole 30-day periods and d the remaining days (Fiscal Service formula).
- Flat fee: added once if the payment is late. Total due = invoice + interest + fee.
Worked examples (from the Bureau of the Fiscal Service)
- Simple interest: a $1,500 invoice due April 1 and paid April 11 is 10 days late. At 6.6%: $1,500 × (0.066 ÷ 360 × 10) = $2.75. (The Fiscal Service page labels the rate 6.625% but computes with .066; at the full 6.625% the result is $2.76.)
- Monthly compounding: $1,500 due April 1, paid June 15 (75 days = two 30-day periods + 15 days), at 6%: $1,500 × (1 + 0.06/12)2 × (1 + 0.06/360 × 15) − $1,500 = $18.83.
Tips for getting paid on time
- Put the due date, any late fee and the interest terms on the invoice and in your contract before the work starts, so there's no dispute later.
- Before paying or re-sending an invoice because of an emailed request to change bank details, confirm by phone using a number you already have. Invoice fraud is a common scam; see our business email compromise guide.
- Keep copies of invoices and payment records; see how long to keep business records.
Sources
- Bureau of the Fiscal Service: Prompt Payment (current rate: 4.75% for July 1 – December 31, 2026)
- Bureau of the Fiscal Service: Prompt Payment simple daily interest formula and example
- Bureau of the Fiscal Service: Prompt Payment monthly compounding interest formula and example
- Bureau of the Fiscal Service: Prompt Payment interest rates
Sources checked October 29, 2026.