Late Fee / Overdue Invoice Calculator
Calculate late fees on overdue invoices with flat fee or interest rate methods.
By Konstantin Iakovlev · Updated September 2026 · Source: SBA — Business Guide
Days Overdue
20
Late Fee
$15.00
Total Owed
$1,515.00
Overdue Details
| Invoice Amount | $1,500.00 |
| Days Overdue | 20 days |
| Late Fee | $15.00 |
| Equivalent Annual Rate | 18.0% |
| Total Amount Owed | $1,515.00 |
Suggested Past-Due Notice
This is a reminder that payment of $1,500.00 was due 20 days ago. A late fee of $15.00 has been applied, bringing the total balance to $1,515.00. Please remit payment at your earliest convenience to avoid further charges.
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. Late payments are a persistent drain on small-business cash flow, which makes consistent enforcement of payment terms a necessity rather than an afterthought. Recovering legitimate late costs keeps your finances steady when clients drag their feet.
Three fee types drive the math. A monthly percentage is prorated by the day: overdue amount × monthly rate × (days overdue ÷ 30). A daily percentage is charged for every day late: overdue amount × daily rate × days overdue. A flat fee is simpler: a fixed charge is added to the original invoice total once the due date passes. Days overdue run from the due date, which comes from the invoice date plus Net 15, 30, 60 or 90 terms or from a date you enter, to the payment date (today if you leave it blank), and the results translate the fee into an equivalent annual rate.
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: Late Fee on an Invoice Paid 20 Days Late
- 1 An invoice for $1,500 dated July 2, 2026 carries Net 30 terms, so it fell due on August 1, 2026. The client paid on August 21, 2026. The terms specify a late fee of 1.5% per month.
- 2 Days overdue: August 1 to August 21 = 20 days. Late fee: $1,500 × 0.015 × (20 ÷ 30) = $15.00. The equivalent annual rate is 1.5% × 12 = 18.0%.
- 3 Total owed: $1,500 + $15.00 = $1,515.00.
- 4 The calculator also drafts a past-due notice with these figures, which makes it easy to apply the fee promptly and tell the client exactly how it was worked out.
Source: SBA — Business Guide · Last updated: September 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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