Invoice numbers: the rules for consecutive numbering

What the law requires

Under the VAT rules every invoice gets a sequential number, based on one or more series, that identifies the invoice uniquely. That means three things: each number occurs only once, the numbers follow each other, and you must be able to explain any missing number. You choose the format yourself. Digits, letters, a year or a code: all of it is allowed, as long as the order is right and nothing appears twice.

The invoice number is on the list of mandatory details, alongside your Chamber of Commerce number, your VAT ID, the invoice date and the VAT breakdown, among others. What else must be on an invoice is covered in making your first invoice. The numbering matters for income tax too: a complete series is the simplest proof that all your revenue is in your records.

Structure: one series, a year prefix or several series

The simplest structure is one continuous series that never restarts: 1001, 1002, 1003. Many freelancers prefer a year prefix, such as 2026-001 and 2026-002. That is allowed: in January you start with 2027-001, because the year keeps the number unique. A date as the invoice number works less well, because two invoices on one day immediately cause a problem.

You may also use several series side by side, as long as each series is continuous in itself and stays recognisable. That is useful if you run several activities under one sole proprietorship, or if your web shop creates invoices automatically while you invoice consulting work yourself. Give each series its own prefix, such as W2026-001 for the shop and A2026-001 for consulting, and record which series you use. That way an auditor can check each series separately.

A gap in the series: explain it, do not fill it

A gap arises faster than you think: a draft invoice that received a number and was then deleted, a switch of accounting software or a typo in manual numbering. A gap is not a disaster in itself, but you must be able to explain it. So record which number is missing and why, for example with a short note in your records: 'invoice number 2026-017 not used, draft deleted on 3 March'.

Do not fill a gap later with a new invoice. The order of numbers and dates then no longer matches, which raises exactly the questions you want to avoid in a tax audit. The best way to prevent gaps is software that only assigns a number once you finalise the invoice; look for that when you choose accounting software. If you switch packages, note the last number from the old one and continue from there in the new one, or start a new, recognisable series.

Credit invoices, platform invoices and e-invoices

A credit invoice always gets its own number. Most packages include it in your normal series; a separate credit series is also allowed, as long as that series is continuous itself. On the credit invoice you refer to the number of the original invoice. The full step-by-step plan is in fixing an error with a credit invoice.

If you sell through a platform that creates invoices on your behalf, that platform uses its own numbering. Treat it as a separate series and export the invoices monthly, so the series is complete in your records. The same applies to an e-invoice: the format changes, the numbering rules do not. And keep everything for seven years, because a complete series only proves something if you can actually show it.

Frequently asked questions