This calculator uses simple daily interest: overdue balance × annual rate × days overdue ÷ 365. It does not compound the fee or add flat charges. The result is an estimate to help check the math before sending a reminder or revised statement.
Only charge a late fee when the contract, invoice terms and applicable law allow it. Caps, notice rules, grace periods and tax treatment vary by jurisdiction and customer type.
Late fee formula and example
A $1,000 overdue balance at 12% annual interest for 30 days produces $9.86 in simple interest: 1,000 × 0.12 × 30 ÷ 365. The updated amount is $1,009.86.
If your agreement uses a monthly percentage or fixed fee, do not treat it as an annual rate. Convert the terms correctly and check any legal maximum before charging the customer.
Turn the result into a real document
The calculator is for the math. Use the free generator to itemize the work, add customer details and download a PDF, Word or Excel file.
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Frequently asked questions
Can I charge a late fee if it was not on the invoice?
That depends on the contract and local law. Adding a fee after the fact can be unenforceable; get legal or accounting advice for your situation.
Does this calculator compound interest?
No. It calculates simple interest on the original overdue balance for the number of days entered.
What annual rate should I use?
Use only a rate permitted by the agreement and applicable law. The calculator cannot determine a lawful rate for you.