Invoice late fee & interest calculator

Work out the late fee or interest on an overdue invoice from the due date and payment date. Choose a flat fee, a one-time percentage, a monthly rate such as 1.5%, or an annual rate.

Overdue invoice

Invoice
Days after the due date with no charge
Your terms
Cap from your contract or local law

Result

Late fee / interest-
New amount due-
Days late-
Days charged-
Per day-
Effective annual rate-

Many US states cap late fees and interest, and some require the terms to be agreed in writing before work starts. Check your contract and local rules before charging.

How to use this calculator

  1. Enter the unpaid balance (not the original total if the client already paid part of it).
  2. Set the due date and the date it was paid. Leave today's date to see what is owed now.
  3. Choose how your contract says late fees work: a monthly rate, an annual rate, a one-time percentage or a flat fee.
  4. Add a grace period or a maximum if your terms include one, then copy the fee onto a reminder or a new invoice line.

How the math works

days late = payment date - due date days charged = days late - grace period (not below 0) monthly % = balance x rate x days charged / 30 annual APR = balance x rate x days charged / 365 one-time % = balance x rate (if any days are charged) flat = flat fee (if any days are charged) fee = MIN(fee, maximum) (if you set a maximum)

Interest here is simple (not compounded), pro-rated by day, which is how most small-business invoice terms are written. "1.5% per month" works out to an 18% annual rate.

Worked example

An invoice for 2,400.00 is paid 40 days late. The contract says 1.5% per month on overdue balances.

  • Fee: 2,400.00 x 1.5% x 40 / 30 = 48.00
  • New amount due: 2,448.00 (1.20 per day)

Under a 12% annual rate the same delay costs 31.56. A flat 25.00 fee would be about a 9.5% annual rate on this balance, which is why flat fees bite harder on small invoices.

Frequently asked questions

What is a typical late fee on an invoice?

Common terms are 1-1.5% per month on the overdue balance, or a flat fee of 25-50. Whatever you choose, put it in your contract or on the invoice before the work starts.

How do I calculate 1.5% interest per month on an invoice?

Multiply the unpaid balance by 0.015, then by the number of days late divided by 30. For 1,000 that is 15.00 for a full month late.

Can I charge a late fee if it was not in the contract?

It is risky. In most places late fees need to be agreed in advance. Without agreed terms, ask politely for payment instead, and add terms to future invoices.

Is there a legal maximum late fee?

Often, yes. Many US states set usury limits or caps on late charges, and the UK and EU have statutory late payment interest rules for business-to-business invoices. Check the rules that apply to you.

Should I charge interest on a partial payment?

Charge it on the remaining unpaid balance only, from the date it became overdue.

Not tax advice. This calculator gives estimates for planning and record keeping only. It is not tax, legal, accounting or financial advice. Fee rates and tax rules change; check the official source (Etsy's fee page, IRS.gov, your state or your contract) or ask a qualified professional before acting on the numbers.

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Last reviewed 2026-11-15.