How to Calculate Interest on Overdue Invoices & Statutory Late Penalties
Master commercial payment terms, calculate simple or compounding late fees accurately, and learn how international legal frameworks protect freelancers and agencies against unpaid bills.
Understanding Commercial Interest Formulae (Simple vs Compound)
When a client misses a payment deadline specified on your invoice (such as Net 15, Net 30, or Net 60 days), you have the legal right in most jurisdictions to charge an annual percentage rate (APR) interest on the outstanding principal balance.
1. Simple Annual Interest Formula
Interest = Principal × (Annual Rate / 100) × (Days Late / 365)
Simple interest adds a flat daily charge proportional to the annual percentage rate specified in your contract terms.
2. Daily Compounding Interest Formula
Total = Principal × (1 + Annual Rate / 365)^(Days Late)
Compounding interest adds yesterday's interest into today's principal balance, accelerating late fees over extended delays.
Global Statutory Late Payment Regulations
Prompt Payment Act & State Usury Caps
Under US commercial law, standard late fee clauses range between 1.5% per month (18% per annum) to 2.0% per month, provided it is clearly stipulated on the original invoice.
Late Payment of Commercial Debts Act
UK legislation allows businesses to charge 8% plus the Bank of England base rate on overdue B2B debts, along with fixed debt recovery compensation (£40 to £100).
Section 50 GST & MSMED Act 2006
Indian MSME registered businesses are entitled to 3x the RBI repo rate for delayed payments. GST late payment interest is charged at 18% per annum under Section 50.
Frequently Asked Questions: Overdue Invoice Interest
Can I charge interest if it wasn't on the original contract?
In jurisdictions with statutory late payment laws (like the UK, EU, and India MSMED Act), statutory interest applies by default. In the US, it is best practice to include "Payment Terms: Net 30. Late payments subject to 1.5% monthly interest" directly on all invoices.
How should I notify clients when late interest is added?
Use our built-in Overdue Email Template Generator to send a polite Level 2 Firm Notice or Level 3 Final Legal Demand attached with the recalculated total statement.