How should I number my invoices?

Give each invoice a unique number that runs in sequence with no gaps or duplicates. A common, practical format is a year prefix with a four-digit number, such as 2026-0001, 2026-0002. Keep one continuous sequence and let your invoicing tool assign the next number automatically.

What number should my first invoice be?

It does not have to be 1. Many businesses start at 0001 within a year prefix, or at a round number like 1001, so an early client cannot tell they are your first. Use leading zeros so every number is the same length and sorts correctly.

Do invoice numbers have to be sequential?

In the UK and EU, a valid VAT invoice must have a unique, sequential number, so for VAT-registered businesses it is required. In the US there is no mandated format, but a clean sequence is expected for bookkeeping and checked in an audit. Keeping numbers unique and sequential is good practice everywhere. This is general information, not tax or legal advice.

Can I use the same invoice number twice?

No. Every invoice must have its own unique number. Reusing a number makes it impossible to tell two bills apart, confuses payment matching, and looks like an error or worse to an auditor. If you need to revise an issued invoice, raise a new one with the next number or a clearly marked credit note.

How do I number invoices for different clients?

You can either keep one continuous sequence across all clients (simplest, recommended for most) or add a client code such as ACME-0007 and run a separate sequence per client. Client codes help if you have a few ongoing clients, but they are more to track, so keep it simple unless you have a clear reason.

What happens if I skip an invoice number?

A gap in the sequence raises questions, because it looks like a missing or deleted invoice. If a number was skipped by mistake, record why in your invoice log; if an invoice was drafted but never sent, mark it void rather than reusing the number. Letting your invoicing tool assign numbers automatically prevents skips in the first place.

Getting paid18 October 2026·7 min read·Written by Remit Team

How to Number Invoices

Invoice numbering sounds like the most boring part of running a business, right up until a client queries a bill, your bookkeeper finds a gap in the sequence, or a tax office asks why two invoices share the same number. A good numbering system takes thirty seconds to set up and saves you all of that. Here is how to number your invoices properly: the one rule that actually matters, the formats worth using, what number to start at, where it is a legal requirement, and how to change systems later without creating a mess.

A freelancer checking figures on a calculator beside printed invoices, choosing an invoice numbering system

The one rule: unique and sequential

Strip away the formatting debates and invoice numbering comes down to a single principle: every invoice number must be unique and run in sequence, with no duplicates and no gaps. Each invoice gets its own number, and the next invoice gets the next number up. That is it.

The reason is simple. The invoice number is the handle everyone uses to refer to that one bill: you, your client, your accountant, and the tax office. If two invoices share a number, or the sequence jumps from 0012 to 0015 with nothing in between, it looks like a mistake at best and like you are hiding income at worst. A clean, unbroken run is one of the first things a bookkeeper or auditor glances at, because a tidy sequence signals tidy books.

So before you worry about any of the formatting below, lock in the habit: never reuse a number, never skip one, and always go up by one.

Formats worth using

Within that rule you have room to choose a format. The common ones, from simplest to most organised:

Plain sequential. 0001, 0002, 0003. Simple and perfectly valid. The only drawback is it tells you nothing at a glance about when or for whom the invoice was raised.
Year prefix. 2026-0001, 2026-0002. The most popular choice, because the year is instantly visible and you can restart the sequence each January if you like. This is a good default for most freelancers and small businesses.
Client or project code. ACME-0007, or 2026-ACME-003. Helpful if you have a handful of ongoing clients and want to see at a glance who a bill belongs to. The catch is that you now run a separate sequence per client, which is more to track, so it suits steady client relationships rather than lots of one-off jobs.
Date-based. 20260118-01 (date plus a daily counter). Useful if you raise several invoices a day, less so otherwise.

There is no single correct answer, only the one that matches how you work. If in doubt, use a year prefix with a four-digit number, and keep one continuous sequence across all clients.

What number should you start at?

You do not have to start at 1, and many people choose not to. If you would rather not advertise that a client is your very first, start at a round number like 1001 (or 2026-1001) instead of 0001. If that does not bother you, 0001 is perfectly fine. No tax authority requires a particular starting number; the only rule is that every number after it is unique and in order.

Use leading zeros so the width stays constant: 0001, 0012, 0123 all line up, whereas 1, 12, 123 sort oddly and look inconsistent. Three or four digits is plenty for most small businesses.

Is sequential numbering a legal requirement?

This depends on where you are, and it is worth knowing rather than guessing. In the UK and across the EU, a VAT invoice must carry a unique number that follows in sequence. Importantly, the EU rules (the VAT Directive, Article 226) allow that sequence to be based on one or more series, for example a separate run per year or per branch, as long as each number identifies one invoice. The UK guidance on what invoices must include lists a unique identifying number that follows on from the last invoice. If you are VAT registered, treat this as required and be ready to explain any gap. In the United States, the IRS does not prescribe an invoice format; it expects records that support your income and expenses, and an unbroken sequence simply makes that easy to show. Other countries vary, and some require strict consecutive numbering for tax, so if you are unsure, check your local rules. This is general information, not tax or legal advice.

The safe move everywhere is the same: keep it unique and sequential. That satisfies the strict regimes and is good practice in the relaxed ones.

How to switch systems without breaking the sequence

At some point you may outgrow your format, for example moving from plain 0045 to a 2027-0001 year prefix. That is fine, as long as you do not break the core rule. Do not renumber old invoices, that only causes confusion with clients and your records. Instead, close out the old sequence and start the new one going forward, ideally at a natural break like the start of a tax year or calendar year.

Keep a short note of where you switched (for example, "invoices up to 0045 used the old format; 2027-0001 onward uses the year prefix") so the one-off jump in style is explained if anyone ever asks. The numbers before and after are each internally sequential, which is what matters.

Corrections, credit notes and voids

The rule that you never reuse or delete a number raises an obvious question: what do you do when an invoice is wrong or cancelled? You do not delete it, because that leaves a gap. Instead you correct it cleanly:

To cancel or refund an issued invoice, raise a credit note. A credit note reverses some or all of an invoice and carries its own number, ideally in its own short series (for example CN-2026-0003) so it is easy to tell credit notes from invoices. Then issue a fresh invoice with the next number if a corrected one is needed.
If an invoice was drafted but never sent, mark it void rather than reusing its number, and keep the voided record.
Never edit a sent invoice in place. Once a client has it, the figures are part of both sides' records, so a change has to be a new document, not a quiet overwrite.

The theme is the same throughout: every number, including cancelled ones, stays in the record. That is what keeps the sequence honest and your books easy to reconcile.

Let the tool do the counting

The single biggest cause of numbering errors is doing it by hand in a spreadsheet, where it is easy to copy an old invoice and forget to change the number, or to lose track after a gap. The fix is to let your invoicing tool keep the sequence for you. Remit's free invoice generator lets you set your format and number once, and then each new invoice carries the next number automatically, so you never duplicate or skip. If you are also still learning what belongs on the document itself, our guide on how to write an invoice walks through every field, numbering included.

Get the system right once and invoice numbering becomes what it should be: invisible. The bills go out in a clean, unbroken run, your records reconcile, and nobody ever has to ask you about a missing number.