Invoice late fee calculator
A late fee is added to an unpaid invoice under the terms you agreed with your client: a flat amount, a one-time percentage, or a monthly percentage that grows with each day late. Enter the unpaid balance, the days past due, a grace period and your rule to see the fee and the new balance.
This calculator runs in your browser and needs JavaScript. The method is written out below if you want to do the sum by hand.
Late fee
- Unpaid balance
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- Days charged
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- New balance
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Nothing you type here is sent anywhere: the calculation runs on this page, in your browser.
How it is worked out
- Flat fee: the amount you set, once, if the invoice is still unpaid after the grace period.
- One-time percentage: balance × rate, once.
- Percentage per 30 days: balance × rate × days charged ÷ 30, where days charged = days past due − grace period. Simple interest, not compounded.
Amounts are worked out in whole cents and the fee is rounded to the nearest cent. Whether you may charge a fee, and how much, depends on your contract and the law where you and your client are; some countries cap late fees or set statutory interest for late business payments.
ReckonOS reminds late payers 3, 7 and 14 days after the due date and can add a one-time late fee you have enabled. How reminders work.
Estimates based on the rules for your contract and local law (no statutory rate applied) as of November 2026. Check with your accountant. This tool gives an estimate, not tax, legal or financial advice.
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Questions
How do I calculate a late fee on an invoice?
Use the rule in your contract or payment terms. A flat fee is added once; a one-time percentage is the invoice balance times the rate; a monthly percentage is usually the balance times the rate times the number of 30-day periods late, often pro-rated by the day.
Can I charge any late fee I like?
No. Late fees should be agreed in your contract or terms before the work starts, and many countries limit them or set statutory interest for late business payments. Check what applies to you and your client before you add one.
Does ReckonOS add late fees automatically?
Only if you turn it on. When a fee is applied it appears as its own dated line on the invoice and is included in the balance, the pay link, the client portal and reminders, and you can waive it.
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