Late Payment Interest Calculator
Work out how much interest an overdue invoice has accrued, and what the new total should be.
How this calculator works
Everything runs in this browser tab: the amount, dates, and rate you enter are never sent anywhere. This tool only does arithmetic on the rate you choose. It never picks a rate for you or gives tax or legal advice. Once the total is worked out, an overdue invoice reminder letter turns it into a complete, ready-to-send letter instead of just a number.
Frequently asked questions
What interest rate should I use?
The tool never suggests or looks up a rate. You enter the rate yourself based on your contract or local law. For UK businesses, statutory interest is the Bank of England Base Rate plus 8 percentage points, but check your jurisdiction for what applies.
Can I calculate interest on partial payments?
No. Enter the full outstanding amount and the number of days late. If only part of an invoice is unpaid, calculate interest on that portion alone by typing the partial amount instead.
Is this interest legally enforceable?
The tool does the math, not the law. Whether you can charge interest, what amount, and under what conditions depends on your contract and local regulations. This is not legal advice. Verify with your accountant or lawyer before sending an interest charge to a client.
Does the calculator handle leap years?
Yes. If you choose “Actual days per calendar year,” the tool accounts for 365 or 366 days depending on whether the period includes February 29th.
Can I use this for invoices from other countries?
Yes, as long as you know the interest rate that applies. Select your currency from the dropdown, enter the rate your jurisdiction allows, and the tool calculates the interest in that currency.
Related tools
Made for this page
Get the Small Business Invoice Aging Tracker
The same days-late math behind the interest, in a workbook that tracks every open invoice's balance and age bucket, so you know exactly which one to calculate interest on next.
Free download
Take a template with you
These tools run in your browser and keep nothing. If you want something that stays on your computer, there is a free invoice template for Excel: download it once, keep it forever.
Next step
Take it further, in a spreadsheet
These tools run in your browser and keep nothing. When you’d rather have the same thing as a file you own, pick the spreadsheet that matches how much you want to track.
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