Invoice Late Fee & Interest Calculator
Calculate total due from late fees and daily interest terms.
Result
17 chargeable days (24 late minus a 7-day grace period) at 12% a year add $30.18 of simple interest, plus the $40 late fee. Check that your contract terms and local law allow these charges.
What the client owes
How to use it
- Enter invoice amount, days late, annual interest rate, fixed late fee, and grace days exactly as your contract describes them. Grace periods of 5-10 days are common when you want a policy that feels firm without charging immediately after the due date.
- Read total amount due, chargeable days, late fee, interest, and effective penalty percent. Annual rates above 30% are flagged because they increase both collection friction and legal-risk questions, especially across jurisdictions.
- Use the effective penalty to judge whether the policy actually changes behavior. A penalty equal to only 1-2% of invoice value may not motivate payment, while a very large combined fee plus interest can damage the client relationship and still be hard to enforce.
- Use the result in reminder emails, final notices, or future contract drafting. Compare fixed-fee-only, interest-only, and combined policies to choose the structure that balances deterrence with collectability.
- Re-run each week an invoice remains unpaid or when you revise payment terms. Track average days late and collected penalty amounts over time because the real goal is faster payment, not larger late fees.
Questions people usually ask
How do you calculate a late fee on an overdue invoice?
Interest accrues on the unpaid invoice balance at your stated rate over the time it is overdue, after any grace period: the common form is invoice amount times the monthly interest rate times the number of months late. If you also charge a one-time fixed fee, add it on top. This calculator takes your annual rate, prorates the interest across the chargeable days past the grace window, and adds any fixed fee to return the total now due.
What is the difference between a late fee and interest?
A late fee is a one-time charge. Interest keeps accruing over time based on the overdue balance and the rate in your terms.
Should I use both a fixed fee and interest?
Sometimes, but only if your contract and local rules support it. The calculator helps you model the math, not the legal enforceability.
Why include grace days?
Because many freelancers and businesses want a policy that feels firm without punishing clients the day after an invoice slips.
Can this tell me what is legal in my jurisdiction?
No. It calculates the amount under the policy you enter. Legal limits and contract wording still need a real review.
Is this tool free and private to use?
Yes. AI Biz Hub tools are free, no-signup browser tools. Inputs stay in your browser unless you choose to share a URL.
Is this professional advice?
No. Outputs are business planning estimates — not legal, tax, or accounting advice.
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Background on the Invoice Late Fee & Interest Calculator
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Articles
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Use it from code
The same calculation as a JavaScript module you can import. It runs where you import it, with no request, key or rate limit.
import { compute } from "https://aibizhub.io/engines/invoice-late-fee-interest-calculator.js";Input and output contract, TypeScript types and the guide for agents.