Invoice numbers
Every invoice needs a unique number in sequence with no gaps — in most countries that's law, not habit. Three schemes that work, and what breaks.
An invoice number is the unique reference on an invoice. It is how you, your client, your accountant and the tax authority all refer to the same document without describing it — and it is the field most likely to stop an invoice being paid, because most accounting systems will not accept an invoice without one, and will not accept two invoices carrying the same one.
The two rules
Unique. No number may ever be used twice, across your entire history. Reusing one makes two documents indistinguishable in every system that holds them, and makes a duplicate payment almost impossible to trace.
Sequential, with no gaps. In most countries this is a legal requirement rather than a convention. A gap in the sequence looks, to an auditor, exactly like a deleted invoice — which is what hidden revenue looks like. The sequence is the audit trail; that is the whole reason it is regulated.
Everything below is a consequence of those two.
How should you number your invoices? Three schemes that work
Plain sequential. INV-001, INV-002,
onwards forever. Simplest, and correct. Its only weakness is that it tells anyone
who receives one roughly how many invoices you have ever issued — which is why
plenty of people start at 100 rather than 1.
Year-prefixed. 2026-001, resetting each January.
Filing becomes obvious and the numbers stay short. Safe as long as the year is
genuinely part of the number, so uniqueness holds across years — 001
in 2026 and 001 in 2027 are the same number if the prefix is
decorative.
Client-prefixed. ACME-014. Useful when you invoice
a handful of large clients repeatedly and want to see at a glance whose invoice
you are looking at. The cost is real: you now maintain several sequences instead
of one, and gaps hide more easily in each. Worth it for a business with five
clients, not for one with fifty.
Whichever you pick, pick it once. Changing scheme mid-year creates exactly the discontinuity the sequence rule exists to prevent. If you must change, change at a year boundary and keep a note of why.
What to do when an invoice is wrong
This is where sequences actually break, and the instinct — delete it and issue a corrected one — is the wrong move.
| Situation | What to do with the number |
|---|---|
| Not sent yet, spotted immediately | Fix the document and keep the number. Nothing has left your hands. |
| Sent, and wrong | Issue a credit note against it, then a new invoice with the next number. Three documents, all accounted for. |
| Cancelled entirely | Keep the number, mark it void, and record why. Do not reuse it and do not skip it. |
| Client says they never got it | Resend the same invoice, same number, same due date. A resent invoice is the same invoice. |
| Duplicate issued by accident | Void the second one. The debt is one debt. |
The pattern behind all five: the sequence records what you did, not what you meant to do. A void invoice with a reason attached is a complete record. A missing number is a question nobody can answer two years later.
Credit notes need their own sequence
A credit note reverses part or all of an invoice, and it is a document in its
own right — so it gets its own number, in its own series
(CN-001), and it should name the invoice it credits. Taking a number
from the invoice sequence for a credit note breaks the invoice sequence, which is
the thing you were trying to protect.
Common mistakes
- Using the date as the number.
20260826is not unique the moment you issue two invoices in one day, and it is not sequential in any useful sense. - Restarting per client without a client prefix. Two clients, two invoices numbered 001. They are the same number.
- Spending invoice numbers on quotes or proformas. Those may never become sales, and each one that does not becomes a gap.
- Letting software and a spreadsheet both allocate numbers. Whatever assigns numbers should be one thing.
- Renumbering on resend. Creates a second document for one debt — which is how work gets paid twice.
Where to find the invoice number on a bill you received
If you are holding somebody else's invoice and looking for the number, it is
almost always in the top right, near the date, labelled Invoice no.,
Invoice #, Ref or Document no.. On a
supplier statement, each line will carry the number of the invoice it summarises
— that is the number to pay against, not the statement's own reference.
Two things it is not. It is not your account number with that supplier, which stays the same across every invoice they ever send you. And it is not the purchase order number, which is your reference, not theirs — an invoice often carries both, and paying against the wrong one leaves the payment unmatched. See how to pay an invoice for why that causes you to be chased for something you have already paid.
If there is no number anywhere on the document, it is not a valid invoice, and most accounting systems will not let you enter it. Ask for a corrected one.
Where the number goes
On the invoice, prominently near the top. In the subject line of the email that carries it. And in the payment reference you ask the client to use, because that is what lets them match the payment to the document — see how to pay an invoice for why an unmatched payment causes the supplier to chase something already paid.
More on the rest of the document in what an invoice must contain and how to write one. The free invoice generator numbers each invoice for you and keeps the sequence unbroken.