Invoice generator › Numbering
How to number your invoices
Sequential, unique, no gaps. Everything else is preference — but the reasons behind that rule decide how you handle mistakes.
The one rule
Invoice numbers must be unique and sequential, with no gaps. Most tax authorities require it, and the reason is simple: a gap in the sequence is indistinguishable from a deleted invoice, so an auditor has to ask what happened to number seven. If you can always answer, you have no problem.
Everything else — the format, whether you reset each year, how many digits — is up to you, as long as it is consistent.
Four formats that work
| Format | Example | Good for |
|---|---|---|
| Plain sequence | 1, 2, 3 |
Simplest and perfectly valid. Reveals how few invoices you have sent, which some people mind |
| Padded sequence | 0001, 0002 | Sorts correctly in a folder, looks established |
| Year prefix | 2026-001 | The most common. Sorts chronologically and makes the tax year obvious |
| Year and month | 2026-08-001 | High volume, or when you want the period visible at a glance |
If you use a year prefix you may reset the counter each January — the sequence has to be unbroken within its series, and the year makes each series distinct. Decide once and stay with it.
What not to do
- Do not put the client in the number.
ACME-001creates a separate sequence per client, which multiplies the series you have to keep unbroken and makes your total invoice count unverifiable. - Do not use the date alone.
19082026collides the moment you issue two invoices in a day. - Do not start at 1000 to look bigger. Harmless in itself, but it is the first step towards inventing numbers, and it fools nobody who has seen an invoice before.
- Do not skip numbers. Not for superstition, not for tidiness. Every gap is a question you will have to answer.
When you have to cancel an invoice
Never delete it and never reuse the number. Two correct routes:
Credit note. Issue a credit note referencing the original invoice, for the full amount if the invoice was wrong in total or partial if only part was. The original stays in the books, cancelled but visible. This is what accountants expect.
Void. Where your jurisdiction allows it, mark the invoice as void, keep it in the sequence, and record why. The number is used, the amount is nil, the trail is intact.
Then issue the corrected invoice with the next number in the sequence. Three documents where you wanted one, and that is the price of a clean record.
Starting out
New freelancers often ask what number to begin with. Start at 1, or at
2026-001. Nobody has ever won or lost a contract over an invoice number, and
a first invoice numbered 4,217 is more likely to raise an eyebrow than to impress.
If you already have a mess
Duplicated numbers, gaps, three overlapping formats — it happens, particularly in a first year. The fix is not to rewrite history. Draw a line: keep the old numbers exactly as they were issued, note what happened, and start a clean series from today, ideally with a year prefix so the break is obvious and explicable. A documented discontinuity is fine. Silently edited invoices are not.
From the makers of InvoiceAbroad
Sequential numbering without thinking about it
Flowzivo increments the number, keeps the series unbroken, and issues credit notes properly when something has to be cancelled — which is the part that gets messy by hand.
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Frequently asked questions
Do invoice numbers have to be sequential?
In most jurisdictions yes, and with no gaps. A gap looks like a deleted invoice, which is what an auditor will ask about.
Can I reset invoice numbers each year?
Yes, if the year is part of the number — 2026-001, 2027-001. The sequence must be unbroken within each series.
What number should my first invoice be?
1, or 2026-001. Starting at an artificially high number to look established fools nobody and pushes you towards inventing numbers later.
What do I do if I need to cancel an invoice?
Issue a credit note referencing it, or void it where that is permitted. Never delete the invoice and never reuse its number — then issue the corrected invoice with the next number.
Can I include the client's name in the invoice number?
Better not to. It creates a separate sequence per client, multiplying the series you must keep unbroken and making your invoice count impossible to verify.