Invoice Late Fee Calculator

An invoice late fee calculator tells you exactly how much is owed on an overdue invoice today, whether your terms use a flat one-time fee or a monthly percentage rate prorated by the day. Enter the invoice amount, due date and rate, and it calculates the days overdue and the total now due automatically.

How the calculation works

For a percentage rate, the calculator prorates your monthly rate by the number of days overdue divided by 30, then multiplies that by the invoice amount. For a flat fee, it simply adds the one-time amount you set once the invoice is at least one day overdue. Days overdue is the exact difference between your due date and the date you are checking as of, so the fee updates precisely as more days pass.

Percentage rate versus flat fee

A percentage rate scales with the invoice size, which suits larger recurring B2B invoices where a flat amount could feel arbitrary either way. A flat fee is simpler to communicate and common on smaller invoices where a percentage would barely register, and it is what many invoicing tools and templates default to. Many freelancers and small businesses simply use whichever their invoice template or accounting software already supports.

What this calculator does not cover

This tool does not apply simple interest that compounds on itself over multiple months, and it does not enforce any legal cap on what you can charge. Late fee limits vary by jurisdiction, contract type, and sometimes by invoice size, and many places distinguish between consumer and business invoices. Confirm your own contract terms and local or state law before charging a late fee; this calculator only does the arithmetic once you tell it the rate.