Invoice late fee calculator ESTIMATE
Apply the late-fee math in your own terms: a flat fee, a one-time percent, or a percent per 30 days.
Result ESTIMATE
The three versions of the math
A flat fee ignores the balance and the day count: $25 is $25. A one-time percent multiplies the unpaid amount once: 1.5% of $1,000 is $15, whether the invoice is 10 days late or 90. A percent per 30 days is prorated here as amount × percent × (days ÷ 30). On $1,000 at 2% for 45 days, the fee is $30. That proration is a definition this calculator uses so the result is predictable. Your contract might instead bill a full 2% at each 30-day anniversary and nothing in between. If that is what you wrote, do not use the prorated mode.
Prorated fee = Invoice × (Percent ÷ 100) × (Days ÷ 30)
Book the fee only if you can collect it
Under the accrual idea, a late fee you have earned and expect to collect is other income, not a reduction of the original sale. Under the cash method, it is income when the customer pays it. Either way, it is not sales tax, and it should not be stuffed into the original revenue account if you want the profit and loss to show service revenue separately from finance income. The starter chart of accounts leaves room for other income. Write the policy on the invoice itself — the invoice guide lists the lines that belong there.
This is not a legal maximum
Some states cap consumer finance charges or require a disclosure before a late fee is enforceable. Business-to-business invoices have their own contract rules. Nothing on this page looks up your state. If you do not have a stated policy, the honest calculator result is zero until the terms exist.