How to number invoices (and why gaps matter)

Invoice numbers sound trivial, but getting them right keeps you compliant and makes your bookkeeping painless. Here’s how to do it properly.

The two rules: unique and sequential

Every invoice needs its own unique number, and for VAT invoices HMRC requires the numbers to be sequential — a continuous run with no unexplained gaps. Gaps look like missing income to an inspector, so keep the sequence unbroken. (See what a VAT invoice must include.)

Even if you’re not VAT registered, sequential numbering is the right habit — it proves your records are complete.

Using a prefix

A short prefix makes numbers readable and sortable:

• INV-0001, INV-0002, …
• Some businesses add the year — INV-2026-001 — and reset the count each year. That’s fine, as long as the combination stays unique.

Zero-padding (0001 rather than 1) keeps them tidy and sortable in folders and spreadsheets.

Keep separate sequences for other documents

Quotes, proformas and credit notes aren’t invoices, so give them their own sequences so nothing collides:

• Invoices — INV-0001
• Quotes — QUO-0001
• Credit notes — CN-0001

The generator handles this for you, keeping invoice and quote numbering separate automatically.

Don’t reuse or delete numbers

If you cancel an invoice, don’t delete it and hand the number to a new one. Keep the original and issue a credit note to reverse it. That leaves your sequence intact and your records self-explanatory.