How to number invoices: formats that keep accountants happy
Invoice numbers must be unique and usually sequential. Good formats, whether to restart each year, what to do with cancelled invoices, and mistakes to avoid.
Published
Tools for this job: Invoice Generator · Sort Lines
Invoice numbers look like an administrative detail until a client pays “invoice 12” and you have two of them, or a tax inspector asks why numbers 40 to 45 are missing. A simple, consistent scheme avoids both problems.
The rules
- Unique: no two invoices ever share a number.
- Sequential: numbers go up without gaps. In the EU, the VAT rules require invoices to carry a sequential number that uniquely identifies them; the UK requires a unique identifying number on VAT invoices; and many other tax systems expect the same.
- Never reused or deleted: a mistaken invoice is cancelled with a credit note, not renumbered.
Formats that work
| Format | Example | Good for |
|---|---|---|
| Simple sequence | 0001, 0002 | Very small businesses |
| Prefix and sequence | INV-0042 | Clear in emails and bank references |
| Year and sequence | 2026-0042 | Yearly bookkeeping |
| Series by type | INV-2026-0042, CN-2026-0003 | Invoices and credit notes |
Pad numbers with leading zeros, such as 0042 rather than 42, so they sort correctly in file lists and spreadsheets. Choose enough digits for several years of growth.
Should you restart each year?
You can, as long as the year is part of the number, so 2026-0001 and 2027-0001 are still unique. Without the year, never restart. A continuous sequence is simplest if you are unsure.
Starting at 1 looks small?
Some new businesses start at 1001 or similar so that the first client does not receive invoice 1. That is fine as long as you continue sequentially from there. Starting high is a presentation choice; skipping numbers later is what causes questions.
Client-specific numbers
Some freelancers put a client code in the number, such as ACME-007. If you do, keep a separate overall sequence too, or make sure your bookkeeping can show every invoice in order. Several parallel series can satisfy the rules, but each series must be sequential and unique on its own.
Keeping track
- Keep a simple log: number, date, client, amount, paid date.
- Fill in the next number in the Invoice Generator, which also uses it in the PDF's title.
- Name each PDF with the number first, such as
INV-2026-0042-acme.pdf, so files sort in order. Sort Lines with natural order sorts a pasted list of numbers correctly.
Mistakes to avoid
- Using the date alone as the number, which fails when you send two invoices in one day.
- Editing a sent invoice and re-sending it with the same number but a different amount.
- Deleting a cancelled invoice instead of crediting it.
- Mixing quotes and invoices in one series; quotes need their own numbering.
For the difference between invoices, quotes and receipts, see invoice vs receipt vs quote. For naming files so they sort, see naming files so they sort correctly.