Late Payment Fee and Interest Calculator
Add interest and a flat fee to an overdue invoice and see the total you can ask for.
A late fee only works if you actually send it
Adding an interest clause to your contract does nothing if you never follow up when a client pays late. The number this calculator gives you is worth the most when it's attached to a real reminder, sent consistently — not just kept as a deterrent you never use. Decide in advance whether you'll charge it every time a payment is late, since applying it selectively can look more like a threat than a policy.
How to use this
- Enter the invoice amount and its due date.
- Set the date you're calculating as of.
- Add your interest rate or flat fee — see the total you can ask for.
- Days overdue
- —
- Original invoice
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- Interest so far
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- Flat late fee
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- Interest per day
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How the total is calculated
Simple interest: amount × yearly rate ÷ 365 × days overdue, plus any flat fee once the invoice is past due.
Can I charge any rate I want?
Not necessarily. Many places cap late fees or interest, and a charge normally needs to be in your contract or invoice terms before the work starts. Check your local rules or ask a lawyer. This tool only does the arithmetic.
Can I charge a late fee if I didn't mention it upfront?
Generally, no — not retroactively on an invoice that's already out, since the client never agreed to it. What you can do is add clear late-fee terms to your next invoice and contract, so it applies going forward. For the overdue invoice you already sent, this calculator can still show you what a fee would have added, which is useful if you're deciding whether to negotiate a firmer deadline instead.
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