Invoice number: formats and rules
An invoice number is the unique ID on each invoice. It lets you and your client match payments, chase the right bill, and find it again at tax time.
Build a numbering scheme
Choose the parts and see what your next invoices, and next year’s first invoice, will look like. The builder flags schemes that could produce duplicates.
Your next three invoices
- INV-2026-0001
- INV-2026-0002
- INV-2026-0003
First invoice next year: INV-2027-0001
✓ Unique, sortable and never repeats.
Common formats
| Format | Example | Good for |
|---|---|---|
| Simple sequence | 0001, 0002 | Sole traders with low volume |
| Prefix + sequence | INV-0042 | Distinguishing invoices from quotes (Q-) and credit notes (CN-) |
| Year + sequence | 2026-017 | Restarting each year while staying unique |
| Year-month + sequence | 202609-03 | Monthly billing cycles |
| Client code + sequence | ACME-2026-004 | Seeing at a glance which client an invoice belongs to |
What the rules say
For VAT-registered businesses in the UK, HMRC lists “a unique invoice number that follows on from the last invoice” among the details a full VAT invoice must show (GOV.UK: Invoices – what they must include). In the EU, Article 226(2) of Directive 2006/112/EC requires “a sequential number, based on one or more series, which uniquely identifies the invoice”. In the US there is no federal invoice-numbering law, but unique numbers make the records the IRS expects you to keep much easier to follow.
General information only. This page summarises official guidance so you know what to look for. It is not tax, legal or accounting advice, and rules change. Check the linked official source or a qualified adviser for your situation.
How InvoiceQuick numbers your invoices
The generator looks at the last document of the same type saved in your browser and adds one, keeping your prefix and zero padding: INV-0009 becomes INV-0010. Quotes, receipts and credit notes each get their own series (Q-, R-, CN-). You can overwrite the number any time.
Questions people ask
What is an invoice number?
A unique identifier given to each invoice so it can be tracked, matched to payments and found in your records. No two invoices you issue should share one.
Do invoice numbers have to be sequential?
For VAT invoices in the UK and EU, yes: HMRC and the EU VAT Directive require a unique number based on one or more series. Outside VAT rules it is still best practice, because gaps and repeats make records hard to audit.
How do I start invoice numbers?
Start anywhere, for example INV-0001 or 1001. Starting at 1001 doesn’t mislead anyone and looks tidier. What matters is that you never reuse a number.
Can I restart invoice numbers every year?
Yes, if the year is part of the number (2026-001, 2027-001), so the full number is still unique.
What if I skip or void an invoice number?
Don’t reuse it. Keep a record of the voided invoice, or issue a credit note if it was already sent.
Where do I find the invoice number on an invoice?
Usually in the top-right corner next to the invoice date, labelled “Invoice No.” or “Invoice #”.