Invoice Numbers: What They Are and How to Number Invoices
The rules are simple (unique, sequential, never reused), but the format you pick now will follow you for years.
An invoice number is the reference that identifies one invoice and only that invoice. You'll use it to match payments, your client's accounts team will use it to file your bill, and a tax inspector will use it to check that nothing is missing. Getting the system right at the start saves a surprising amount of hassle later.
The three rules
- Unique. No two invoices share a number, ever. Not across years, not across clients.
- Sequential. Each new invoice gets the next number. Some countries (the UK and EU member states among them) require a sequential series for VAT invoices, and it's good practice everywhere.
- Never reused. If you cancel an invoice, the number stays used. You issue a credit note to cancel it, not a new invoice with the same number.
That's the whole legal core. Everything else is choosing a format you'll be happy with.
Five numbering systems, with examples
1. Plain sequence
0001, 0002, 0003… or INV-0001, INV-0002…
The simplest option and completely acceptable. The prefix INV- helps when the number sits next to a PO number or a quote number, because people can tell at a glance which is which. Pad with zeros (four digits handles 9,999 invoices) so files sort correctly.
Downside: your first client receives invoice #0001. Some people don't love that. You're allowed to start at any number, so starting at 1001 is fine as long as you continue sequentially from there.
2. Year-based
2026-001, 2026-002… 2027-001
Very popular with freelancers and small firms. The number tells you roughly when it was issued, and it resets each year, which keeps numbers short. The combination of year and sequence is still unique.
Watch out: in some jurisdictions, resetting is fine as long as the year prefix keeps each number unique. If you're unsure, don't reset: use 2026-001 then 2027-048, which keeps one continuous sequence.
3. Year and month
202608-01, 202608-02, 202609-01
Useful if you send many invoices a month and reconcile monthly. It gets long, and the per-month reset means you need to be careful about uniqueness if you ever issue from two devices.
4. Client code
ACME-001, ACME-002, BLUE-001
Easy to see how many invoices a client has had. The problem is that you now have several sequences, not one, which some tax authorities don't like for VAT invoices. A common fix is to combine it with a global sequence: 0147-ACME.
5. Project or job number
J2291-1, J2291-2
Works well in trades and construction where one job gets several invoices (deposit, progress payments, final). Again, pair it with a global sequence if you're VAT-registered.
| System | Example | Best for | Watch out for |
|---|---|---|---|
| Plain sequence | INV-0148 | Anyone | Starting at 0001 looks new |
| Year-based | 2026-017 | Freelancers | Check reset rules for VAT |
| Year and month | 202609-04 | High volume | Long numbers |
| Client code | ACME-003 | Few, regular clients | Multiple sequences |
| Job number | J2291-2 | Trades, construction | Needs a global sequence too |
What about quotes, receipts and credit notes?
Give each document type its own series. QUO-0012 for a quote, INV-0148 for the invoice that follows it, CN-0003 for a credit note. If you use the same sequence for everything, every quote that doesn't turn into a sale leaves a gap in your invoice numbers, which is exactly what an auditor looks for.
The generator does this for you: switch the document type and the prefix changes from INV to QUO, EST, RCT, PRO or CN.
Start at any number you like. The “Start the next invoice” button bumps it automatically.
Open the invoice generatorFixing mistakes
You skipped a number. It happens. Make a note in your records of why (for example, "INV-0151 not issued, created in error"). A single explained gap isn't a problem; a pattern of unexplained gaps looks like missing income.
You used the same number twice. Keep the first invoice as it is. Reissue the second with the next available number, tell the client which one to pay, and note the change.
The invoice itself was wrong. Don't edit and resend it under the same number once it's been sent. Issue a credit note against it and a new invoice. See how to correct or void an invoice.
Where the number goes
Top right is the convention, near the date, where accounts payable staff look for it. It should also be in the email subject line when you send it and in the PDF filename: Invoice-INV-0148-Acme.pdf. And ask clients to quote it as the payment reference so you can match the money when it arrives.
Keep reading
How to Write an Invoice (With a Worked Example)
How to write an invoice that gets paid: the 10 details to include, a line-by-line worked example, payment terms that work, and the mistakes that delay payment.
What to Include on an Invoice: The Complete Checklist
Every detail an invoice needs, which ones are legally required in the US, UK, EU and Australia, and the optional extras that get you paid faster.
How to Correct, Cancel or Void an Invoice
What to do when an invoice is wrong: correcting unsent invoices, credit notes for sent ones, cancelling duplicates, fixing tax errors, and what never to do.