Late Fee Calculator for Unpaid Invoices
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Simple (non-compounding) calculation, prorated by day. Only charge late fees your contract or invoice terms allow — rules vary by place.
How late-fee math works
A late fee is almost always quoted as a monthly percentage — 1.5% per month is the most common figure in freelance contracts, which works out to 18% per year. The fee is then prorated by how late the payment actually is: the formula is invoice amount × monthly rate × (days overdue ÷ 30). So a $2,500 invoice that's 30 days late at 1.5% monthly accrues exactly $37.50. At 45 days, the same invoice accrues $56.25. This calculator uses simple interest — each month's fee is calculated on the original invoice amount, not compounded onto previous fees — because that's how most freelance late-fee clauses are written.
But the math is the easy part. The important part is permission: you can only fairly charge a late fee if the client agreed to it in advance — in your contract, your proposal, or the payment terms printed on the invoice itself. A fee invented after the invoice goes overdue is very hard to enforce and poisons the relationship. That's why experienced freelancers add a one-line clause from day one, something like "a 1.5% monthly late fee applies to overdue balances." You may never need to use this calculator — its real power is that the clause exists at all, quietly moving your invoice to the top of the client's payment queue.
When you do apply a fee, communicate it plainly: state the original amount, the days overdue, the rate from your agreed terms, and the new total. Then invoice the fee as its own line item so the paper trail stays clean. Our guide on how to charge late fees walks through the reminder sequence — friendly nudge, firm restatement, formal notice — that gets most overdue invoices paid before fees even enter the picture.
Frequently asked questions
What is a reasonable late-fee rate?
1% to 2% per month (12–24% annually) is typical for freelance and small-business contracts, with 1.5% the most common single figure. Some places cap the maximum by law, so check your local rules before setting a rate.
Can I charge a late fee if my invoice didn't mention one?
It's much harder. Late fees are generally enforceable only when the client agreed to them beforehand — in a contract or in the invoice terms. Add the clause to your terms now so future invoices are covered.
Is the fee compounded or simple?
This calculator uses simple interest: each period's fee is based on the original invoice amount. Unless your contract explicitly says fees compound, simple is the fair and standard assumption.
Should I actually enforce the fee on a good client?
Often the clause alone does the job — many freelancers mention it in the reminder and the payment appears. For a genuinely good client with a one-off delay, waiving the first fee can buy more loyalty than the dollars are worth. Reserve enforcement for repeat offenders.