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
Due date and calculation date were carried over from the days past due calculator. You can edit them. Enter the amount, currency and interest rate yourself.
Worked example
1,000 EUR, 30 days late, 8% a year
Due date September 1, calculation date October 1 is 30 overdue calendar days. Simple interest at a rate you entered as 8% is 1,000 × 8% × 30 ÷ 365 = 6.58 EUR (rounded half up to the cent), so principal plus estimated interest is 1,006.58 EUR. The 8% is only an illustration, not a rate to use.
What this means for an overdue invoice
What this means for an overdue invoice
The original invoice stays exactly as issued. The estimate is context for your own decision and for the reminder you may prepare; it is not added to the invoice or to the reminder amount, and it does not show that interest is owed. If you need only the number of days late, use the days past due calculator.
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.
Where statutory rules come from: in the EU, late payment in commercial transactions between businesses and with public authorities is the subject of Directive 2011/7/EU, but the interest rates, deadlines and compensation that apply are set by each country's national law and by your contract. Look up the rule for your country before you enter a rate here. This page names the directive only for orientation and does not apply it.
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 InvoiceCraftly determine the legal interest rate?
No. You enter the rate. The tool never infers a rate from your country, currency, customer or dates, and it does not label the result legally owed or statutory interest.
Should I automatically add interest to a payment reminder?
Not by default. The calculation is an estimate, not proof that interest can be charged. When you continue to the payment reminder generator, the original amount is carried over and the estimate is shown only as context.
Does this add the amount to my invoice?
No. Review the estimate and enter any agreed amount on your invoice yourself.
Reviewed 2026-09-29. This is a deterministic simple-interest calculation using a rate you enter; it does not cite or apply any statutory rate.
Understand the related terms: Invoice and Payment reference.