Invoice Late Fee Calculator
A client missed the due date — now what do you charge? This free invoice late fee calculator applies a flat late fee, a monthly percentage, or a daily percentage to an overdue invoice and instantly shows the fee plus the new invoice total, so you can quote an overdue balance with confidence.
Enter your numbers
Your results
| Original invoice amount | $5,000.00 |
|---|---|
| Days overdue | 30 |
| Fee structure | Flat fee of $50.00 |
| Effective annualized rate | — |
Estimate only. Percentage modes use simple interest (no compounding). What you can actually charge is governed by your contract terms and applicable state usury and late-fee laws — this is not legal advice.
How to use this calculator
Enter the invoice amount and how many days overdue the payment is, then choose your late fee type: a flat dollar fee, a percentage per month, or a percentage per day. Type in the fee or rate, and the calculator instantly shows the late fee and the new invoice total.
The fee-type selector changes the default rate field: flat fees default to $50, monthly percentages to 1.5%, and daily percentages to 0.05% — common starting points you can adjust to match your contract. If you're not sure what to charge, 1–1.5% per month is a typical small-business late fee.
How it works
Flat fee: the fee is simply the fixed amount you enter — fee = flat amount. A $50 flat fee on a $5,000 invoice overdue 30 days gives a $5,050 total.
% per month: fee = invoice amount × (rate ÷ 100) × (days ÷ 30). Each month is treated as 30 days, so a 1.5% monthly rate on a $5,000 invoice overdue 30 days gives $5,000 × 0.015 × (30 ÷ 30) = $75. The equivalent simple annual rate is the monthly rate × 12 (1.5% × 12 = 18% per year).
% per day: fee = invoice amount × (rate ÷ 100) × days overdue. A 0.05% daily rate on a $5,000 invoice overdue 30 days gives $5,000 × 0.0005 × 30 = $75. The equivalent simple annual rate is the daily rate × 365.
Simple vs. compound: this calculator uses simple interest — the fee is always calculated on the original invoice amount, never on previously accrued fees. Compounding (charging interest on unpaid interest) is rare for invoice late fees and is more likely to run afoul of usury laws, so simple interest is the safer, standard assumption. Results are estimates, not legal advice — your contract terms and state fee laws govern what you can actually charge.
Frequently asked questions
How do you calculate a late fee on an invoice?
It depends on the fee structure in your contract. A flat late fee is a fixed dollar amount (e.g. $50). A monthly percentage fee is invoice amount × (rate ÷ 100) × (days overdue ÷ 30). A daily percentage fee is invoice amount × (rate ÷ 100) × days overdue. Most late fees use simple (not compounding) interest.
What is a typical late fee percentage?
A common late fee is 1% to 1.5% per month (about 12% to 18% annualized) on the overdue amount, or a flat fee of $25 to $50. What you can actually charge depends on your contract terms and the laws in your state or country.
Can I charge interest on overdue invoices?
In most cases, yes — but only if the late fee was agreed to in advance, usually in your contract or written payment terms. Your state's usury and late-fee laws cap what you can charge, and some jurisdictions require written notice before penalties apply. This is not legal advice.
Are late fees taxable income?
Yes. Late fees you collect are generally treated as ordinary business income and must be reported on your tax return, just like the invoice payment itself. This is general information, not tax advice.
What is the difference between a flat late fee and a percentage late fee?
A flat late fee is a fixed dollar amount charged once the invoice is overdue, regardless of the balance. A percentage late fee grows with the invoice amount and the number of days overdue. Flat fees are simple and predictable; percentage fees scale with how late the payment is.