How to Calculate Invoice Late Fees: Formulas and Examples

The standard late fee is 1.5% per month (18% annually) on the unpaid balance — so a $2,000 invoice 45 days late accrues $45. The exact formula depends on whether you charge monthly, daily, or flat fees. Get the number instantly with our invoice late fee calculator; the full math is below.

The three ways to calculate a late fee

Every late fee policy boils down to one of three formulas, and you should pick the one that matches what your contract actually says. Method one: monthly percentage. Late fee = invoice amount x monthly rate x months overdue. At the standard 1.5% per month, a $3,000 invoice that is 45 days late (1.5 months) accrues $3,000 x 1.5% x 1.5 = $67.50. Method two: daily rate from an annual rate. Late fee = invoice amount x (annual rate / 365) x days overdue. A $5,000 invoice at 12% annually, 45 days late: $5,000 x 0.12 / 365 x 45 = $73.97. Method three: flat fee. A fixed amount per month late, regardless of invoice size — $25 or $50 per month is common. The monthly-percentage method dominates professional services and consulting; the daily-rate method suits businesses quoting annual interest rates in their terms; flat fees work best when your invoices are small, because 1.5% of a $200 invoice is only $3 — too small to motivate anyone. Some freelancers combine approaches: '1.5% per month or $25, whichever is greater.' Whatever you choose, the calculation must match your written terms exactly, or the fee will not survive a dispute. Skip the manual math and run your numbers through our invoice late fee calculator, which handles all three methods.

What rate is standard — and what is legal

The most common late fee in freelance and small-business invoicing is 1.5% per month, equivalent to 18% annually. It is widely recognized, easy to calculate, and sits inside the legal limits of most US states. Construction and B2B services often charge 2% per month (24% annually), which is defensible when payment cycles run long and project financing is expensive. But your ceiling is not your preference — it is your state's usury law. Most states cap interest on commercial debts somewhere between 10% and 25% annually, and a handful of states set much lower ceilings for consumer transactions. If your 2%-per-month clause pushes past your state's cap, the entire fee is unenforceable and in some states the clause can be voided altogether. The safe play: default to 1.5% per month unless you have checked your state's specific cap, and note that some states also require specific disclosure language for the fee to stick. Our state-by-state late fee law guide walks through where the ceilings sit; if your invoices go to clients abroad, the UK's Late Payment of Commercial Debts Act sets a statutory rate of 8% above the Bank of England base rate plus fixed compensation of 40 to 100 pounds.

Worked example: a $2,000 invoice, 45 days late

Run the full lifecycle of one overdue invoice so the mechanics are concrete. You send a $2,000 invoice on March 1 with Net 30 terms, making it due March 31. The client pays nothing. On April 15 — 15 days late — your first reminder goes out with the late fee accruing. On May 15, the invoice is 45 days late, or 1.5 months. At 1.5% per month: $2,000 x 1.5% x 1.5 = $45.00 in late fees, so the total now due is $2,045. If your terms instead use a $35 flat monthly fee, the charge is $35 x 1.5 months = $52.50 (round partial months up if your contract says so — many do). Now the compounding question: most small-business terms use simple interest, meaning the fee accrues only on the original $2,000. If your contract specifies monthly compounding, month two's fee is calculated on $2,030, adding $30.45 instead of $30 — a small difference at 45 days but a meaningful one at 180. When you issue the revised invoice, add the fee as its own line item ('Late fee: 45 days at 1.5%/month — $45.00') and reference the original invoice number and the clause that authorizes it. To pin down exact day counts between the due date and payment, our Unix timestamp converter turns both dates into comparable numbers. For numbering the revised invoice cleanly, see how to number invoices for small business.

Simple vs compound interest on overdue invoices

The difference between simple and compound late fees seems academic until a client is six months late. With simple interest, the fee each period is always calculated on the original invoice amount: a $5,000 invoice at 1.5% monthly accrues $75 every month, so after 6 months the fee is $450. With monthly compounding, each month's fee is calculated on the growing balance: month one is $75, month two is $76.13, and after 6 months the total fee is about $467 — roughly 4% more. Stretch that to a year and compounding adds nearly 8% more than simple interest. Most courts and most small-business contracts default to simple interest unless the agreement explicitly says the fee compounds, and several states restrict compounding on late fees as a penalty. The practical advice: use simple interest, state it plainly in your terms ('late fees accrue as simple interest, not compounded'), and you will never have to defend your math in a dispute. The bigger lever is not the interest method but enforcement — a 1.5% fee you actually charge beats a 2% fee you waive every time. Our invoice late fee calculator shows both simple and compound totals so you can compare before you write your terms.

When to charge, when to waive, and how to collect

Having a late fee clause and using it well are different skills. Charge it when the client is habitually late, when the delay is long (30+ days), or when the invoice is large enough that the wait genuinely costs you — the fee exists to change behavior, and waiving it for a chronic late payer teaches them that your terms are decoration. Waive it for a first-time delay of a few days from a good client, when the relationship is worth more than the fee, or when the late payment was arguably your fault (unclear invoice, wrong amount). The middle path — and the professional one — is to state the fee on every reminder but offer to waive it for prompt payment: 'A late fee of $67.50 has accrued per our agreement; pay by Friday and I'll remove it.' That preserves the relationship and the incentive. For collection mechanics: add the fee as a line item on a revised invoice, send it with a reference to the original terms, and keep every reminder in writing. If 90 days pass with no payment, the fee has done its job of documenting the debt — at that point you are choosing between a collections agency (typically 25 to 40% of the recovered amount) and small claims court. For keeping your books clean through all of this, how to write an invoice covers the fundamentals that prevent disputes before they start, and our word counter can help keep your reminder emails short and firm.