About the Late Fee Calculator
Late payment fee on an overdue invoice: flat fee plus monthly interest, calculated daily from the due date.
Also known as: late payment interest, overdue invoice interest, late charge.
How to use this calculator
- Enter invoice amount, due date, payment date, monthly late interest and the other fields.
- The result updates instantly as you type — press Calculate (or Enter) at any time.
- Review the breakdown shown under the main result.
- Use Copy result or Share to save the answer or send a link with your inputs.
Formula
Fee = flat + amount × monthly rate × days/30
Worked example
Using the values pre-filled in the calculator above:
- Invoice amount = 2,500
- Due date: 2026-08-08
- Payment date: 2026-09-22
- Monthly late interest = 1.5 %
- Flat late fee = 25
the calculator returns:
- Late fee: 81.25
- Days late: 45
- Total due: 2,581.25