How to Use This Calculator
This tool helps freelancers, contractors, small businesses, and service providers calculate the interest and fees owed on late or overdue invoices.
- Enter the invoice amount: The original amount due before interest.
- Set your interest rate: This is typically defined in your contract or invoice terms. Common rates range from 1.5% to 18% annually, or you may use statutory rates for your jurisdiction.
- Enter the due date: When payment was originally expected.
- Enter the payment date: When the payment was (or will be) made. Defaults to today.
- Choose calculation method: Simple interest applies a flat rate; daily compound interest accrues daily.
- Add a grace period (optional): Days after the due date before interest starts accruing.
- Include a flat late fee (optional): Some contracts allow a one-time flat fee in addition to interest.
Example: A $5,000 invoice due on January 1st, paid on February 15th (45 days late), with a 10% annual interest rate and no grace period, would accrue approximately $61.64 in simple interest.
Understanding Late Payment Interest
Simple vs. Compound Interest
Simple interest calculates interest only on the principal amount. Formula: Interest = Principal × Rate × (Days / 365)
Daily compound interest calculates interest on the principal plus previously accrued interest, compounding daily. This results in slightly higher totals over time.
Typical Interest Rates
- Contractual rates: Often 1.5% per month (18% annual) or 1% per month (12% annual)
- U.S. statutory rates: Vary by state; often 6%–10% annually
- UK statutory interest: Bank of England base rate + 8%
- EU late payment directive: European Central Bank rate + 8%
Grace Periods
A grace period is a number of days after the due date during which no interest is charged. Common grace periods are 5, 10, or 15 days, depending on your contract terms.
Disclaimer: This calculator is for informational and estimation purposes only. It does not constitute legal, financial, or tax advice. Late payment terms must be clearly stated in your contract or invoice terms to be enforceable. Consult a legal or financial professional for guidance specific to your jurisdiction and situation.
Frequently Asked Questions
Can I legally charge interest on late invoices?
In most jurisdictions, yes—but only if your contract, invoice terms, or local statutory law allows it. Always include clear payment terms and late fee clauses in your agreements.
What interest rate should I use?
Use the rate specified in your contract. If none is specified, consult your jurisdiction's statutory late payment interest rate. Rates typically range from 6% to 18% annually.
Should I use simple or compound interest?
Simple interest is more common and easier to explain. Compound interest is typically used in banking and long-term debt. Check your contract terms or industry norms.
Can I charge both interest and a flat late fee?
Yes, if your contract allows it. Many businesses charge a flat fee (e.g., $25 or $50) plus ongoing interest to cover administrative costs and delayed payment.
How do I enforce late payment charges?
Send an updated invoice showing the original amount, interest accrued, and total due. If payment is still not received, you may need to pursue collection or legal action. Keep all documentation and communications.
Do I need to notify clients before charging interest?
It's best practice to include late payment terms on the original invoice and send a reminder before applying charges. Some jurisdictions require advance notice.