Invoice Late Fee Calculator
Estimate the fee and updated balance for an overdue invoice using the terms agreed with your client. Calculations run in your browser.
Calculate an overdue invoice
Use the rate and method stated in your contract or invoice. This tool provides a mathematical estimate, not legal advice.
Your estimate
How does an invoice late fee calculator work?
A late fee calculator estimates the additional amount that may be due when an invoice passes its due date. Enter the unpaid balance, due date, calculation date, grace period, and the fee method stated in your payment terms.
Late fee formulas used
Monthly simple rate
Estimated fee = unpaid balance × monthly rate × chargeable days ÷ 30. This prorates a monthly rate by chargeable days and does not compound earlier fees.
Annual simple interest
Estimated interest = unpaid balance × annual rate × chargeable days ÷ 365.
One-time flat fee
Estimated fee = the flat fee entered, once the due date plus any grace period has passed.
These methods are not interchangeable. Follow the method in your agreement and check whether local rules limit or prohibit particular charges.
How to use this calculator
- Enter the original invoice amount and subtract payments already received.
- Choose a currency and enter the invoice due date.
- Select the date through which you want to calculate the fee.
- Enter any grace period and choose the agreed fee method.
- Review the estimate and copy the summary for your records.
Frequently asked questions
Can I add a late fee if it was not agreed in advance?
This calculator cannot determine your legal rights. Check the contract, invoice terms, and applicable local rules before adding a charge.
Is a 1.5% monthly fee always allowed?
No. A rate that appears in one contract or market may not be allowed in another. Use only the rate permitted by your agreement and applicable law.
Does this calculator compound interest?
No. The monthly and annual methods use simple, non-compounding calculations on the unpaid principal. If your agreement uses another method, calculate according to that agreement.
Does this tool store invoice information?
No. The calculation runs locally in your browser; the values are not sent to GenFreeInvoice by this page.
Late fee examples for different billing situations
The estimate depends on the unpaid amount, the number of chargeable days, and the fee method. For example, if 900 units remain unpaid and the agreed monthly simple rate is 1.5%, a 20-day period produces an arithmetic estimate of 9 units using the 30-day prorating method. This example explains the calculation only; it does not say that rate is valid for your transaction.
Before you rely on an overdue invoice estimate
- Confirm the actual due date and how the contract counts days.
- Subtract all payments and credits already received.
- Use the rate and method agreed with the customer; do not mix monthly and annual rates.
- Check any cap, grace period, notice requirement, or restriction that applies to your transaction.
- Keep the calculation with your invoice records and clearly label it as an estimate.
More calculation details
Which balance does this calculator apply the rate to?
It uses the original invoice amount minus payments received. It does not add previous fees to the principal, so the monthly and annual methods are not compounded.
Can I calculate a fee before an invoice becomes overdue?
Yes. If the selected calculation date is on or before the due date, the calculator shows zero chargeable days and no fee estimate.
Why might the result differ from my accounting software?
Different systems may count days differently, use business days, apply a different day-count convention, or follow a contract-specific formula. Confirm the agreed method and compare the inputs.
Does selecting a currency convert the amount?
No. Currency selection changes how the entered amount is displayed; it does not perform an exchange-rate conversion.