How to number your invoices (simple formats that work)

Why invoice numbers matter

• You and your customer can refer to the exact invoice ("please pay invoice 0042").
• You can see at a glance how many you have sent.
• A clean sequence makes accounting and tax time easier.
• It prevents duplicates, which cause payment confusion.

Simple formats you can use

• Plain running number: 0001, 0002, 0003. The simplest option.
• Prefix + number: INV-0001, INV-0002. The prefix tells you it is an invoice.
• Year + number: INV-2026-001. The sequence restarts each year, and the year is clear.
• Client code + number: ADA-001 for a regular client. Useful for retainers.

Use a different prefix for each document type, such as QUO- for quotations, RCT- for receipts, CN- for credit notes and PO- for purchase orders.

Rules to follow

1. Never reuse a number, even if you cancel an invoice. Issue a credit note instead.
2. Keep the sequence in order, without gaps you cannot explain.
3. Do not start at 1 if you want to look established. Starting at 0101 or 1001 is fine, as long as you are consistent.
4. Put the number clearly at the top of every invoice.
5. Match receipts to invoices. A receipt should name the invoice number it pays.

Common mistakes

• Using the date as the only number. Two invoices on the same day then clash.
• Changing the format halfway through the year.
• Different numbering in different places (a notebook and a phone).
• Deleting an invoice and reusing its number.

Let the app count for you

If you number by hand, mistakes are easy. Aurum numbers every invoice, receipt and quotation for you in the right sequence, so you never repeat or skip one.

Questions people ask