Guide
Invoice number format: how to number your invoices
An invoice number is the reference you and your client use for one invoice, for good. Here is how to choose a format, run a series year after year, and deal with gaps, mistakes and other documents.
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The basics
Whatever format you choose, a good invoice number is:
- Unique. No two invoices ever share a number, not even across years or clients.
- In order. Each new invoice gets the next number, so the series shows what was issued and when.
- Consistent. Pick one format and keep to it. Changing styles halfway through makes records harder to follow.
- Short and easy to type. Clients copy it into payment references and emails; long codes get mistyped.
Rather not start at 0001? Starting higher, such as at 1001, is common and fine. What matters is that you go up from there and never reuse a number.
Common invoice number formats
Any of these works. The last column shows the number that follows.
| Format | Example | Next |
|---|---|---|
| Plain sequenceThe simplest choice for a small business | 0042 | 0043 |
| Prefix + sequenceTelling invoices apart from quotes and receipts | INV-0042 | INV-0043 |
| Calendar yearRestarting the count each January | 2026-042 | 2026-043 |
| Financial yearFollowing a year that runs April to March | 2026-27/042 | 2026-27/043 |
| Year and monthSeeing at a glance which month an invoice is from | 202611-07 | 202611-08 |
| Client codeA few large clients who each want their own count | ACME-007 | ACME-008 |
You can combine them, as in INV-2026-0042, but every part you add is something to type and to keep consistent. Two parts are usually enough.
Prefixes and leading zeros
A short prefix such as INV- tells anyone reading a bank statement or an email subject that the number belongs to an invoice. Use letters and digits with a hyphen or a slash; avoid spaces, which tend to get lost when the number is copied.
Leading zeros (0042 rather than 42) keep the numbers the same length, so they sort in the right order in a spreadsheet or a folder of PDFs. Choose enough digits for the invoices you expect: four digits last for 9,999 invoices.
Leave client names and amounts out of the number. They change, and the number should not.
A new series each year
Many businesses start counting again each year and put the year in the number, so the numbers still never repeat: 2026-001, 2026-002 … then 2027-001 in January.
If your business follows a financial year rather than the calendar year, use that: a year that runs from April to March, as it does in India, is often written 2026-27, giving 2026-27/001, 2026-27/002 and so on, then 2027-28/001 from April.
Only restart the count when the year is part of the number. Without it, 001 next year would repeat an invoice you already sent.
Gaps, mistakes and cancelled invoices
- Never reuse a number, even for an invoice you deleted before sending. Give the new invoice the next number instead.
- Don’t delete a sent invoice. If it was wrong, keep it on file and correct it (usually with a credit note against it and a new invoice, or by marking it cancelled), following the practice where you work.
- Explain gaps. If a number was skipped or cancelled, keep a short note of why. A tax office or an accountant may ask, and a recorded reason is easy to show.
- Don’t number backwards. An invoice dated after another should not have a lower number in the same series.
Separate series for quotes, receipts and credit notes
Quotes, proforma invoices and receipts are not invoices, so they are easier to track with their own series and prefix. A quote that is accepted becomes an invoice with the next invoice number, not the quote’s number.
| Document | Series starts |
|---|---|
| Invoices | INV-0001 |
| Quotes and estimates | Q-0001 |
| Proforma invoices | PF-0001 |
| Receipts | RCT-0001 |
| Credit notes | CN-0001 |
A credit note should also mention the number of the invoice it corrects. If you run more than one business, shop or branch, give each its own prefix so their series never collide.
Local rules
Some countries set rules for invoice numbers: for example, that they run in one or more unbroken series, or that a number may be only so long. If you are registered for VAT, GST or sales tax, check your tax authority’s guidance or ask an accountant before you settle on a format.
In India, GST tax invoices have their own numbering rule. If you are registered for GST, read invoice numbering in the GST invoice format guide and make your invoices in the GST invoice generator.
Numbering in the invoice generator
- Type any format in the number field, up to 40 characters. The first invoice starts as
INV-0001; change it to the format you want. - New starts the next invoice with the next number: the generator adds one to the last group of digits and keeps the zeros, so
INV-0009becomesINV-0010,2026-27/099becomes2026-27/100. - At the start of a new year, type the first number of the new series once (
2027-28/001); the next invoices follow on from it. - For a quote or a receipt, choose the document type and give it a number from its own series, such as
Q-0001.
More on what goes on an invoice in the guide to writing an invoice, and on due dates in the guide to payment terms.