Silo 1 · Global Late Payment Law
How to Charge Late Fees on Invoices Legally
By Santanu Sarma — Mathematical Economics & Public Finance · Updated June 2026
A late fee is not automatically enforceable because you wrote it on an invoice. It is enforceable only if it clears three independent tests: prior disclosure, a reasonable relationship to actual harm, and — if it is structured as interest rather than liquidated damages — compliance with the usury ceiling of the governing state. Get any one of these wrong and a court can strike the entire charge, not just the excess.
Interest vs. Liquidated Damages: The Distinction That Decides Everything
US usury statutes regulate "the loan or forbearance of money." A late fee on a commercial invoice is not a loan — you never extended credit by agreement, the buyer simply failed to pay on time. Courts have consistently treated this distinction as outcome-determinative. New York's Department of Financial Services has stated the rule directly:
a late charge is considered liquidated damages, not interest, on a delinquent account
This reclassification matters because New York's civil usury ceiling under General Obligations Law § 5-501 caps loan interest at 16% per annum, with criminal usury at 25% under Penal Law § 190.40 — yet a properly drafted late fee can exceed both, because it is compensating for collection cost and capital displacement rather than charging for the use of money. The same logic underpins late-fee enforcement nationally. It is also why General Obligations Law § 5-521 matters for B2B invoicing specifically: corporations and LLCs cannot raise a usury defense at all, civil or otherwise, which is the default posture for almost every invoice this site generates.
The tradeoff: liquidated damages must still be reasonable. Courts borrow the U.C.C. § 2-718(1) standard by analogy even outside pure goods sales — a fee "grossly disproportionate" to the creditor's actual cost of delay is void as an unenforceable penalty, regardless of what the contract says.
The Three-Part Enforceability Test
| Test | Requirement | Failure Mode |
|---|---|---|
| 1. Prior Disclosure | Rate stated in the contract or on the original invoice, before the buyer accepted goods/services | Fee added retroactively after the due date passes — void for lack of consideration (pre-existing duty rule) |
| 2. Reasonableness | Rate approximates actual cost of delay (cost of capital, collections overhead) | Punitive multiples (e.g. 10%/month) reclassified as an unenforceable penalty |
| 3. Usury Compliance | Only applies if structured as interest rather than liquidated damages | Rate exceeds the state's interest ceiling with no liquidated-damages framing to fall back on |
The Math: Prorated Daily Interest
Whichever doctrine applies, the accrual math is identical. Late fees accrue daily, not as a flat monthly penalty, unless your contract explicitly states otherwise:
Interest Accrued = Principal × Daily Rate × Days Overdue
Worked example: A $3,200 invoice at the common 1.5%/month rate (18% APR), 60 days overdue.
Interest = $3,200 × 0.00049315 × 60 = $94.69
Note the convention: 18% ÷ 365 × 60 days, not 18% ÷ 12 × 2 months. Courts and collections software both default to the 365-day actual/actual method unless the contract specifies a 360-day banker's-year convention — which itself must be disclosed to be enforceable.
Simple vs. Compounding Interest
Absent an explicit compounding clause, US common law defaults to simple interest on commercial debts — interest calculated only on principal, never on previously accrued interest. If you want compounding (monthly is most common for invoice late fees), it must be stated in the originating contract:
where r is the annual rate, n the compounding periods per year, and t the time in years. Daily-compounding late fees are technically draftable but invite heightened judicial scrutiny under the reasonableness test above — most jurisdictions have never tested whether a daily-compounding invoice penalty survives a penalty-doctrine challenge, and as the party drafting the clause, you carry that litigation risk.
Usury Ceilings by Jurisdiction (Reference Table)
| Jurisdiction | Governing Statute | Commercial Ceiling | Key Note |
|---|---|---|---|
| California | Cal. Const. art. XV, § 1 | Greater of 10% or Federal Reserve discount rate + 5% | Most institutional lenders exempt; B2B late fees generally analyzed as liquidated damages |
| Texas | Tex. Fin. Code § 302.001 (legal rate); judgment rate per § 304.002 | 6% default legal rate; contract rate enforceable to the 18% judgment ceiling | Construction invoices fall under Tex. Prop. Code § 28.004 instead — fixed 1.5%/month |
| New York | GOL § 5-501; Banking Law § 14-a | 16% civil (individuals only) / 25% criminal (all parties) | Corporations cannot assert civil usury defense — GOL § 5-521 |
| U.S. Government as debtor | 31 U.S.C. § 3902 | 4.125% per annum (H1 2026, Treasury-set) | Statutory, not contractual — covered in our Prompt Payment Acts guide |
Drafting Checklist
- State the periodic rate (e.g. "1.5% per month") and the annualized equivalent — courts disfavor ambiguity between the two.
- Specify the accrual start date: invoice due date, delivery date, or a defined grace period.
- State simple interest explicitly, or define the compounding period if you intend to compound.
- Disclose the clause in the contract or signed terms — not solely on the invoice template generated after the fact.
- Cap the fee at a rate you can defend as reasonable relative to your actual cost of delayed payment, not an arbitrary round number.
None of this requires a separate collections contract. A single disclosure clause — present before the work begins — combined with the daily-proration formula above is sufficient in the overwhelming majority of US commercial disputes.
This article is informational and reflects general statutory mechanics as of June 2026. It is not legal advice. Usury and liquidated-damages doctrine vary by state and by transaction; confirm current statutory language and rates with counsel or your state's official code before relying on a specific figure.