This late payment interest calculator finds the interest owed on an overdue amount from the annual rate and the number of days late. Enter the values above.
Formula
Interest = Amount × Annual rate × Days ÷ 365
Example
$1,000 overdue, 10% annual, 30 days late: interest = 1,000 × 0.10 × 30/365 = $8.22.
Good to know
These results are estimates for educational purposes only and are not financial advice. Rates, fees, and terms vary — confirm figures with a licensed professional before making decisions. Last updated: August 2026.
Frequently Asked Questions
How do I calculate late payment interest?
Multiply the overdue amount by the annual rate and by the days late over 365.
What late interest rate can be charged?
It depends on the contract and local law; many invoices use a stated annual rate or a statutory maximum.
Is late interest simple or compound?
Usually simple interest over the overdue period, unless the agreement says otherwise.