Late Payment Fee Calculator
Calculate late payment fees on overdue invoices
Enter the invoice amount, days overdue, your annual interest rate, and any flat penalty fee to calculate total amounts owed.
How late payment fees work and when to apply them
Late payment fees are charges added to an invoice when a customer pays after the agreed due date. They serve two purposes: they compensate you for the cost of delayed cash and they create a financial incentive for customers to pay on time. Without any consequence for late payment, some customers will consistently delay, treating your invoice as an interest-free loan.
There are two common structures for late payment fees. An interest-based fee applies a daily or monthly rate to the outstanding balance for each day it is overdue. An annual rate of 18 percent, for example, works out to approximately 0.049 percent per day. A flat fee is a fixed amount added to the invoice regardless of how many days late the payment is. Some businesses use both: a flat administrative fee for any late payment plus daily interest on the outstanding balance.
Before applying late payment fees, you must ensure they are disclosed in your payment terms and agreed to by the customer at the outset of the relationship. Including late payment terms in your original invoice, contract, or engagement letter is essential. Surprising a customer with a fee they did not know about will damage the relationship and may not be legally enforceable depending on your jurisdiction.
What annual interest rate is standard for late payments?
In the United States, rates typically range from 1 to 1.5 percent per month, which equals 12 to 18 percent annualised. In the United Kingdom, the Late Payment of Commercial Debts Act sets a statutory rate of 8 percent above the Bank of England base rate. In Australia, the Penalty Interest Rate Act sets a statutory rate that varies quarterly. You can charge higher or lower rates than statutory defaults if agreed in writing, within legal limits.
Should I actually charge late payment fees?
This is as much a relationship decision as a financial one. Many businesses include late payment terms in their contracts but choose not to enforce them for good long-term clients who have a genuine one-off delay. However, for clients who consistently pay late, enforcing late payment fees signals that you take your terms seriously and may change behaviour. Some businesses find that merely mentioning the fee on the invoice causes late payers to prioritise payment.
How does this calculator work?
The interest fee is calculated as: invoice amount multiplied by the annual rate divided by 365, multiplied by the number of days overdue. This gives you the simple interest charge for the overdue period. If you also enter a flat fee, it is added on top. The total amount owing is the original invoice plus both fee components.
Can I waive a late fee once charged?
Yes. Many businesses calculate the fee, show it on a revised invoice, and then offer to waive it in exchange for immediate full payment of the original amount. This approach preserves the relationship while still sending a firm message about payment expectations. Document any waiver in writing so there is no ambiguity about what was agreed.
