How to number your invoices
Use a short prefix and a number that goes up by one each time, such as INV-0001, INV-0002, INV-0003, across all your clients. Never reuse a number, and keep cancelled invoices rather than deleting them, so every number in the series is accounted for.
Written by Flowfetti's founder, a working Pilates and Yin yoga instructor who invoices studios every week. Updated 3 October 2026.
Why the number matters
The invoice number is how money finds its way back to the right invoice. When a studio pays with the number as the reference, you can match the payment in your bank account in seconds. When two invoices share a number, you cannot, and neither can your accountant.
Three formats that work
- Plain sequence: INV-0001, INV-0002. The simplest, and the right choice for most people.
- Year first: 2026-001, 2026-002. Handy at tax time, as long as you never restart a year's numbering partway through.
- A prefix of your own: your initials, such as JS-0001. Fine, as long as there is still one series for everyone.
Leave room for growth: four digits (0001) covers years of weekly invoices without the numbers changing length.
Rules that keep the series clean
- Never reuse a number, even for a cancelled invoice.
- Never delete an invoice you have sent. Mark it cancelled and keep it.
- One series for every client, rather than one per studio or family.
- Ask for the number as the payment reference, on the invoice itself.
Correcting a mistake
If an invoice was wrong, keep the original, mark it cancelled, and send a new one with the next number. If you are registered for GST and the original was a tax invoice, the ATO calls the document that corrects it an adjustment note. Your accountant can tell you whether you need one.
Everything else that belongs on an invoice is in the free invoice maker, which starts the numbering at INV-0001 for you.
General information, not tax advice. For your own situation, check with the ATO or your accountant.
Questions people ask
What is a good invoice number format?
A short prefix and a number that goes up by one each time, such as INV-0001, INV-0002, INV-0003. It is easy to read aloud, easy to search for, and never repeats.
Is an invoice number a legal requirement in Australia?
The ATO’s list of what a tax invoice must show does not include an invoice number. But a unique number lets you and your client match each payment to its invoice, so almost everyone uses one.
Should each client have its own numbering?
One series across all your clients is simpler and is what most accounting tools expect. A separate series per client makes it easy to issue the same number twice.
What do I do with the number of a cancelled invoice?
Do not reuse it or delete the invoice. Mark it cancelled and keep it, so there is no unexplained gap or duplicate in your records.
Rather not keep count?
Flowfetti numbers every invoice in order for you, so a number is never skipped or used twice.
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