Formula
How the estimate is calculated
The tool uses simple interest on an actual/365 calendar-day basis: principal × your annual rate × (overdue calendar days ÷ 365). It does not compound and does not add collection fees.
Free overdue interest estimate
Late payment interest calculator
Estimate simple overdue interest on a late invoice. You supply the annual interest rate; this tool does not determine whether interest is legally due or pick a statutory rate for you.
Overdue interest
Enter the overdue invoice details
You choose the rate
You supply the interest rate
You enter the annual interest rate you want to use. The calculator does not select a statutory rate for you, and it does not distinguish B2B, B2C, or public-authority rules automatically.
Before you invoice
Confirm before charging a customer
This estimate does not determine whether interest is legally due for your situation. Confirm the applicable rate and rules with your customer agreement, local law, or an adviser before charging interest, and enter any agreed amount on your invoice yourself.
Late payment interest FAQ
Does this tell me the legal interest rate?
No. You enter the rate you want to use. The calculator does not choose a statutory rate or determine whether interest is legally due for your transaction.
Does it handle B2B, B2C, or public-authority rules?
No. Those distinctions vary by jurisdiction and contract, so confirm them separately before charging interest.
Does it add collection fees or compound interest?
No. The estimate is simple interest only, with no added fees and no compounding.
Does this add the amount to my invoice?
No. Review the estimate and enter any agreed amount on your invoice yourself.
Understand the related terms: Invoice and Payment reference.