A late payment interest clause tells a debtor exactly what it costs to pay you late: the rate, when it starts running, and how it is calculated. To hold up in court, the clause needs four specifics locked in, a rate that stays below your state’s usury cap, and a number that a judge would accept as a reasonable estimate of your loss rather than a punishment. Get any of those wrong and the clause can be reduced, struck, or, in some states, cost you the interest entirely.
The Four Elements Every Clause Needs
Ambiguity in any of these gives a court room to rewrite or void the provision, so pin each one down in the contract itself.
The Rate
State an annual percentage. Contracts either fix the rate (1.5% per month, or 18% per year) or peg it to a benchmark such as a set number of points above the prime rate or the weekly average one-year Treasury yield published by the Federal Reserve. Floating rates keep multi-year agreements current when market conditions shift. Whichever method you use, the rate has to stay inside the usury ceiling of the governing state.
The Trigger Date and Grace Period
Say exactly when interest begins. That can be the day after the invoice due date or a set number of days later. Commercial contracts often build in a grace period of 10 to 20 days, and consumer credit rules usually require one. For high-cost mortgages, a lender generally cannot assess a late charge until at least 15 days after the payment due date.1Consumer Financial Protection Bureau. Truth in Lending Act (Regulation Z) A consumer clause with no grace period risks being struck as unfair.
The Day Count Convention
Interest math depends on how many days you treat as a year. The two options are actual/365 and actual/360. A 360-day year produces a slightly higher daily rate for the same headline annual percentage, so the choice is not cosmetic. Short-term dollar-denominated instruments traditionally use 360; many other contracts default to 365. Name the convention in the clause. Leaving it out invites disputes over basis points that add up on large balances.
Simple or Compound Interest
State whether interest compounds, and if so, how often. Simple interest accrues only on the original overdue amount. Compound interest recalculates each period on principal plus previously accrued interest, and the balance grows faster. Some states prohibit compounding on consumer contracts, so a compound clause in a consumer deal may not survive. Federal post-judgment interest, by contrast, compounds annually by statute.2Office of the Law Revision Counsel. United States Code Title 28 – 1961
Staying Under the Usury Cap
Every state caps interest, and the caps vary enough that a rate legal in one state can be void in another. Consumer and commercial deals are usually treated differently, with tighter limits on consumer debt.
General consumer usury ceilings typically fall between 6% and 15% per year, with 10% the most common default. Those headline numbers can mislead, though, because many states set higher ceilings for specific products such as small-dollar loans or retail installment contracts, where caps run from 16% to well above 30%. Several states use floating caps tied to Treasury bill rates or the Federal Reserve discount rate, so the limit moves quarterly or annually.
Commercial contracts often get more room. Some states exempt business-to-business loans from general usury caps altogether, letting the parties agree on any rate. Where caps do apply commercially, the range runs from roughly 5% to 25%, clustering around 10% to 12%. When a contract crosses state lines, the choice-of-law provision decides which cap governs, so that clause deserves as much attention as the rate itself.
Charging above the cap can cost more than just the excess. Depending on the jurisdiction, the creditor may forfeit all interest on the debt, not only the amount over the ceiling. Federal law governing national banks lets a borrower who paid usurious interest recover twice the interest paid if the suit is filed within two years.3Office of the Law Revision Counsel. United States Code Title 12 – 86 Usurious Interest Penalty for Taking Some state statutes void the entire loan, principal included. A court can also declare an excessive rate unconscionable and strike it while enforcing the rest of the agreement.
The Penalty Problem
A rate that clears the usury cap can still be thrown out if a court decides it functions as a penalty rather than a reasonable estimate of the creditor’s loss. This catches drafters off guard. Under the Restatement (Second) of Contracts, a clause fixing damages for breach is enforceable only if the amount is reasonable relative to anticipated or actual loss, considering how hard the real damages would be to prove.4H2O (Harvard Law School). Restatement Second Contracts 356
In practice, a late payment rate of 24% per year on a commercial invoice where the creditor’s actual cost of capital is 6% can be struck as a penalty even if state usury law allows the rate. Courts look at whether the number was a genuine pre-estimate of harm or a threat to coerce timely payment. Flat late fees get the same scrutiny; a $500 fee for being one day late on a $1,000 invoice is almost certainly a penalty.
The safer path is to tie the rate to something concrete, such as the creditor’s borrowing cost plus a reasonable margin, and to document the reasoning. A paper trail showing how you arrived at the number improves the odds of enforcement if a judge ever has to look at the clause.
Doing the Math
Once the rate, trigger, and day count are set, calculation is straightforward. Simple daily interest is the most common commercial method.
Simple Interest
Divide the annual rate by 365 or 360, then multiply by the outstanding principal and the number of days past due. On a $10,000 balance at 10% per year using a 365-day convention:
- Daily rate: 10% ÷ 365 = 0.0274% per day
- Daily charge: $10,000 × 0.000274 = $2.74
- 30 days late: $2.74 × 30 = $82.19
- 90 days late: $2.74 × 90 = $246.58
Switch to a 360-day year at the same rate and the daily charge becomes $2.78, or $83.33 over 30 days. Small on $10,000. On a $500,000 invoice, the 360-day convention adds roughly $700 more over 90 days. That is why the convention belongs in the contract.
Compound Interest
Compounding recalculates the balance at each interval, so interest accrues on prior interest. Monthly compounding on the same $10,000 at 10% per year:
- Month 1: $10,000 × (10% ÷ 12) = $83.33 → balance $10,083.33
- Month 2: $10,083.33 × (10% ÷ 12) = $84.03 → balance $10,167.36
- Month 3: $10,167.36 × (10% ÷ 12) = $84.73 → balance $10,252.09
After three months, compounding produces $252.09 against $246.58 simple. The gap widens on larger balances and longer delinquencies. Check local law before writing a compound clause into a consumer contract.
Late Fees Are Separate
Flat late fees and running interest serve different purposes and belong in separate lines. A late fee is a one-time charge when a payment crosses the deadline. Late interest is a daily charge that grows until the balance clears. Some contracts include both. When you send a demand letter, break the total into original principal, accrued interest (with rate, start date, and days), and any flat fee. A clean breakdown is easier to enforce and harder to dispute.
Consumer Contract Rules to Watch
Late interest in consumer transactions triggers federal disclosure requirements. For credit card accounts, Regulation Z requires every periodic statement to show the payment due date, the amount of any late fee, and any increased interest rate that will apply on a late payment.5eCFR. 12 CFR 1026.7 – Periodic Statement Changing a penalty fee or rate counts as a significant change to account terms and requires 45 days’ written notice.1Consumer Financial Protection Bureau. Truth in Lending Act (Regulation Z) For closed-end mortgages, the Loan Estimate must state the late charge as a dollar amount or percentage and the number of days a payment must be late before the charge applies.
The FTC’s Credit Practices Rule also prohibits pyramiding for creditors under FTC jurisdiction, where a single missed late fee is treated as a shortfall on the next timely payment, generating a new late fee on that period and every one after.6Federal Trade Commission. Complying with the Credit Practices Rule A late payment clause that permits this pattern is unenforceable to that extent.
What Happens If You Leave the Clause Out
Silence on late interest does not mean the creditor recovers nothing. Every state has a default rate that kicks in when money is owed and the contract sets no rate of its own. These statutory rates typically run from about 5% to 12% per year. Some states fix a flat number (California 7%, Connecticut 10%, Idaho 12%); others use a formula tied to the Federal Reserve discount rate or Treasury yields, recalculated quarterly or annually.
Federal courts apply their own default on money judgments: the weekly average one-year constant maturity Treasury yield for the week before the judgment, compounded annually.2Office of the Law Revision Counsel. United States Code Title 28 – 1961 That rate moves with the market, so interest on a federal judgment entered in January can differ from one entered in June.
Under the Uniform Commercial Code, a seller whose buyer refuses to pay for accepted goods can recover the contract price plus incidental damages, which courts have read to include finance charges the seller incurred while waiting. The UCC sets no specific rate, so the state default fills the gap. You can still claim interest without a clause, but you will almost always recover less than a well-drafted clause would have given you. That gap is the reason to write one.