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A client is past due. If your contract allows it, you can add a late charge — a one-time flat fee, or interest that accrues the longer they sit on it. This works out the fee and the new balance, and shows the math so you can put a defensible number on the follow-up.
Nothing you type leaves your browser. What late fee you can actually charge depends on your contract and local law — this is not legal advice.
Late fees do two jobs: they compensate you for being kept out of your own money, and — more usefully — they give a client a reason to pay now instead of whenever it's convenient. But a late fee is only collectible if your contract said it was coming, so the number you land on here is the arithmetic; whether you can charge it is a matter of what you agreed to and where you operate.
There are two ways freelance contracts usually write the charge, and this calculator does both:
The most common clause is a percentage per month — something like "1.5% per month on overdue balances." This tool treats that as simple interest: the fee is the rate applied to the original unpaid amount, scaled by how many periods (here, fractions of a month) have actually passed. So a $2,000 invoice at 1.5% per month that's 45 days late has been overdue for 1.5 months, and accrues 1.5% × 1.5 = 2.25% of $2,000 — a $45 late fee, for a new balance of $2,045. A "month" is counted as 30 days, a week as 7, a day as 1, so partial periods are prorated rather than rounded up to a whole one.
A flat fee ("$40 if not paid within 30 days") is simple, predictable, and reads as fair on a small invoice. Its weakness is that it stops mattering — once it's been charged, there's no further cost to the client for stalling another two months. A percentage per period keeps applying pressure: the longer the invoice sits, the more it grows, which is exactly the incentive you want on larger or chronically-late accounts. Some contracts combine the two (a flat fee and ongoing interest); if yours does, run each here and add them. Whichever you use, the rate has to be in the signed agreement to be enforceable — adding a fee a client never agreed to rarely survives a dispute.
For percentage interest, 1% to 1.5% per month (roughly 12–18% a year) is the range most freelance and B2B contracts use, and it reads as standard rather than punitive. Flat fees are usually a modest fixed amount scaled to the invoice. The ceiling, though, isn't set by what's "reasonable" — many places cap the maximum interest you can charge by law, so a rate that's fine in one state or country may be unenforceable in another. Check the limit where you operate before you set it.
Usually not — and that's the single most important thing to get right. A late fee is generally only collectible if the client agreed to it in writing before the work, typically in your contract or on the invoice terms they accepted. Adding a surprise fee after the fact is hard to enforce and can sour the relationship. The fix is to put a clear late-fee clause in every agreement going forward, so the charge this calculator produces is one you actually have the right to bill.
This calculator uses simple interest — the rate is applied to the original unpaid amount, not to a balance that itself grows each period. That's how the vast majority of late-payment clauses are written ("1.5% per month" almost always means simple), and it's the more conservative, easier-to-defend figure. Compounding overdue interest is unusual in freelance contracts and is restricted or prohibited in some jurisdictions, so unless your agreement explicitly says the interest compounds, simple is the right default.
No. Every calculation runs entirely in your browser — nothing you type is sent to a server or stored. The link in your address bar updates with your numbers so you can bookmark or share a scenario, but that link only contains the figures you chose.
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