Sequential, unique, unbroken. Boring — until an accountant or HMRC asks why invoice 47 does not exist.
13 August 2026 · 4 min read
Invoice numbers are one of those requirements nobody explains until something goes wrong. The rule is simple: every invoice needs a unique identifying number, and the sequence should be continuous.
A missing number invites the obvious question: was there an invoice there that has been removed? Usually the answer is innocent, but you will be the one explaining it. Keeping the run unbroken avoids the conversation entirely.
Do not delete it and reuse the number. Issue a credit note against it, or mark it cancelled and keep it in the sequence. The record shows what happened, which is the entire point.
You do not have to start at 1. Continue from where your old system stopped, or start a clearly different series. What matters is that the new sequence is consistent from then on.
Invoices are numbered automatically and sequentially, so the sequence stays intact without you tracking it. See the invoices guide for the full flow from quote or job through to a numbered PDF.
If you are VAT registered, an invoice needs additional details including your VAT number and the VAT breakdown — the VAT invoice template shows the required layout.
Each invoice needs a unique identifying number and the sequence should be continuous, without unexplained gaps. It is a record-keeping requirement rather than an arbitrary rule — the sequence is what shows nothing has been quietly removed.
Keep it in the sequence and mark it cancelled, or issue a credit note against it. Never delete an invoice and reuse its number — that creates exactly the gap that raises questions.
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