When an invoice passes its due date, a freelancer or small business may want to estimate the additional amount under payment terms agreed with the client. The calculation depends on the method in the agreement: a one-time flat fee, a monthly percentage, or annual interest. This guide explains the math; it does not determine whether a fee is legally permitted.
Use the fee method and grace period specified in your agreement.
1. Calculate the unpaid balance
Start with the original invoice amount and subtract payments already received.
Unpaid balance = original invoice amount − payments received
Example: if the invoice was $1,200 and the client paid $300, the unpaid balance is $900. Use the unpaid amount as the principal unless your agreement specifies a different basis.
2. Choose the method stated in your terms
One-time flat fee
A flat fee is a fixed amount charged once when the conditions in the agreement are met. Example: a $900 unpaid balance plus an agreed $25 flat fee gives an estimated balance of $925. This is an arithmetic example, not a claim that the fee is allowed everywhere.
Monthly percentage using simple, prorated interest
Fee = unpaid balance × monthly rate × chargeable days ÷ 30
Example: $900 × 1.5% × 20 ÷ 30 = $9. This assumes a 1.5% monthly rate, 20 chargeable days, a 30-day month, and no compounding. Follow the exact method in your agreement if it differs.
Annual simple interest
Interest = unpaid balance × annual rate × chargeable days ÷ 365
For example, $900 at 12% per year for 30 days gives estimated simple interest of about $8.88. The actual method may differ depending on your contract and applicable rules.
3. Account for the due date and grace period
Count days from the invoice due date to the date through which you are calculating. If your agreement includes a grace period, apply it exactly as written. A calculator may count calendar days, business days, or use another convention; check the agreement before relying on the result.
4. Keep the calculation transparent
- Keep the original invoice and payment record.
- Show the due date, unpaid principal, agreed rate or flat fee, and the period used.
- State the calculation method and updated amount clearly.
- If the client has paid part of the invoice, ensure the remaining balance is correct.
- Do not add a fee retroactively without checking your agreement and local rules.
5. Use the free invoice late fee calculator
Enter your invoice amount, payments received, due date, calculation date, grace period, and the method from your agreement. The result is an estimate calculated in your browser. It does not assess whether the charge is enforceable.
Frequently asked questions
Is a 1.5% monthly late fee allowed everywhere?
No. Contract terms and legal limits vary. Check the rules that apply to your transaction before charging a fee.
Should I compound late fees?
Only use compounding if your agreement and applicable law permit the method. The linked calculator uses simple, non-compounding calculations.
What if my invoice does not mention late fees?
Do not assume you can add one after the fact. Review your contract and local rules, and consider qualified legal advice if the amount matters.
What payment terms should I put on future invoices?
State the due date or number of days to pay, accepted payment methods, and any late-payment provision clearly before work begins. Read our invoice payment terms guide.
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Disclaimer: This guide and calculator provide general information and mathematical estimates, not legal, tax, or accounting advice.