Invoice Late Fee & Interest Calculator

Invoice Late Fees: Fair Terms That Get You Paid

The invoice went out on March 1st with Net 30 terms. By June, the client still had not paid. When the collections call finally happened, they disputed the $847 in late fees and interest. No one could explain where the number came from because the bookkeeper had used three different methods across four months. The client paid the original $12,000 and nothing more.

Clear late fee policies protect cash flow and maintain relationships. This calculator shows exactly how flat fees, percentage charges, and interest compound over time so you can quote precise figures, include correct amounts on statements, and defend your numbers if challenged. Slow-paying customers don't just cost you interest, they tie up cash you were counting on. If stretched receivables are eating your runway, model the effect in the cash burn and runway calculator.

Late Fee vs Interest

Late payment charges come in two forms that serve different purposes and accrue differently.

Flat and Percentage Fees

One-time charges applied after the grace period expires. Flat fees work well for small invoices where a percentage would yield trivial amounts. Percentage fees scale with invoice size. Many policies use both: "$25 or 2% of invoice, whichever is greater."

Interest Charges

Ongoing charges that accrue daily for as long as the balance remains unpaid. Interest compensates you for the cost of money tied up in receivables. Common B2B terms state "1.5% per month" which equals 18% APR.

Combined Structures

Professional policies often combine a one-time fee plus ongoing interest. The initial fee covers administrative cost of the late payment. The interest compensates for continued delay. Both must be disclosed in your original terms.

How Late-Fee Interest Is Calculated

Late-fee interest is a daily rate applied to the unpaid balance for the number of days the invoice sits past due. You need three numbers: the balance, the annual rate (APR), and the days late after any grace period. Everything else is one of three formulas.

Start by converting the APR to a daily rate. Divide by 365.

Work the standard case. A $10,000 invoice at 18% APR, 30 days late, simple interest. The daily rate is 0.18 / 365 = 0.000493. Multiply by the balance and the days: 10,000 x 0.000493 x 30 = $147.95. That is the figure a search for "0.18/365" is trying to reproduce, and it is the number the calculator above returns.

Compounding changes little over a month and more over a quarter. On the same invoice, daily compounding gives $149.01 at 30 days, about a dollar more than simple. The classic B2B term "1.5% per month" is the same 18% APR expressed monthly. Use simple interest when you want a number that is easy to defend in a reminder email. Reserve compounding for accounts that have run long past due.

Daily vs Monthly Compounding

How often interest compounds affects the total amount owed, especially on long-overdue invoices.

Avoiding Disputes

Disputes over late fees waste time and damage relationships. Most can be prevented with clear documentation and consistent application.

Disclose Terms Before the Transaction

Late fee policies must appear in your contract, service agreement, or terms of service before work begins. Fees added after the fact are often unenforceable. Include specific language: "Payments received more than 5 days after due date are subject to a $25 late fee plus 1.5% monthly interest on the unpaid balance."

Print Terms on Every Invoice

Each invoice should restate your late payment terms. Customers process many invoices and may not remember contract language. A line at the bottom reminding them of consequences reduces late payments and supports enforcement.

Send Itemized Statements

When adding late charges, provide a clear breakdown: original invoice amount, days late, flat fee applied, interest calculation, and total due. A single unexplained number invites questions. An itemized statement shows exactly how you arrived at the figure.

Apply Policies Consistently

Waiving fees for some customers but not others creates legal risk and relationship problems. If you waive fees, document the reason. Better yet, apply the same policy to everyone and negotiate upfront on payment terms rather than after-the-fact on penalties.

Legal Limits Vary by State

There is no single national ceiling. Usury caps are set state by state, and they differ for consumer versus commercial transactions. A rate that is routine in one state can be unenforceable in another, and charging above the cap can cost you the right to collect any late fee at all. Some states void the entire debt when usury is proven. Consumer transactions face stricter limits than business-to-business terms, and many states exempt commercial loans over a size threshold entirely.

The per-state table below gives the starting figure and the statute for each state, and the calculator flags a rate that runs past the cap for states that set a firm one. Fees a court reads as "disproportionate to actual damages" can still be voided even when the rate is legal, so keep the charge tied to the real cost of the delay.

Example Invoice Calculations

Sources

• SBA: Managing Business Finances and Receivables
• FTC: Fair Debt Collection Practices Act
• CFPB: Debt Collection and Interest Regulations
• UpCounsel: State Interest Rates and Usury Limits (50-state survey), cross-referenced with the state statutes named in the per-state table.

Late Fee Interest Caps by State

The rate you can charge on an overdue invoice is set by state law, not by a single national number. Most competitors tell you to "check your state" and stop. This table gives you the starting figure and the statute so you can actually do it. Pick your state in the calculator above and it flags any rate that runs past the cap.

Two things matter before you read a row. First, consumer and commercial transactions are treated differently: consumer debts usually face stricter limits, while business-to-business terms often qualify for exemptions or a higher ceiling. The figures below are the general or maximum-contract rates that most apply to a B2B invoice. Second, a cap is only the ceiling. A rate can be legal and still unenforceable if it wasn't disclosed in your terms before the work, or if a court reads it as a penalty rather than compensation for the delay.

How to read the table

Capped states have a clear, stable statutory ceiling, and the calculator warns you when your rate goes above it. No fixed cap means the state exempts most commercial transactions, so the agreed contract rate governs. Keep it reasonable and disclosed. Benchmark-tied states peg the cap to a moving federal or prime rate, so there is no single fixed number to hardcode. Check the current benchmark. Where a stable commercial figure could not be confirmed, the row reads Verify and points you at the statute rather than inventing a number.

Figures are commonly cited general usury or maximum-contract rates drawn from the named statutes and cross-checked against a secondary 50-state survey (UpCounsel, State Interest Rates and Usury Limits (50-state table)), reviewed July 2026. Usury statutes change and carry many exemptions for loan size, corporate borrowers, and licensed lenders. Treat this as a starting point for research, confirm against the cited statute, and get legal advice before you rely on a number in a dispute.

What is the maximum late fee I can charge in my state?

It depends on the state and the transaction. A handful set a firm ceiling: California caps most contracts at 10%, New York at 16% civil usury, Florida at 18% (25% over $500,000), Massachusetts at 20%, and New Jersey at 30% for individuals. Many states, including Nevada, Indiana, and New Hampshire, set no fixed cap on the agreed rate, and several exempt business loans outright once they pass a size threshold (Connecticut over $10,000, Virginia over $5,000, Mississippi over $5,000). Find your state in the table, then confirm against the statute before setting your rate.

Are late fees legal?

Yes, in every state, provided the fee was disclosed in your contract or terms before the transaction, is reasonable relative to the harm caused by late payment, and stays within your state's interest ceiling. Fees added after the fact, or rates that look punitive, are the ones courts strike down. A flat fee plus a stated monthly interest rate, written into the agreement and printed on each invoice, is the pattern that holds up.

What is a reasonable late fee?

Common B2B practice is a flat fee in the $15 to $50 range, or 1% to 2% of the invoice, applied once after a grace period, plus interest of 1% to 1.5% per month (12% to 18% APR) on the unpaid balance. That interest range sits inside the cap in most states and reads as compensation for the delay rather than a penalty. On small invoices a flat fee does more work than a percentage. On large invoices the percentage and the monthly interest carry the weight.