Late Fee / Overdue Invoice Calculator
Calculate late fees on overdue invoices with flat fee or interest rate methods.
By Konstantin Iakovlev · Updated April 2026 · Source: SBA — Business Guide
Use the Late Fee / Overdue Invoice Calculator above to calculate your results. Enter your values and see instant results — all calculations run in your browser.
Disclaimer: This calculator is for informational purposes only and does not constitute tax, financial, or legal advice. Results are estimates based on the information you provide and current rates. Always consult a qualified tax professional or financial advisor for advice specific to your situation.
How It Works
Unpaid invoices carry real costs, and this tool puts a number on them whether you charge a flat penalty or accrue interest over time. The pressure is rising: in 2026, the average U.S. small business is seeing a 7% increase in overdue payments compared to 2025, which makes consistent enforcement of payment terms a cash-flow necessity rather than an afterthought. Recovering legitimate late costs keeps your finances steady when clients drag their feet.
Two methods drive the math. Under the interest approach, the daily rate comes from dividing the annual interest rate by 365, and that figure is then multiplied by both the overdue amount and the number of days the invoice has gone unpaid. The flat-fee approach is simpler: a fixed charge is added to the original invoice total once the due date passes.
Late fees hold up best when they appear plainly in your original invoice terms and stay within local limits, since some jurisdictions cap how much you can charge. Fees pitched too high risk being treated as punitive and unenforceable, which defeats their purpose. The aim is to nudge clients toward paying on time, not to turn penalties into a revenue stream.
Example: Calculating a Late Fee for a 2026 Overdue Invoice
- 1 Imagine an invoice for $1,500 due on October 1, 2026. As of October 21, 2026, it remains unpaid. Your terms specify a 1.5% monthly interest rate for overdue invoices.
- 2 The invoice is 20 days overdue (October 21 - October 1). The annual interest rate is 1.5% * 12 months = 18%. The daily interest rate is 18% / 365 = 0.049315%. The late fee is calculated as: $1,500 * 0.00049315 * 20 days.
- 3 The calculated late fee is $14.79. The new total amount due on the invoice is $1,500 + $14.79 = $1,514.79.
- 4 This $14.79 represents the additional cost incurred due to the delay in payment. Promptly applying and communicating these fees helps encourage faster payment behavior and mitigates the financial impact of overdue invoices on your business's 2026 cash flow projections.
Source: SBA — Business Guide · Last updated: April 2026
Frequently Asked Questions
How much can I charge for late payment on an invoice?
When can I start charging a late fee on an invoice?
How do I calculate interest on an overdue invoice?
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