Invoice Late Fee Calculator
What an unpaid invoice costs you per day, and when escalating starts to pay for itself. You supply every rate. Runs with no network at all - double-click this file.
Fill in the invoice and press Calculate.
The report prints the exact expression behind every number. Money is handled as whole cents (BigInt), never as decimal floating point.
Questions this page answers
How do I calculate a late fee on an overdue invoice?
You need three numbers: the amount owed, the day it became due, and the fee basis your own agreement states. This calculator takes a percent per year (simple or compounded), a percent per month, or a flat amount per day, and lets you set a 365 or 360-day year, calendar or 30-day months, and whether a partial month is charged in full, prorated, or ignored. Every figure in the report prints the exact expression behind it, so you can check the arithmetic line by line.
Is this late fee calculator free, and does it send my invoice details anywhere?
It is free and it makes no network request at all - the maths runs in the page, so you can save the file and open it offline. Nothing is uploaded, there is no account, and no amount, due date or invoice label leaves your machine.
What if my contract does not state a late fee?
Choose "no fee asserted" as the fee basis. The page still works: it shows the time value of the money you are waiting on and the cost of your own follow-up minutes, without inventing a charge you never agreed with the client.
Is the number it shows the money I am legally owed?
No. The output is arithmetic on the rates you typed in. Whether a late fee or statutory interest applies, and at what rate, depends on your contract and your jurisdiction, and the page says so plainly - it is not legal advice and not a finding that you are owed anything. Verify before quoting any figure to a client.
Next step, if the fee is not the issue but the silence is: what to send on day 1, day 7, day 11 and day 30.