Past-Due Invoice Interest Calculator

Enter the invoice amount, your annual interest rate, and how many days the payment is late. The calculator applies simple daily interest, adds any flat late fee, and shows the new total owed — updating live as you type.

📖 How it works & FAQ

Invoice amount ($)

Annual interest rate (%)

Days past due

Flat late fee ($, optional)

How late-payment interest is calculated

This calculator uses the simple daily interest method most small businesses put in their payment terms: the invoice amount is multiplied by a daily rate (your annual rate divided by 365), then multiplied by the number of days the payment is overdue. Interest = amount × (annual rate ÷ 365 ÷ 100) × days late. Because it is simple interest, nothing compounds — a $2,500 invoice at 18% per year accrues the same $1.23 each day whether it is 10 days late or 100. If your terms also include a one-time late fee, add it in the flat fee box and it is stacked on top of the accrued interest to produce the new total owed. These figures are estimates only, not professional, financial, tax, or legal advice; allowable rates & fees vary by state, country, and contract.

How to use it

Enter the original invoice amount in dollars.
Enter the annual interest rate from your payment terms — 18% per year (1.5% per month) is a common benchmark.
Type the number of days the invoice is past its due date.
Add a flat late fee if your contract charges one, or leave it at zero.
Read the results instantly: interest accrued, the fee, the new total owed, and the daily rate being applied.

FAQ

What interest rate can I legally charge?: It depends on your jurisdiction and contract. Many businesses use 1–2% per month, but state usury laws & local rules cap what is enforceable. Whatever you charge should be written into your payment terms before the sale.
Is this simple or compound interest?: Simple. Interest accrues only on the original invoice amount, never on previously accrued interest. That matches how most late-payment clauses are written and keeps the math easy to defend.
When do the days start counting?: Typically the day after the due date on the invoice. If terms are Net 30 and the invoice was issued June 1, day one of lateness is July 2.
Does my data leave the browser?: No. Everything is computed locally on your device — nothing is uploaded, stored, or shared.