Number every invoice with something nobody else has used, and keep going up. That is the whole idea. The rest is choosing a format you will still like in two years.
1Keep it simple
Start at INV-0001 and add one each time. Four digits leaves room for ten thousand invoices and keeps the numbers in order when you sort them. A prefix such as INV for invoices and EST for estimates keeps the two kinds apart.
2Do not reuse a number
If you make a mistake before you send, fix it. If you already sent it, make a new invoice or a credit note and give that its own number, so the first one can still be found. A gap is better than a repeat.
3Should the year be in the number?
It can help, as in 2026-0012, because the number tells you when it was made. It also means starting again each January. Pick one style and stay with it. Do not change halfway through the year.
4Do not start at 1 for a new client
One list for all your clients is easier to keep and easier for an accountant to follow. A client who sees INV-0001 and INV-0002 knows you are new, so some people start at a higher number. That is a matter of taste, not a rule.
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Questions people ask
Do invoice numbers have to be in order?
In some places they must be. Even where they do not, going up by one is the easiest way to see that none are missing. Ask your accountant about your local rules.
What number should my first invoice have?
INV-0001 is fine. The tool on this site starts there and adds one for you the next time you use it in the same browser.