Invoice Late Fee Calculator

Works out the late fee and total now due on an overdue invoice from the terms you put on it, and shows what that fee comes to as a yearly rate.

Your estimate

Late fee owed...

Total now due...

Added for each further day...

Fee as an annualized rate...

Estimates only. Assumptions are listed below, and you can change every input.

You put a late fee on your invoices so clients have a reason to pay on time. Then an invoice actually goes 30, 45 or 60 days past due and you need to state exactly what is owed. Doing that by hand means remembering whether your rate is monthly or yearly, whether it started at the due date or after a grace period, and whether a flat charge applies on top.

This calculator takes the terms as you wrote them and returns the fee, the total now due and how much more the invoice grows each day. It uses simple interest pro-rated by day on a 30-day month, does not compound, and does not apply any cap. It also shows the fee as an annualized rate, which is useful for checking whether your terms look reasonable and sit within the limits that apply to you before you send the revised invoice.

How to use this tool

Enter the original invoice amount and how many days it is past the due date, plus any grace period you offer before fees start.
Pick how your late fee works: a percentage per month, a flat charge, or both, and enter the rate and flat amount from your invoice terms.
Read the late fee, the total due today, the amount added for each further day, and the annualized rate to sanity check your terms.

What the math assumes

Interest is simple, not compounded: the monthly rate is pro-rated by day using a 30-day month and is charged on the original invoice amount only, never on earlier fees.
Days inside the grace period accrue nothing; the fee starts the day after the grace period ends.
A flat fee is charged once, as soon as the invoice is past the grace period, however late it then becomes.
The annualized rate is the fee divided by the invoice amount, scaled to 365 days; it is a comparison figure, not legal advice.
No fee cap or state maximum is applied. The default 1.5% and $25 are placeholders to replace with your own terms, not recommendations.

Frequently asked questions

Should I charge a percent per month or a flat fee?

A flat fee is simple and predictable on small invoices but can look punitive relative to the amount. A monthly percentage scales with the invoice and keeps growing, which matters more on large balances. Whichever you choose, it needs to be written into your terms before the first invoice goes out.

Does the calculator compound interest?

No. It charges the monthly rate on the original invoice amount only, pro-rated by day, and never charges interest on earlier fees. If your terms say fees compound, your actual figure will be slightly higher.

Can I legally charge any late fee I want?

No. Late fees generally need to be agreed in writing before the work starts, and some states cap the rate that can be charged on certain kinds of invoices. The annualized rate shown here helps you compare, but check the rules that apply to you or ask a lawyer.

What counts as the due date?

The date your payment terms set, for example 30 days after the invoice date for net 30. Days past due means days after that date, not days since the invoice was sent.

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