How to Calculate Late Fees (Legally)
Published June 2026 · Freelance Finance · 6 min read
Your client is late. Again. You want to charge a late fee but you're not sure if it's legal, how much to charge, or how to calculate it.
Here's everything you need to know.
📜 Is It Legal to Charge Late Fees?
Yes. In most jurisdictions, you can charge late fees as long as:
- The fee is stated in your contract or invoice before the work begins
- The fee is "reasonable" (not punitive)
- You comply with any state-specific limits
⚠️ Important: Some states have maximum allowable interest rates. Charging more could make your fee unenforceable or even illegal. Check your state's usury laws.
🧮 The Formula: Simple Interest
The standard formula for late fees is simple interest:
Late Fee = Principal × Rate × (Days Late ÷ 365)
Example Calculation
Let's say you invoiced $5,000 and the client is 30 days late. Your late fee rate is 1.5% per month (18% APR):
Late Fee = $5,000 × 0.18 × (30 ÷ 365) = $73.97
So the total due is $5,073.97.
📊 Standard Late Fee Rates
| Rate | APR | Common Use |
|---|
| 1% per month | 12% APR | Conservative, low-risk |
| 1.5% per month | 18% APR | Industry standard |
| 2% per month | 24% APR | Aggressive but common |
| 0.05% per day | 18% APR | Daily calculation |
💡 Pro tip: 1.5% per month (18% APR) is the sweet spot. It's high enough to motivate payment but not so high that clients feel gouged or that courts would consider it punitive.
📝 What to Include in Your Invoice
Your late fee policy should be clearly stated on every invoice. Here's the language I use:
A late fee of 1.5% per month (18% APR) will be applied to invoices paid more than 15 days after the due date.
📧 Email Templates for Late Payments
Day 1 (Due Date Passed)
Hi [Client Name],
Just a friendly reminder that invoice [INV-001] for $[Amount] was due on [Date].
Please let me know if you need anything from me to process this payment.
Thanks!
[Your Name]
Day 15 (Late Fee About to Apply)
Hi [Client Name],
Invoice [INV-001] for $[Amount] is now 15 days overdue. Per our agreement, a late fee of 1.5% per month will begin accruing tomorrow.
Current total: $[Amount]
After late fee (if paid today): $[Amount + Fee]
Please let me know if there's an issue with the invoice or if you need a payment plan.
Thanks,
[Your Name]
Day 30 (Firm)
Hi [Client Name],
Invoice [INV-001] is now 30 days overdue. Late fees have been applied per our agreement.
Current total due: $[Amount + Fees]
Late fees accrued: $[Fee Amount]
Please remit payment by [Date] to avoid further escalation. I'm happy to discuss a payment plan if needed.
[Your Name]
🧮 Free Late Fee Calculator
I built a free late fee calculator because I was tired of doing this math in Excel:
- Enter invoice amount
- Enter rate (e.g., 1.5%)
- Enter days overdue
- Get exact fee + new total
- Copy-paste the result
Built by a freelancer who got tired of chasing payments. Open source on GitHub.