Invoice Late Fee Calculator

Choose the method your agreement actually allows

This calculator supports two common structures: simple annual interest and a one-time flat fee. It does not decide whether either charge is enforceable. Start with the signed contract, accepted terms, purchase order, and invoice language. A fee that appears for the first time after payment is late may not be collectible.

In annual-interest mode, daily accrual equals the unpaid principal multiplied by the entered annual rate, divided by 365. Fee owed is that daily amount multiplied by days overdue. The calculation does not compound, change the principal after partial payments, or use a 360-day commercial convention. If your agreement specifies monthly interest, compounding, a grace period, or a different day-count basis, this result will not match it.

Flat-fee mode applies the entered amount once when the invoice is at least one day overdue. It does not repeat monthly. The total now due is always original unpaid principal plus the calculated fee.

For a worked 2026 collection example, assume an accepted agreement permits simple annual interest of 18% on a $5,000 invoice and the full principal is 30 days overdue. Daily accrual is $5,000 × 18% ÷ 365 = $2.4658. The fee is $2.4658 × 30 = $73.97 after rounding, and total due is $5,073.97. The 18% is a user-entered contract term, not a Numerandi recommendation or a universal 2026 legal rate. If the customer paid $1,000 before the period being calculated, use the remaining $4,000 principal only if the agreement and applicable law support that allocation.

There is no general federal late-payment rate for ordinary private commercial invoices. Contract and state law usually do the work. Article 2 of the Uniform Commercial Code governs many sales of goods, while state contract, liquidated-damages, interest, and usury rules vary. UCC § 2-718 permits reasonable agreed liquidated damages but treats an unreasonably large amount as a void penalty; it does not supply a universal percentage.

Rates current as of September 2026. The entered rate is supplied by the user, not recommended by Numerandi.

Why state law and customer type matter

Some jurisdictions distinguish interest on a debt from a reasonable liquidated-damages charge. Others may treat an excessive late charge as an unenforceable penalty or apply a usury ceiling. In Solomon v. Davisson, 2018-Ohio-2011, the court required a writing assented to by both parties rather than interest terms added unilaterally on an invoice. The result can also differ for consumer, residential, government, and business-to-business transactions. A governing-law clause is relevant but not automatically conclusive.

State statutes show why a national default would be misleading. For example, New York's general judgment-interest statute specifies a rate in CPLR § 5004, while California contract interest rules appear in Civil Code § 3289. Those provisions have their own scope and are not permission to apply the same rate to every invoice.

Before invoicing, state the due date, any grace period, the fee method, the rate or dollar amount, whether charges repeat or compound, and how partial payments are allocated. Obtain acceptance before providing the goods or services. Keep the language consistent across the proposal, contract, and invoice.

Common mistakes and edge cases

• Adding a fee that the customer never accepted can make collection harder and may be unenforceable.
• Counting from the invoice date instead of the day after the due date overstates days overdue.
• Applying interest to the original invoice after a partial payment overstates principal.
• Entering a monthly percentage as an annual percentage produces a much smaller result; converting it without legal review can be equally risky.
• Consumer, government, judgment, bankruptcy, and cross-border debts may follow rules different from an ordinary business invoice.

Using the number in a collection message

Recalculate from the remaining unpaid principal after credits or partial payments. State the original amount, due date, days overdue, contractual clause, fee calculation, and resulting total. For interest, showing the daily accrual makes the demand easier to audit. Preserve invoices, delivery evidence, approvals, and communications.

Do not use the calculated amount as a threat or add collection costs that the agreement or law does not authorize. Government customers, prompt-payment statutes, bankruptcy, and court judgments can involve different rules. When the amount is significant or the customer disputes the fee, get advice in the relevant jurisdiction.

What to do next

Confirm the accepted terms and the governing jurisdiction before sending a fee demand. Reconcile credits and partial payments, save the exact calculation, and send a factual statement showing principal, dates, formula, fee, and total. Ask for a specific payment date and preserve delivery and approval records. For future work, put payment timing, grace periods, and permitted charges in the contract before work begins.

Disclaimer: This calculator is informational only and is not legal or tax advice. It does not determine enforceability, applicable law, or a lawful maximum rate.