Overdue bills

Late Payment Interest Calculator

Find out exactly how much interest and fees have accrued on an overdue invoice or bill, based on the annual rate and number of days late.

—Total amount owed
—Interest accrued
—Interest per day

Calculated on your device. Nothing is uploaded or stored on a server.

How to use the late payment interest calculator

Enter the overdue amount, the annual interest rate that applies (check your contract, invoice terms, or statutory late-payment rate), how many days the payment is late, and any flat late fee charged separately from interest.

How late payment interest accrues

This calculator uses simple daily interest: it converts your annual rate into a daily rate (dividing by 365) and multiplies by the overdue amount and the number of days late. Any flat late fee is added on top, since fees and interest are usually charged separately.

Check your actual terms

Some contracts compound interest, use 360 days a year, or apply a grace period before interest starts. This tool models the common simple-daily-interest case — confirm your specific terms for an exact figure.

Fees vs. interest

A flat late fee is a one-time charge regardless of how late the payment is, while interest keeps growing every additional day the balance goes unpaid.

Late payment interest calculator FAQ

Common questions, answered.

How is late payment interest calculated?

Interest = overdue amount × (annual rate ÷ 365) × days late. This is simple daily interest on the outstanding amount, added to any flat late fee.

What interest rate should I use?

Use the rate specified in your invoice, contract, or loan terms. Many jurisdictions also set a default statutory interest rate for late commercial payments if none is specified.

Does interest compound if I stay late longer?

This calculator uses simple (non-compounding) daily interest. Some agreements do compound overdue interest — check your specific terms if the debt has been outstanding for a long period.

Can I use this for invoices I'm owed, not just bills I owe?

Yes, the same calculation works whether you're the one who is late paying or you're calculating interest owed to you on a late-paying customer's invoice.

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