Invoice Number Formats: 10 Patterns With Examples

This page is a gallery of invoice number formats you can copy, with example sequences, who each suits and what to watch for. For the rules behind numbering (gaps, corrections, credit notes and what tax authorities require), see invoice number best practices.

What makes up an invoice number format?

Most formats are built from the same few segments joined by a separator. Choose the segments you need and leave the rest out.

The counter is the part that does the legal work. In the UK, a VAT invoice needs "a sequential number based on one or more series which uniquely identifies the document" (HMRC VAT Notice 700, 16.3.1), and the EU VAT Directive uses almost the same words (Article 226(2)). Everything else is there to make the number useful to you and your client.

Ten formats with example sequences

Each format below shows three consecutive numbers so you can see how the sequence moves.

Format 6 follows India's rule: a GST tax invoice number may be up to 16 characters, use letters, digits, hyphens and slashes, run in one or more series and must be unique for the financial year (CGST Rules, Rule 46(b)). INV/26-27/0001 is 14 characters, so it fits.

For other document types, use their own prefix so nobody mistakes them for an invoice: CN- for credit notes, EST- or Q- for estimates and quotes, PRO- for pro forma invoices. A pro forma invoice is an offer, not a tax invoice, so it should never take a number from your invoice series. In Spain, corrective invoices (facturas rectificativas) must be issued in their own specific series (Real Decreto 1619/2012, article 6.1.a).

Which format suits your business?

Pick the simplest format that answers the question you most often ask about an invoice.

Two practical limits apply to all of them:

• Pad the counter to four digits (0001) unless you issue more than a few thousand invoices a year. Padding keeps numbers in order when sorted as text; without it, INV-10 sorts before INV-9.
• Keep the whole number short enough to type. Clients quote it on bank transfers, and some banking apps limit the reference field. Sixteen characters or fewer is a sensible ceiling, and India makes that a legal limit.

Worked example: one year of invoices in three formats

A designer issues seven documents in 2026 and early 2027: five invoices, one credit note and one invoice in January. Here is the same list in three formats.

All three are valid ways to give each invoice a unique number in a sequence. Format 2 is the easiest to run. Format 3 tells you the year but means two INV-...-0001 numbers exist, separated only by the year. Format 7 groups by client but runs two invoice series at once, plus a credit note series, which is more to check at year end.

How do you create these formats in a spreadsheet?

Keep the counter in its own column and build the number with a formula, so you never type a number by hand. With the counter in column A and the issue date in column B:

Use the issue date column rather than TODAY(), which changes every day and would rewrite old numbers. Turn the formula results into fixed values once an invoice is sent. To generate a batch of numbers in any of the first three formats, the free invoice number generator builds them in your browser and exports a CSV.

Which formats does InvoiceQuickly support?

InvoiceQuickly numbers invoices as a prefix plus a four-digit counter, for example INV-0042 (format 2). In settings you can change the prefix (up to 10 letters, numbers, hyphens or underscores) and raise the next number, but not lower it, so a number can never be issued twice. The counter keeps growing past 9999.

• To show the year, set the prefix to INV-2026 and change it to INV-2027 in January. The counter does not restart, so you get format 4: INV-2026-0118, then INV-2027-0119.
• Month, client, project, branch and date segments are not added automatically, and one account is one business, so formats 5 and 7 to 10 are not available. A fiscal-year label works with hyphens (prefix INV-26-27 gives INV-26-27-0001), because the prefix cannot contain a slash.
• Estimates and credit notes now use separate EST- and CN- counters without consuming invoice numbers. Historical gaps remain, so keep the document trail and cutover note; see invoice number best practices.

Which invoice number formats should you avoid?

• Random or hashed IDs (7F3K-Q9). They are unique but not sequential, which UK and EU VAT invoices require, and they make missing invoices impossible to spot.
• Full client names (INVOICE-ACME-INDUSTRIES-LTD-3). Too long, and the number changes meaning if the client renames.
• A date alone (2026-11-27). The second invoice that day has no number.
• Starting every client or year at 1 without a distinguishing segment. Two documents called INV-001 is a duplicate.
• Numbers that look like amounts or phone numbers. 2400 next to a $2,400 total invites confusion on a remittance.

Frequently asked questions

What is a good invoice number format?

A prefix plus a zero-padded counter, such as INV-0001, suits most small businesses. Add the year (INV-2026-0001) if you want to see when an invoice was issued, and add a client, project or entity code only if you regularly need to sort by it.

Is the bill number the same as the invoice number?

From the seller's side, yes: the number on the bill you send is your invoice number. Your customer may record it in their own system as a "bill" or "vendor invoice" and give it an internal reference as well, so quote your invoice number on every reminder.

Can an invoice number have letters?

Yes. UK and EU VAT rules ask for a sequential number that uniquely identifies the invoice, and letters in a prefix do not change that. India's GST rules explicitly allow letters, digits, hyphens and slashes, up to 16 characters.

Should my first invoice number be 1?

It does not have to be. Many businesses start at 1001 or INV-0100 so the first invoice does not look like the first. What matters is that the numbers then run in sequence and never repeat.

How many digits should an invoice number have?

Pad the counter to at least four digits (0001 to 9999), which covers a few thousand invoices a year. Use five if you expect more. The total length including prefixes is best kept to 16 characters or fewer.

Can I use a different format for each client?

You can run a separate series per client (ACME-001, BLUE-001), as UK and EU rules allow one or more series. Each series must still be sequential, and you need to keep track of every series you have opened. A single series with the client name recorded on the invoice is usually simpler.

Should invoice numbers restart every year?

They can, if the year is part of the number so that INV-2026-0001 and INV-2027-0001 stay distinct. Restarting without the year creates duplicate numbers. Some countries are stricter about continuity, so check the rules before you reset.

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Last reviewed: October 27, 2026. Legal wording is quoted from HMRC VAT Notice 700, the EU VAT Directive, India's CGST Rules and Spain's Real Decreto 1619/2012. This is general information, not tax or legal advice.

Related: invoice numbering systems, the invoice number glossary entry, what to include on an invoice and the InvoiceQuickly blog.