Late Payment Interest Calculator

About the Late Payment Interest Calculator

When payments are overdue, the actual charge often depends on more than one annual rate. Some agreements use simple daily interest, others compound monthly or daily, many include a grace period before interest starts, and some layer on a flat late fee or a contractual cap on how much interest can accrue.

This calculator models those moving parts in one worksheet. It calculates the overdue interest, adds any flat late fee, applies an optional cap, and shows a 30-day accrual schedule so the numbers are easier to audit. It is meant as a planning and comparison tool, not as a statement of what is legally recoverable in every jurisdiction.

When This Page Helps

Overdue-payment clauses are easy to misread when compounding, grace periods, fees, and caps are buried in contract language. This page makes those assumptions visible so you can compare the different charging structures before using the number in a budget, demand, or negotiation.

How to Use the Inputs

1. Enter the overdue principal amount (the unpaid balance).
2. Enter the annual interest rate (check your contract or statutory rate).
3. Enter the number of days the payment is overdue.
4. Choose the compounding method and add any grace period, flat late fee, or interest cap.
5. Review the accrued interest, total penalty, and full balance due.
6. Use the 30-day schedule to compare how the charge grows over time.

Example Calculation

Tips & Best Practices

• Check your contract for the agreed-upon interest rate before using a statutory default rate.
• Statutory and contractual interest rules vary widely by jurisdiction and transaction type.
• Document the start date of the overdue period clearly to avoid disputes.
• Check whether the clause actually allows compounding before using a compounded method.
• Include per diem interest in demand letters so the debtor knows the cost of continued delay.
• Courts may reduce contractual interest rates they deem unconscionable.

Simple Versus Compounded Charges

Simple interest grows only from the original principal. Monthly or daily compounding adds prior interest back into the balance, which changes the path of the charge over time. This page lets you compare those methods directly instead of assuming every overdue-payment clause works the same way.

Why Grace Periods and Fees Matter

A short grace period can erase the first several days of accrual, while a flat late fee can matter more than the interest in short-delay scenarios. That is why the worksheet keeps the fee and the interest separate before combining them into the total penalty.

Use the Result as a Contract Worksheet

This page is useful for checking arithmetic and comparing clause structures. It does not determine which rate is legally permitted, whether compounding is enforceable, or whether a court will award the same amount.

Frequently Asked Questions

• What is a per diem interest rate?

  Per diem means "per day." The per diem reference is the annual rate divided by 365, applied to each day the balance remains unpaid. This page shows that simple daily reference even when you are comparing it against a compounded method.

• What is the statutory interest rate?

  That depends on the governing law, the type of obligation, and whether the charge is contractual, prejudgment, post-judgment, consumer, or commercial. This page helps with the math once you know the applicable rate; it does not determine which jurisdictional rate applies.

• Is this simple or compound interest?

  The page can model simple interest, monthly compounding, or daily compounding. Simple interest is still common in many overdue-payment and judgment contexts, but some contracts expressly allow compounding.

• Can I charge interest on top of a late fee?

  Sometimes, but not automatically. Whether both charges can be collected depends on the contract, the governing law, and any reasonableness, usury, or consumer-protection limits that apply to the transaction.

• How do I calculate per diem interest for a court judgment?

  If a judgment or contract uses simple daily accrual, multiply the principal by the applicable annual rate, divide by 365, and multiply by the number of chargeable days. This page does not determine which legal rate applies; it only helps with the arithmetic once you know it.

• Does interest accrue on weekends and holidays?

  Yes, interest accrues every calendar day, including weekends and holidays. Unlike business-day calculations, interest runs continuously from the date the payment became overdue.