Invoice Numbers: How to Number Invoices

An invoice number is the one field that ties an invoice to everything else: your records, your client's accounts payable, and the payment that eventually arrives. Get the numbering right once and you rarely think about it again. Get it wrong and you spend hours matching payments to the wrong documents.

This guide covers what a good number looks like, the formats people use, what to do about gaps and cancelled invoices, and where the rules come from.

Why invoice numbers matter

A number gives every invoice a unique name. Your client quotes it when they pay, your accountant uses it to match the payment to the sale, and you use it to answer the question "which invoice is this about?" in a single word.

Many tax systems also expect invoices to be numbered in a unique, ordered way, so that an inspector can see that none has been quietly removed. Requirements differ by country, so check the official guidance for yours (see the sources below).

What makes a good invoice number

• Unique. No two invoices from the same business share a number, ever.
• Sequential. Each number follows from the one before it, so the order of issue is obvious.
• Permanent. Once an invoice has been sent, its number never changes and is never reused.
• Readable. Short enough to quote over the phone, with a prefix so it cannot be mistaken for a purchase order number.

Common numbering formats

Whichever you choose, use one scheme for the whole business and stick to it.

Rules worth following

• Start at a number that looks natural to you (0001, or 1001 if you would rather not show how new the business is). Either is fine.
• Never edit the number on an invoice you have sent. If something is wrong, issue a correction instead.
• Never reuse a number, even for an invoice you cancelled.
• Keep a separate sequence for each legal entity you invoice from.

Gaps, cancellations and corrections

Sooner or later a draft gets abandoned or an invoice goes out with a mistake. The safe approach is not to delete it and renumber the rest.

• Unsent draft you no longer need: void it and keep a note, so a missing number can be explained.
• Sent invoice with an error: cancel it and issue a credit note, then send a new invoice with the next number. See credit notes explained.
• A skipped number: write down why. A gap you can explain is a very different thing from one you cannot.

Quotes, credit notes and several businesses

Give each type of document its own prefix and its own sequence: INV- for invoices, QUO- for quotes, CN- for credit notes. That keeps an accepted quote from being mistaken for an invoice. See quotes, estimates and invoices for how the documents fit together.

What tax authorities say

Rules differ by country and by whether you are registered for VAT or GST. In the UK, GOV.UK's guidance on VAT invoices sets out what a VAT invoice has to show, so check it for the numbering rules that apply to you. Elsewhere, look at your tax authority's guidance, or ask an accountant. Zap Bill does not file anything with a tax authority and this guide is not tax or legal advice.

Numbering in Zap Bill

In a Zap Bill account, saved invoices are numbered in sequence and you can set your own prefix and starting number in your profile. In the free generator that needs no account, the invoice number is a field you can overwrite, so replace the suggested number with one from your own sequence.