How to Work Out Late Fees on an Overdue Invoice

An invoice for £2,400 was due on the 14th. It’s now the 9th of the following month, the client has gone quiet, and your contract says “late payments accrue interest at 1.5% per month.” You’re about to send a firm reminder and you’d like it to contain an actual figure rather than a vague threat — but you’re not certain whether that 1.5% applies to the whole month, to the days elapsed, or compounds.

Getting that number right matters more than it looks. A late fee you can explain line by line reads as professional. One that’s obviously guessed reads as a bluff, and clients who pay late are usually experienced at spotting bluffs.

What a late fee actually is

A late fee is interest charged on money that should already be in your account. Two things define it: the rate and how the rate is applied.

Rates are usually quoted per month or per year. A per-month rate like 1.5% means 18% annually. Some contracts instead reference a statutory rate — in many jurisdictions, a base rate plus a fixed percentage — and some add a flat administrative charge on top of the interest.

The application matters just as much. Daily accrual means you take the annual rate, divide by 365, and multiply by the days overdue. Monthly accrual may mean the full month’s charge applies the moment the invoice tips a day past due, which is why a payment three days late can attract the same fee as one thirty days late. Read which one your terms actually specify before you calculate anything.

Why people get stuck here

• Vague contract wording. “Interest may be charged on overdue amounts” gives you no rate to apply and no basis to insist.
• Counting days wrong. Is the due date itself day zero or day one? Consistency matters more than the choice, but be consistent.
• Simple versus compound. Most commercial terms use simple interest on the outstanding principal. Compounding monthly is a different, and much larger, number.
• Tax on the fee. Whether late payment interest is subject to sales tax or VAT varies by jurisdiction, and it’s worth checking before you add it to an invoice.
• Reluctance to charge at all. Plenty of freelancers write late fees into contracts and never invoke them, which teaches clients the clause is decorative.

What a good late fee calculation looks like

It’s traceable back to the contract

Every element of the number — the principal, the rate, the day count, the accrual basis — should map to a clause you can quote. If a client queries it, you want to answer in one line.

It separates principal from interest

Your reminder should show the original amount, the days overdue, the rate applied, and the interest as separate figures. Lumping them into one total invites a dispute about the whole invoice, not just the fee.

It’s proportionate

A £14 interest charge on a £2,400 invoice is a nudge. A £400 charge on the same invoice looks punitive and gets escalated to someone whose job is to argue about it.

Illustrative figures on a $2,400 principal; your contract’s terms govern.

Common mistakes to avoid

• Charging a rate that isn’t in the contract. A fee you never agreed is unenforceable and damages your position if the debt is escalated.
• Applying the fee before any grace period expires. If your terms allow seven days’ grace, day one of accrual is day eight.
• Backdating the calculation to the invoice date. Interest runs from the due date, not the issue date.
• Sending the fee without a payment link. Every extra step between the reminder and the payment is another week.
• Adding the fee silently to a new invoice. Show it as a distinct line with the days and rate spelled out.

How to do it with Invoice Late Fee Calculator

Online Tool Store’s Invoice Late Fee Calculator does the interest arithmetic in your browser, so the invoice figures never leave your machine.

1. Enter the outstanding invoice amount — the principal only, excluding any fee already added.
2. Enter the due date and today’s date so the tool can count the days overdue.
3. Enter the rate exactly as your contract states it, and pick the matching monthly or annual basis.
4. Read the interest figure and the new total separately, and check the day count against your own calendar.
5. Paste both figures into your reminder as distinct lines: the original amount, then the interest with its rate and day count shown.

If you’re rebuilding the invoice itself, the Invoice PDF Generator and the Business Days Calculator are both in the tools directory.

Frequently asked questions

Can I charge a late fee if my contract doesn’t mention one?

In many jurisdictions there’s a statutory right to claim interest on late commercial payments even without a contractual clause, but the rate and conditions vary considerably. Check your local rules before invoking it, and add an explicit clause to future contracts either way.

Should the late fee include tax?

That depends on your jurisdiction — in some places interest on late payment is outside the scope of sales tax, in others it isn’t. Ask your accountant once and apply the answer consistently rather than deciding per invoice.

Does charging interest hurt the client relationship?

Applied consistently and explained clearly, it usually doesn’t. What damages relationships is inconsistency — waiving it for one client and enforcing it for another makes the charge look arbitrary rather than contractual.

Final thought

The point of a late fee is to make paying on time the easier option, not to earn interest. Calculate it properly, show your working in the reminder, and most clients simply pay the original invoice.

Try the free Invoice Late Fee Calculator