Late fee calculator for overdue invoices
Key takeaways
1.5% a month on a $4,000 invoice is $60 for every 30 days late.
Prorate by day: days late divided by 30, times the monthly fee.
Only charge a fee your terms set out before the work started.
State law can cap late fees and interest. Check before you raise the rate.
Loading the calculator. The worked examples below use the same formula.
How is a late fee worked out?
The calculator uses the plain method most small businesses put in their terms: a monthly percentage, prorated by the day and not compounded, or a single flat fee once the due date passes.
| Invoice | Terms | Days late | Late fee |
|---|---|---|---|
| $4,000 | 1.5% a month | 45 | $90.00 |
| $2,000 | 1.5% a month | 20 | $20.00 |
| $12,500 | 1% a month | 90 | $375.00 |
| $800 | $50 flat | any | $50.00 |
What should my invoice terms say?
Write the fee where the customer sees it before they agree: the contract, the quote and the invoice itself. One line is enough, for example: "Invoices are due in 30 days. Overdue balances carry a late fee of 1.5% per month." Then show the fee as its own line on any reminder, so the customer can check the math. Our payment reminder email templates show where it fits.
When does a late fee stop helping?
A fee nudges a customer who is simply slow. It does little against one who has stopped answering. Past 30 days with no reply, a final notice letter or a demand letter does more than a growing fee. If it goes to small claims, bring your terms and this calculation to the hearing.
Last updated October 2026. Easy Small Claims is not a law firm and this page is general information, not legal advice.
Questions, answered
How do you calculate a late fee on an invoice?
Start from the rate in your terms. For a monthly percentage, multiply the invoice amount by the rate, then by the days late divided by 30. A $2,000 invoice at 1.5% a month that is 20 days late carries a $20 fee. For a flat fee, add the fixed amount once the due date has passed.
Can I charge a late fee if it was not in the contract?
It is hard to. A late fee usually has to be agreed before the work, in the contract, the quote or invoice terms the customer accepted. Adding one afterwards gives the customer a reason to dispute the whole invoice. State law may still allow some interest on a late debt, so ask a lawyer if the amount matters.
Is a late fee the same as interest?
Not quite. A flat late fee is a one-off charge for paying late. A monthly percentage works like interest and grows the longer the invoice sits. Some states treat a percentage fee as interest under their usury rules, which can cap it, so a very high monthly rate can backfire if the dispute reaches court.
Can I add late fees to a small claims case?
Yes, if your terms allowed them. Show the judge the terms, the invoice and how you worked out the figure, and list the fee separately from the principal on your claim. A late fee is part of what you are claiming, so count it when you check your amount against the small claims limit.
A small business owner himself, Nitin has been on the wrong end of an unpaid invoice and knows how easy it is to give up on one. He built Easy Small Claims so owners can get that money back. He is also a practicing barrister.