Invoice number: how to number your invoices

Every invoice you send needs a number. It is a small detail, but a sensible numbering system saves you time when a client pays, when you look for an old invoice, and when anyone reviews your records. Here is how to set one up and keep it tidy.

What is an invoice number?

An invoice number is a unique reference assigned to each invoice you issue. It identifies that invoice and no other. Your client uses it when they pay (“payment for INV-0042”), your bookkeeping uses it to match payments to invoices, and you use it to find the right document when someone has a question.

It is one of the standard parts of an invoice, alongside the date, the parties, the line items and the total. Our guide to what an invoice is covers the rest.

Why invoice numbers should be unique and sequential

Unique is the essential part. If two invoices share a number, payments get applied to the wrong one, and it becomes hard to tell which document is the real one.

Sequential means each new invoice takes the next number in the series. That gives you:

• A complete trail. Anyone looking at your invoices can see that none are missing.
• Easy ordering. Sorting by number is the same as sorting by date of issue.
• Fewer mistakes. You always know what the next number should be.

In many places it is also a legal requirement. Under EU VAT rules, for example, an invoice must carry a sequential number, based on one or more series, that uniquely identifies it. The UK has a similar rule for VAT invoices. Requirements differ by country, and some are stricter than others, so check what applies where you work.

Common invoice number formats

There is no single correct format. Pick one that suits you and stick to it.

A note on client prefixes: if you number per client, each series must still be unique as a whole. ACME-001 and GLOBEX-001 are different numbers, which is fine, but keep the rule consistent so you never issue the same number twice.

Rules of thumb

• Never reuse a number. Not even for a cancelled or deleted invoice.
• Pad with zeros. INV-0009 and INV-0010 sort correctly as text; INV-9 and INV-10 often don’t.
• Leave room to grow. Four digits covers most small businesses for years.
• You don’t have to start at 1. Starting at 1001 or similar is common and perfectly acceptable, as long as you count up from there.
• Keep the format consistent. If you change it, start the new format cleanly (for example at the start of a year) and keep a note of when and why.
• Use the number everywhere. In the PDF file name, the email subject, and as the payment reference you ask clients to quote.

What to do with a voided or incorrect invoice

Mistakes happen: a wrong price, the wrong client, a duplicate. How you fix one depends on whether it was sent.

If you haven’t sent it yet, you can usually correct it and keep the number. If you decide not to use it at all, mark it as void and keep a copy, rather than deleting it and handing the number to the next invoice.

If you have already sent it, don’t edit it and don’t reuse its number. Instead:

1. Issue a credit note that refers to the original invoice number and cancels all or part of it.
2. If something is still owed, issue a new invoice with the next number in your series.

This leaves a clear record on both sides: the original invoice, the credit note that reverses it, and the correct invoice. Credit notes usually get their own series, such as CN-0001.

What about gaps?

Sometimes a number gets skipped: a draft deleted by accident, a typo that jumped from INV-0041 to INV-0043. An occasional gap isn’t the end of the world, but in places with strict rules, an unexplained gap can prompt questions during a review. The safe habit is to keep a short note of any skipped or voided numbers and why. If a voided invoice exists, keep it on file rather than deleting it.

Multiple numbering series

One sequence doesn’t have to cover everything. Common reasons to run several:

• Different document types. Quotes (QUO-), estimates (EST-), purchase orders (PO-), receipts (REC-) and credit notes (CN-) each get their own series, separate from invoices. A quote that never turns into a sale then doesn’t leave a hole in your invoice numbers.
• Different branches or brands. A separate prefix per location or business line, such as NW-0001 and NS-0001.
• Yearly series. Restarting at 2027-001 on January 1. The year keeps each number unique.

Where tax rules apply, each series should still be sequential on its own, and every number should be unique across all of them. Some countries have specific rules about when multiple series are allowed, so check if you’re unsure.

How KeenInvoice handles numbering

You can type any number and format you like into an invoice. After that, KeenInvoice keeps count for you: click New and the next document picks up where you left off, so INV-0009 is followed by INV-0010. Your business details, logo, currency, tax settings, payment details and design carry over at the same time.

Every document you create is listed in My documents, where you can search your documents, see which invoices are paid or overdue, and export a CSV for your records. Because everything is stored in your browser rather than an account, it’s worth exporting a backup now and then.

For the full process from blank page to sent PDF, see how to make an invoice.

Start numbering properly

If you’re setting up your numbering for the first time, pick a format from the table above and create your first invoice with the invoice generator. From then on, each new invoice counts on from the last.