Invoice Late Fee Calculator

How to use this calculator

Enter the amount still outstanding on the invoice — the original total minus anything already paid. Then enter how many days late the payment is, counting from the day after the due date you gave the client. Finally, enter the annual late-fee interest rate you specify in your own terms.

That last input matters. This tool does not know, and does not guess, what rate you are entitled to charge. Late payment rules differ by country and by contract, and a rate only binds a client if it was agreed before the work started — typically as a line in your terms and conditions and repeated on the invoice itself. Enter the rate you actually wrote down. If you never wrote one down, treat the result as a private estimate of what the delay has cost you rather than something you can invoice.

The calculation is simple interest: the annual rate divided by 365 to get a daily rate, then multiplied by the days overdue. It doesn't compound, which keeps the figure conservative and easy to defend in an email.

What the output means

The headline is the total you'd ask for today: the original invoice plus accrued interest. The daily interest line is the most useful number in the breakdown, because it tells you what one more day of silence costs — and it's an effective thing to state plainly when you follow up. "The balance accrues $0.82 per day from the due date" reframes the conversation from a favour to an arithmetic fact.

The monthly equivalent shows the same rate expressed the way many contracts write it (for example, an annual 18% is 1.5% per month). If your terms are written monthly, multiply that figure by twelve before entering it here.

Getting paid, not just calculating

A late fee is leverage, not revenue. Most freelancers who successfully collect overdue money do it with a predictable sequence rather than an angry email: a polite reminder on the due date, a firmer one a week later restating the terms and the accruing fee, a formal notice at thirty days with the total from this calculator attached, and a decision at sixty about whether to escalate, pause work, or write it off and stop taking that client's jobs.

You always retain the choice to waive the fee. Waiving it explicitly — "I'll drop the $61 in interest if the balance clears this week" — is often more effective than never having charged it, because it gives the client a reason to act now. What you can't do is invent a fee at the end of a job that was never mentioned at the start, which is why the single most valuable habit here is putting your payment terms in writing before the first hour of work.

This calculator performs arithmetic only. It is not legal advice on what you may charge in your jurisdiction.