Late Payment Interest Calculator
Our interest & day count calculator computes late payment interest instantly. Get useful results with practical tips and recommendations.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Interest = Principal x Rate x (Days Overdue / Days in Year)
Simple late payment interest is calculated by multiplying the unpaid amount by the annual interest rate and the fraction of the year the payment is overdue. The day count convention determines whether the year divisor is 360 or 365 days. Compound interest uses the formula: Interest = P x (1 + r/n)^d - P.
Worked Examples
Example 1: Standard Commercial Invoice 60 Days Late
Problem:A $5,000 invoice due on January 15 is paid on March 16 (60 days late). The contract specifies 8% annual interest using actual/365 day count with simple interest.
Solution:Principal: $5,000\nAnnual rate: 8%\nDays overdue: 60\nDay count basis: Actual/365\nInterest = $5,000 x 0.08 x (60 / 365)\nInterest = $5,000 x 0.08 x 0.16438\nInterest = $65.75
Result:Interest Owed: $65.75 | Total Due: $5,065.75
Example 2: Large Invoice with Flat Penalty and 30/360 Convention
Problem:A $25,000 invoice due on March 1 is paid on June 1 (92 days late). Rate is 12% annual with 30/360 convention, simple interest, plus a $100 flat penalty.
Solution:Principal: $25,000\nAnnual rate: 12%\nDays (30/360): 30x(6-3) + (1-1) = 90 days\nInterest = $25,000 x 0.12 x (90 / 360)\nInterest = $25,000 x 0.12 x 0.25\nInterest = $750.00\nFlat penalty: $100\nTotal additional: $850.00
Result:Interest: $750.00 | Penalty: $100 | Total Due: $25,850.00
Frequently Asked Questions
How is late payment interest calculated?
Late payment interest is calculated by applying an annual interest rate to the unpaid invoice amount for each day the payment is overdue. The basic formula for simple interest is: Interest = Principal x Annual Rate x (Days Overdue / Days in Year). For example, a $5,000 invoice that is 60 days late at an 8% annual interest rate would accrue approximately $65.75 in interest using a 365-day year. The calculation can vary depending on the day count convention used (actual/365, actual/360, or 30/360), whether interest compounds, and any flat late fees that may also apply. Many commercial contracts and jurisdictions specify which calculation method must be used to ensure consistency and legal enforceability of the charges.
What is a typical late payment interest rate for commercial invoices?
Late payment interest rates vary by jurisdiction, contract terms, and the type of transaction. In the United States, commercial contracts typically specify rates between 1 and 2 percent per month, which translates to 12 to 24 percent annually. Many states have usury laws that cap the maximum allowable rate, typically between 6 and 25 percent per year. In the European Union, the Late Payment Directive sets a statutory interest rate equal to the European Central Bank reference rate plus 8 percentage points for commercial transactions. In the United Kingdom, statutory interest for late commercial payments is set at 8 percent above the Bank of England base rate. If no rate is specified in a contract, the applicable statutory rate for the jurisdiction typically applies automatically.
Can I charge both late payment interest and a flat penalty fee?
Yes, many commercial contracts include both a flat late payment fee and ongoing interest charges, and this is generally permissible under most legal systems as long as the combined charges are reasonable and not considered punitive. A common structure is a flat fee of 25 to 50 dollars or 1 to 5 percent of the invoice amount as an administrative charge, plus a per-day interest rate on the outstanding balance. However, the enforceability of these charges depends on several factors including whether they were clearly disclosed in the contract, whether they comply with applicable usury laws, and whether they are proportionate to the actual damages suffered. Some jurisdictions require specific notice before late charges can be imposed, and consumer transactions are typically subject to stricter rules than commercial ones.
What is the legal basis for charging late payment interest?
The legal basis for late payment interest comes from multiple sources depending on the jurisdiction and context. Contractual provisions are the most common basis, where the original agreement explicitly states the interest rate and conditions for late payments. Statutory provisions provide default interest rates when contracts are silent on the matter. In the United States, the Uniform Commercial Code governs commercial transactions, while individual state statutes address maximum allowable rates. The European Union Late Payment Directive (2011/7/EU) establishes minimum rights for creditors in commercial transactions across all member states. Common law also provides remedies for creditors through breach of contract claims and equitable principles. To ensure enforceability, businesses should always include clear late payment terms in their contracts, invoices, and purchase orders.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy