Charging Interest on Overdue Invoices: Legal Guide

Interest on an overdue invoice compensates you for waiting for money you have earned, and it signals that due dates are real. It is also regulated: statutes, usury limits and penalty rules vary by country and state. What works for a freelancer in one place may be unenforceable in another.

This article is general information, not legal advice. For the rules where you and your client are based, check the official sources linked below or speak to a lawyer. Last reviewed: October 28, 2026.

Contract first, invoice second

Interest should be agreed in your contract or standard terms before work begins. Surprising a client with a high rate on their first late invoice damages trust and may not be enforceable.

Repeat the same wording in your invoice terms or notes, consistent with your invoice payment terms. For example: "Invoices are due 30 days from the invoice date. Overdue amounts accrue simple interest at 1.5% per month (18% a year) from the day after the due date, or the maximum rate permitted by law if lower."

Statutory vs contractual interest

Some jurisdictions give you a default statutory rate for commercial debts even when the contract is silent; others rely almost entirely on the contract. Check whether your client is a business or a consumer, because consumer rules are often stricter and several statutory regimes cover business-to-business debts only.

Late fees vs interest

A flat late fee is easy to understand; interest builds up over time. Some businesses use both, but stacking them can look punitive and, in some places, runs into penalty or usury rules. Whatever you choose, show the arithmetic. The late fee calculator works out simple interest for any amount, annual rate and number of days.

Calculation hygiene

State:

• Rate: annual or monthly, and say which
• Start date: usually the day after the due date, unless your contract or the statute says otherwise
• Method: simple interest on the unpaid balance. Compounding raises the effective rate and can push you over a usury limit, so take advice before using it.

Relationship management

Before you add interest, run your follow-up strategy. A dated plan for the balance often recovers the principal faster than arguing about interest; see partial payments. If the client has paid part of the invoice, charge interest only on what is still owed.

Tax and accounting

Interest and late fees you collect are generally taxable income, so track them separately in your books and ask your accountant how to report them. In the UK, HMRC treats statutory late-payment interest as outside the scope of VAT (HMRC VAT Supply and Consideration manual, VATSC06810). Keep your calculations with the invoice in your audit trail.

If you operate internationally

Read our notes on international clients and currency. Calculate interest in the invoice currency. Which country's late-payment rules apply depends on the law that governs the contract, so name the governing law in your terms.

Operational consistency

If your contract allows interest, put the clause in the terms on every invoice, not just some. When you waive interest for a good reason, record it as a one-time waiver so nobody later assumes the policy has lapsed. Offer a dated payment plan when a client has a temporary cash problem; a documented plan beats silent resentment. If you ever bill individuals, check consumer rules separately. Save the calculator output with the invoice so the figures can be checked later.

Charging interest in InvoiceQuickly

InvoiceQuickly does not add late fees or interest to invoices automatically, and a sent invoice cannot be edited (only drafts can). To charge interest, create a separate invoice with its own number, a line such as "Late payment interest on INV-0091, 18% a year, 1 October to 15 November 2026", and the amount from your calculation. This also follows GOV.UK's advice to send a new invoice if you decide to add interest. Put your interest clause in the terms field of every invoice so it is agreed in advance.

Closing checklist

Once a year, recheck the statutory references in your templates against the official sources. Keep a list of clients with active waivers. Reconcile interest income in your books with the interest invoices you issued. Make sure the payment terms on your invoices still match your contracts after renewals. Don't improvise penalty wording in emails.

Metrics and cadence

Track interest billed versus collected; a large gap means the policy is symbolic rather than operational. Review active waivers twice a year. Compare days to pay before and after you tightened your terms. Log legal escalations separately from routine follow-ups, and use the late payment guide for the wider recovery process.

Statutory interest rates by jurisdiction

These are default rates that apply when the contract sets no rate of its own. In the UK, a contract can replace statutory interest only with a "substantial contractual remedy" for late payment (section 8 of the 1998 Act). They are not recommended contract rates.

For more states and how penalty rules affect fixed late charges, see late fee laws by state. Rates and base rates change; check the source before relying on a figure.

Step-by-step: setting and collecting interest

Step 1: Put the interest term in every contract and invoice

Sample wording: "Payment due within 14 days of the invoice date. Overdue amounts accrue simple interest at 1.5% per month from the day after the due date, or the maximum rate permitted by law if lower." Include it in the contract and in the terms on every invoice. Check that the rate is lawful where your client is based.

Step 2: Apply the policy consistently

If you charge interest on some late invoices and not others without a reason, the policy looks optional. Charge it consistently, and record any waiver and why.

Step 3: Calculate interest correctly

At 1.5% a month, a $5,000 invoice accrues $75 for each full month overdue. For a part month, use a daily figure: 18% a year ÷ 365 × days. Worked example: $5,000 paid 45 days late at 18% simple interest is $5,000 × 18% × 45 ÷ 365 = $110.96.

Step 4: Communicate interest in writing

Send the interest invoice with a short note: "Per our contract terms, interest of $110.96 applies to invoice #X for 45 days past due. The total now owed is $5,110.96." That gives you a paper trail if the client disputes it.

Step 5: Decide when to pursue or write off

If the debt keeps ageing, move to formal steps: a final notice, a collection agency, or a small-claims claim. In the US, a business bad debt is deductible only if the amount was already included in your income, which generally rules out cash-method sole proprietors (IRS Topic 453).

Common scenarios

Solo freelancer, $5,000 invoice now 45 days late: with an 18% a year simple-interest clause, the interest is $110.96 (see Step 3). Send a separate interest invoice with the calculation and a firm date for the total.

Monthly retainer, $4,000 a month, client missed a payment: pause the service if your contract allows it, apply interest per the contract, and resume when the payment clears. Interest on several missed months adds up quickly, so deal with the first missed payment promptly.

Large client, $50,000 invoice 60 days late: a late fee may not move a large accounts payable team, where the delay is usually process rather than cash. Escalating to the right person matters more. Still put the interest in writing so it can be collected with the payment.

UK supplier billing a business client: the Late Payment of Commercial Debts (Interest) Act 1998 gives statutory interest and fixed recovery sums even when the contract is silent, provided UK law governs the contract. Worked example: £5,000 paid 30 days late at 11.75% is £5,000 × 11.75% × 30 ÷ 365 = £48.29, plus the £70 fixed sum.

Frequently asked questions

Are 18% a year late fees legal?

It depends on where you and your client are and what the contract says. There is no single US cap: New York's civil usury ceiling is 16% a year (though a corporation cannot use it as a defence), while Florida treats simple interest above 18% a year as usurious. In the UK, a contract term that replaces statutory interest is void unless the contract gives a substantial remedy for late payment. Check late fee laws by state or local rules before choosing a rate.

What if my client refuses to pay late fees?

Point to the clause in the contract and the invoice terms, and show the calculation. If they still refuse, decide whether the interest is worth pursuing on its own; many businesses collect the principal first and then pursue or waive the interest. If you go to court, include the interest in your claim.

Can I waive late fees as a goodwill gesture?

Yes, occasionally. Record why: "Waiving interest on invoice #X as a one-time courtesy because payment was made immediately after notice. Our standard policy continues to apply." Don't waive it routinely, or the clause loses its force.

Is it worth chasing interest on a small invoice?

Often not on its own. Court fees and your time can exceed the interest on a small invoice. For small, repeat clients, deposits or shorter terms usually work better than interest.

How do I handle a client who declares bankruptcy?

In the US, stop collection action once you are notified (the automatic stay), and file a proof of claim with the bankruptcy court by the deadline stated in the court's notice. As an unsecured creditor you may recover only part of what you are owed, or nothing. Talk to a lawyer about larger claims.

Model late fees and interest before you change policy. Start free and pair calculators with clear invoices.