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Free Late Fee Calculator
Calculate late payment fees and interest on an overdue invoice.
Calculate late payment fees and interest on an overdue invoice. It shows what a delay adds up to.
How to use the Late Fee Calculator
- Select your country at the top of the page so the currency, tax names and rates match where you do business.
- Enter your numbers. Every result updates instantly and each line of the working is shown so you can check it.
- Use the PDF or print button to keep a record, and change one input at a time to compare scenarios.
What is the Late Fee Calculator?
When a client pays late, you can add interest, a flat fee or both, if your contract or invoice allows it. This calculator shows the fee for any overdue amount and the new total due, and converts a monthly rate to its yearly equivalent.
How the calculation works
The interest is the overdue amount, multiplied by the monthly rate, multiplied by the days overdue divided by 30. Add any flat fee to get the total late fee, and add that to the invoice to get the amount now due.
Example: an invoice of 2,000 is 20 days overdue with a late fee of 1.5% per month. Interest is 2,000 x 0.015 x (20 / 30) = 20. With no flat fee, the total late fee is 20 and the amount now due is 2,020.
Can you charge a late fee at all?
Only if it was agreed in advance or the law allows it. Put the rate or the fee on the invoice and in your contract before the work starts. A fee you add afterwards is hard to collect. Some places set limits or rules, so check yours. In the United States many states cap interest charges. In the United Kingdom, businesses can claim statutory interest and fixed compensation on late commercial payments under the Late Payment of Commercial Debts (Interest) Act, even without a contract term. In Canada the Interest Act requires that a rate stated per month also be shown as a yearly rate, which this calculator does for you. In Australia there is no general statutory late fee for business invoices, so the contract rules.
Monthly rate or flat fee?
A monthly rate grows with the amount and the delay, so it is fairer on large invoices and keeps pressure on a long overdue payment. A flat fee is simple and works well for small invoices. Many businesses use both. Keep the total reasonable, because an excessive penalty can be unenforceable.
Getting the payment before you charge the fee
A friendly reminder the day after the due date often works, and many clients pay without the fee. Send a second reminder with the fee and the new total, and keep copies. Use the Invoice Generator to reissue an invoice that shows the late fee as its own line.
Tips for better results
Treat every result as an estimate. Real tax depends on your full circumstances such as other income, credits and deductions.
Try best and worst cases. Run the numbers with a slightly lower price or higher cost to see how safe your margin is.
Check the tax year shown under the result. Rates and thresholds change, and the calculator notes which year its figures come from.
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Privacy and where your data goes
Documents and files you use here are processed on your own device. Signplify does not receive what you type or the files you open, which makes it suitable for sensitive information. If you clear your browser data, any saved draft is removed. Last updated: October 26, 2026. This page is general information, not legal or tax advice.
Frequently asked questions
How do I calculate a late fee?
Multiply the overdue amount by the monthly rate and by the days overdue divided by 30, then add any flat fee.
What is a reasonable late fee?
Rates between 1% and 2% per month are common, but the limit depends on your law and contract. Excessive fees can be unenforceable.
Can I charge a late fee if it is not in the contract?
Usually not, unless the law gives you a right to interest, as in the United Kingdom for commercial debts. Put the fee in writing before the work starts.
What is a 1.5% monthly late fee per year?
It is 18% per year as a simple rate. The calculator shows the yearly equivalent of any monthly rate.
Should I charge a flat fee or interest?
A flat fee is simpler for small invoices and interest suits larger or very late ones. You can use both if the contract says so.
What do I enter in the late fee calculator?
You enter Invoice Amount, Days Overdue, Late Fee Rate (% per month) and Flat Late Fee. The result updates as you type, so you can change one number and see the effect straight away.
What result does the late fee calculator give for example numbers?
With the starting example (Invoice Amount $2,000, Days Overdue 20, Late Fee Rate (% per month) 1.5 and Flat Late Fee $0), it shows Amount now due of $2,020.00, along with Interest on overdue amount $20.00, Flat fee $0.00, Total late fee $20.00 and Equivalent annual rate 18.00%. These are the US defaults; change the inputs to match your own situation.
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