How do I write a demand letter for an unpaid invoice?
Write a short letter of facts: the work, the delivery, the acceptance, the invoice, the amount, one deadline, and the next step if the deadline passes. Review the receipt and actual deliverable before accepting a handover.
Scope: general information and a template, not legal advice. Late-payment rights, interest, and formal pre-action requirements differ by jurisdiction; the notes under the template flag the US and UK differences at a general level only.
A demand letter fails in two ways: too soft to change behaviour, or so heated it hardens a late payer into a disputing one. The effective letter is a document, not a vent: six factual blocks in order, written as if it will be read later by a mediator, a court, or the client's own finance department, because it may be.
The record is what makes this letter different from the reminders that already failed. The delivery block does not say "work was completed"; it links the handover, whose steps, saved results, and the client's own recorded acceptance settle what was delivered and when. A letter that arrives with its evidence attached moves the conversation from "did they do the work" to "when is payment scheduled," which is the conversation that pays.
Send it once, by a channel that shows receipt, and keep the deadline real: the stated next step must be one you will actually take. The template below includes the two common jurisdictional flavours at a general level. In the US, interest and fees follow your contract unless a state rule says otherwise. In the UK, the Late Payment of Commercial Debts (Interest) Act 1998 supplies statutory interest for business-to-business debts where the contract does not, and formal pre-action practice expects a clear letter before proceedings. Neither note is advice; both are reminders to check your position before the letter goes.
Template: the demand letter (fill in the brackets)
[Your name or business, address, date.] To: [client name and address]. Re: Unpaid invoice [number], [amount], due [date]. Dear [name], 1. Agreement: On [date] you engaged me to [work] for [amount]; the terms are in [contract or proposal reference]. 2. Delivery: The work was delivered on [date] and accepted on [date]; the delivery record is here: [handover link]. 3. Invoice: Invoice [number] was issued on [date] and fell due on [date]; [number] days have passed. 4. Amount due: [amount], plus [interest or fee per contract, or delete]. 5. Deadline: Please pay in full by [date, 7 to 14 days out] to [payment details]. 6. Next step: If payment is not received by that date, I will [pause all work / begin recovery proceedings / instruct a collection service, pick the true one] without further notice. Yours, [name]. Enclosed: invoice copy; delivery record link.
Worked example
Block 2's link, in public form: https://zambo.dev/handover/45434a17-4afc-486a-9646-e689bcee4297 shows a delivered job with the client's acceptance recorded on it. A demand letter whose delivery block points at a record like this rarely needs a second letter, because the only open question left in it is the payment date.
Frequently asked questions
How many reminders should come before a demand letter?
Two polite reminders is the common pattern: one at the due date, one a week later naming the new date. The demand letter is the third contact and the first document; its power comes from being different in kind, not louder in tone.
Should I threaten legal action in the letter?
State the next step you will genuinely take, once, factually. An empty threat teaches the client to ignore the next letter; a true statement of consequences, kept, usually makes the next letter unnecessary.
The client disputes the work itself, not the payment. Is this the right letter?
Send the record first and answer the dispute item by item; the demand letter follows once the delivery facts are on the table. The template's block 2 does the heavy lifting either way, because it attaches the evidence instead of describing it.
By Brennan Zambo, founder of Zambo.
Last verified: 2026-10-17.
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Try a handover
A handover gives your client one proof link with verifiable receipts to review, then Accept or Flag.
try it: say 'use zambo to hand this over to <client>' in your ai