Quick answer
An invoice number is a unique reference you give each invoice, usually in a sequence such as INV-0001, INV-0002. It lets you and your customer find the invoice, match payments to it, and keep clean records. In many countries, VAT and GST invoices must be numbered in a unique sequence.
Examples of invoice numbers
- INV-0001: a simple sequence, fine for most small businesses
- 2026-001: year plus a sequence that restarts each year
- ACME-003: a client code plus a sequence
Pick one format and stick to it. Our guide on how to number invoices compares more formats.
Need one now? Open the invoice template. It’s free and makes a PDF in a minute.
Rules to follow
- Never reuse a number, even for a cancelled invoice.
- Go up in order. Gaps can raise questions from tax authorities.
- Fix mistakes with a credit note, then issue a new invoice with the next number. See what is a credit note.
- Ask customers to quote the number when they pay, so you can match the payment.
The free invoice generator numbers invoices for you and remembers where you left off.
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Invoice templateFrequently asked questions
Where does the invoice number go?
Near the top of the invoice, next to the invoice date, so it is easy to find and quote.
Can my first invoice number be 1?
Yes. You can start at 1, 100 or any number, as long as each new invoice is numbered higher than the last.
Is an invoice number the same as a purchase order number?
No. You create the invoice number; the customer creates the purchase order (PO) number. If a customer gives you a PO number, add it to the invoice’s reference field.
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