Invoice Late Fee Calculator

Calculate legal, APR-based late fees on overdue invoices. All math runs locally — no data leaves your browser.

Common: 12–18% APR (1–1.5% per month). Check your state usury cap.

How to use this calculator

  1. Enter the original invoice amount (principal).
  2. Enter the annual late fee rate you disclosed on your invoice (APR). If you use a monthly rate, multiply it by 12.
  3. Enter the number of days past the due date.
  4. Choose simple (default) or compound daily interest.

Legal note: Always verify your state or country usury laws before charging. Many US states cap commercial interest at 10–18% APR. In the UK, the statutory rate is 8% plus the Bank of England base rate.

Formula

Simple interest: fee = principal × (APR ÷ 365) × days

Compound daily: fee = principal × ((1 + APR÷365)^days − 1)

Late Fee FAQ

How is invoice late fee calculated?

Multiply the overdue balance by the daily interest rate, then multiply by the number of days overdue. Daily rate equals annual rate divided by 365. Example: $1,000 at 18% APR overdue 45 days = 1000 × (0.18/365) × 45 = $22.19.

What is a reasonable late fee for invoices?

Common practice in the US is 1.5% per month (18% APR) on overdue balances, matching the UCC maximum in many states. In the UK, statutory interest is 8% plus the Bank of England base rate under the Late Payment of Commercial Debts Act. Always verify against local usury caps.

Can I charge late fees without a written contract?

Enforceability varies by jurisdiction. Best practice is to disclose late fees on the invoice itself and in any engagement agreement before services begin. Without clear disclosure, courts may refuse to enforce the fee.

Are late fees tax deductible as business expense?

Late fees paid by your business are generally deductible as ordinary business expense. Late fees you charge clients are ordinary income. Consult a tax professional for jurisdiction-specific guidance.

Does this calculator store my invoice data?

No. All calculations run locally in your browser using JavaScript. No amounts, dates, or client details are transmitted to any server or stored anywhere.