Knowing how to charge interest on overdue invoices is one of the most practical steps an AR manager or business owner can take to reduce late payment losses. The process is straightforward, but it only holds up if three things are in place before the invoice goes out: a disclosed rate, a written agreement, and a policy applied consistently across your debtor book. Get those right, and interest charges become a legitimate, enforceable tool. Skip them, and you may find the charges unenforceable when you need them most.
For B2B companies dealing with slow-paying accounts, understanding late payments in B2B is the starting point. Once that foundation is clear, the mechanics of charging interest are easier to apply at scale.
Can You Legally Charge Interest on Overdue Invoices?
Yes. Charging interest on overdue invoices is legal in the United States, provided the terms were disclosed in writing before any goods or services were delivered. The critical word is before. Courts have consistently held that interest charges applied retroactively, without prior agreement, are not enforceable.
Three conditions must be met for the charge to hold:
- The rate and trigger date must appear in your contract or written terms.
- The client must have received and acknowledged those terms before work began.
- The charge must comply with your state’s usury ceiling (more on this below).
For B2B transactions specifically, there is no federal cap on late payment interest. The ceiling is set state by state. As long as your rate is within your state’s limit and was disclosed upfront, the charge is enforceable. If you are invoicing across multiple states, build your policy around the most conservative rate in your portfolio.
How Much Interest Can You Charge on Overdue Invoices?
The standard monthly rate across US B2B transactions runs between 1% and 2% per month, which equals 12% to 24% annually. Most AR policies land at 1.5% monthly (18% annually) because it sits above the cost of capital while remaining below the threshold most state courts consider excessive.
The formula to calculate the charge is straightforward. Display it, apply it consistently, and document each calculation on the updated invoice.
Interest = Invoice Amount x Monthly Rate x Months Overdue
Example: $5,000 x 1.5% x 2 months = $150.00
A worked example: an AR manager holds a $5,000 invoice unpaid for two months at 1.5% monthly. The interest charge is $150. That figure is added as a separate line item on a revised invoice sent to the debtor, referencing the original invoice number and the agreed payment terms.
A few practical notes on rate selection:
- 1% monthly (12% annually) is the most conservative and is enforceable in every US state.
- 1.5% monthly (18% annually) is the most common choice for B2B AR policies.
- 2% monthly (24% annually) is the ceiling in several states and should be verified against local law before use.
Compound interest is permitted in some states and not others. If your policy uses compound interest, this must be stated explicitly in the contract.
How to Include an Interest Charge on Your Invoice
Adding interest to an overdue invoice is a two-step process: first, confirm the original agreement authorizes it; second, issue a revised invoice with the charge itemized clearly.
On the revised invoice, include:
- Original invoice number and amount
- Payment due date from the original invoice
- Number of days or months overdue
- Monthly rate applied
- Interest amount as a separate line item
- Updated total balance due
Send the revised invoice via the same channel used for the original, and follow up with a formal overdue letter citing both the original and revised invoice numbers. Keeping a consistent paper trail matters for any dispute or escalation later.
Sample Late Fee Clause for Payment Terms
The clause below is designed for B2B service agreements. Insert it into your contract’s payment terms section before any work begins. Both parties should sign or acknowledge it in writing.
Sample Late Fee Clause
Invoices not paid within [30] days of the invoice date will accrue interest at a rate of [1.5]% per month (18% per annum) on the outstanding balance. Interest begins accruing on the first day following the payment due date. This late payment policy applies to all outstanding balances and has been agreed upon in writing prior to the commencement of services.
Replace the bracketed values with your chosen payment window and rate. Have the client countersign before any work begins.
One important distinction: the clause must specify whether the charge is a late fee (a fixed penalty) or interest (a recurring charge on the outstanding balance). Courts treat these differently. A contract that authorizes a late fee cannot be used to enforce interest charges, and vice versa. If you want both options available, include both explicitly in your terms.
State-Level Considerations for Overdue Invoice Charges
No single federal law governs late payment interest between private businesses in the US. Each state sets its own usury limit, and a handful have specific rules for commercial transactions. Before finalizing your rate, check your state’s position on:
- Maximum annual interest rate for commercial contracts
- Whether compound interest is permitted
- Any required grace period before interest begins accruing
- The statute of limitations on debt collection for unpaid invoices in your state
The statute of limitations on debt collection varies from three to ten years depending on the state and the contract type. Knowing this limit is relevant when deciding whether to pursue old overdue accounts or write them off.
If your business invoices clients in multiple states, it is worth reviewing two or three of the states where your largest receivables sit. California, Texas, and New York each have distinct rules. A legal advisor familiar with commercial collections in your jurisdiction can confirm your rate is compliant before you publish terms.
Frequently Asked Questions
No. Interest on overdue invoices is only enforceable if the rate and terms were agreed upon in writing before the invoice was issued. Applying interest retroactively, without a prior written agreement, is not legally enforceable in US courts.
The most commonly used rate in US B2B transactions is 1.5% per month, equivalent to 18% annually. Rates between 1% and 2% per month are standard, though the enforceable ceiling varies by state. Always verify your chosen rate against the usury limit in the debtor's state.
Multiply the outstanding invoice amount by the monthly interest rate, then by the number of months overdue. For example, a $5,000 invoice at 1.5% per month overdue by two months accrues $150 in interest. Add this as a separate line item on a revised invoice.
Not if the policy was disclosed upfront. Clients who agreed to the terms in writing have no grounds for dispute. Applying the policy consistently, and pairing it with a brief explanatory note on the revised invoice, keeps the interaction professional and reduces the chance of a relationship breakdown.
If a client continues to ignore the invoice after interest has been applied, the next step is to follow up with a formal overdue letter or a formal demand letter. If those also fail, next steps when interest still does not prompt payment include escalating to a third-party collection agency.
When Interest Charges Are Not Enough
A well-structured late fee clause recovers money in most cases. Some accounts, however, will not respond to interest, letters, or reminders. When that happens, the practical next step is understanding late payments in B2B and knowing when to escalate.
Kollecta works with B2B businesses on a contingency basis, taking over delinquent accounts when internal efforts have stalled. No recovery, no fee. If adding interest charges has not resolved the account, Kollecta’s team can take over collection with no upfront cost, preserving the client relationship where possible while recovering the outstanding balance.
Get a free consultation to discuss your overdue accounts.
