How to Number Invoices: Formats, Gaps and Credit Notes

An invoice number looks like a small detail until something goes wrong. A client pays "the November invoice" and you sent two. Their accounts system rejects a number it has already seen. Or, a year later, someone asks why invoice 0047 does not exist.

A good numbering system prevents all three. This guide covers the rules, the common formats, year-based and multiple series, gaps and voided invoices, credit notes, and how to fix a numbering mistake.

The rules below come from official pages in the UK, the EU, Ireland, Australia and the US, read on 6 November 2026 and listed at the end. This is general information, not tax or legal advice. If you are registered for VAT or GST, check the page for your country or ask your accountant.

The short answer

Whatever format you choose, four rules keep you safe:

1. Every number is unique. No two invoices ever share a number.
2. Numbers follow a sequence. Each new invoice takes the next number in its series.
3. A number is never reused, even when the invoice that had it was cancelled.
4. Every number you issue stays in your records, cancelled invoices included.

What tax authorities say

The rules depend on where you are and whether you charge VAT or GST:

Where	Source	What it says about the number
UK, every invoice	GOV.UK	Each invoice needs a unique identification number
UK, VAT invoices	HMRC, VAT Notice 700 (16.3.1) and 700/21	A sequential number, from one or more series, that uniquely identifies the document
European Union	European Commission	A full VAT invoice carries a unique sequential number that identifies it
Ireland	Revenue	A VAT invoice shows a unique sequential number
Australia	ATO and business.gov.au	The ATO's list for tax invoices has no invoice number; business.gov.au calls a unique number a good idea on tax invoices and common practice on regular ones
United States	IRS	The IRS page on records we read sets no numbering format; invoices are among the supporting documents for your gross receipts

HMRC's wording for VAT invoices is "a sequential number based on one or more series which uniquely identifies the document". Two parts of it matter in practice. "Sequential" means numbers follow an order. "One or more series" means you may run several sequences, as long as each number stays unique.

The European Commission's explanatory notes on the invoicing rules go further. They say a series may include letters, and that a business may keep separate series, for example per branch, per type of supply or per customer. The notes are informal guidance and not legally binding, and each EU country writes the rules into its own law, so check your country's tax authority page.

For Canada, New Zealand and the other countries we cover, see our invoice template pages.

Common invoice number formats

Format	Example	Good for	Watch out for
One running sequence	INV-0001, INV-0002	Most freelancers and small teams	Nothing, as long as you never restart it
Year, then a sequence	2026-001, then 2027-001	Seeing at a glance which year it belongs to	The year is what keeps 2027-001 unique: never drop it
Year prefix, continuous number	2026-0141, 2027-0142	The year at a glance, without a reset	Longer numbers
A series per client	ACME-001, ACME-002	A few large clients with their own records	Each series must stay sequential and never overlap
A series per document type	INV-0001, CN-0001	Keeping invoices and credit notes apart	Use each prefix for one type only
Date-based	20261106-01	Several invoices on the same day	Gaps are harder to spot

Three habits help with any format:

• Pad with zeros. Spreadsheets sort numbers stored as text character by character, so INV-10 lands before INV-9. With INV-0009 and INV-0010, the order stays right.
• Pick a starting number and keep going. None of the official pages we read sets a starting number. Some freelancers start at 1001 so a first client does not see invoice 1. What matters is that the sequence continues from there.
• Ask clients to quote the number as their payment reference, so each payment matches one invoice.

Year-based series or one continuous sequence?

Both fit the "one or more series" wording, if the full number stays unique. A series that restarts every January starts a new series each year, and the year in front is what tells 2026-001 and 2027-001 apart.

One continuous sequence	A series that restarts each year
Nothing to change on January 1	The year is visible in every number
Gaps are easy to spot across years	The number shows how many invoices you sent that year
Numbers get longer over time	A dropped year prefix creates duplicates
Works the same in any tool	Your tool must support a yearly reset, or you do it by hand

For most freelancers, one continuous sequence is the simpler choice. If you prefer a yearly series, switch on January 1, not mid-year, keep your old numbers as they are, and make sure the new format cannot produce a number you have already used.

Multiple series: when they make sense

You can run more than one series, but each one is something else to keep in order. They make sense when:

• You run two businesses or trading names. Each keeps its own sequence, the same way it keeps its own records.
• You issue different documents. Quotes and credit notes are not invoices, so give them their own prefixes (Q-0001, CN-0001) and they never take an invoice number.
• A client asks for it. Some large clients prefer a dedicated series. Keep it sequential, and keep a list of every series you use.

A converted quote becomes an invoice with the next invoice number. Mention the quote number on the invoice ("Based on quote Q-0012") so the client can match the two.

Gaps and voided invoices

A gap is a number in your sequence with no invoice behind it. None of the pages we read says a gap is forbidden, but anyone checking your records may ask what happened to the missing number. Keep the answer ready.

Gaps usually come from three places:

• A deleted draft. Many tools give a draft its number when you create it, so deleting the draft leaves a hole. A draft that never left your hands was never an invoice, but write the reason down.
• A typo. You typed 0048 instead of 0047. Leave the gap and note it; do not renumber invoices a client already has.
• A cancelled invoice. This one is not really a gap: the invoice exists and must stay in your records.

A voided (cancelled) invoice is one you issued and then cancelled, because it went to the wrong client, duplicated another one or should never have been sent. Do not delete it, and do not give its number to a new invoice. Mark it cancelled, keep a copy, and if the client received it, send a credit note that cancels it.

Credit notes

A credit note reduces or cancels an invoice you already issued. It is a document in its own right, with its own number and date:

• UK (VAT): HMRC lists what a valid credit note shows, including its own identifying number and date of issue, the number and date of the original VAT invoice, and the VAT credited, in sterling (VAT Notice 700, 18.2.3).
• EU: in the European Commission's list of invoice details, a credit note issued as a simplified invoice must refer specifically and unambiguously to the original invoice and to the details being changed. National rules can add to this, so check your country's tax authority.
• Australia (GST): corrections go through adjustment notes. The ATO says you generally need a valid one before you can make a decreasing adjustment, unless the adjustment is for GST of $75 or less.

Many businesses number credit notes in their own series, such as CN-0001, so they never take an invoice number. Whichever you choose, keep the series sequential, and keep each credit note with the invoice it corrects.

What to do after a numbering mistake

Mistake	What to do
An error in a draft you have not sent	Fix the draft. Nothing else to record
A sent invoice charged too much	Send a credit note that refers to it, then a correct invoice with the next number if the work is still billable
A sent invoice charged too little	Send a supplementary invoice for the difference, or cancel with a credit note and reissue
A sent invoice has the wrong client or details	Cancel it with a credit note and issue a new invoice. In Australia, replace an incorrect tax invoice when the buyer asks for it
Two invoices with the same number	Keep the first. Cancel the second with a credit note, reissue it with the next free number, and tell the client which to pay
A skipped number	Leave the gap and note why. Do not renumber invoices you already sent
The sequence restarted at 1	Go back to the highest number you used and continue from there. Cancel and reissue any duplicates

For VAT invoices, HMRC says a credit note or supplementary invoice that corrects an error should refer to the number and date of the original invoice, and show both the correct and incorrect VAT amounts (VAT Notice 700, 19.8). For anything involving VAT or GST, check the correction with your accountant before you send it.

Key takeaways

• Every invoice gets a unique number from a sequence, and a number is never reused.
• UK and EU VAT invoices need a sequential number; the series may be one or several, if each number stays unique.
• The ATO's tax invoice list has no invoice number, but business.gov.au recommends one.
• One continuous sequence is the simplest choice; a yearly series works if the year stays in the number.
• Never delete a sent invoice: cancel it with a credit note, keep it in your records, and reissue with a new number.
• Write down the reason for every gap.

For the rest of the invoice, see how to make an invoice as a freelancer. For the December routine, our year-end invoicing checklist includes a numbering check.

Sources

Read on 6 November 2026:

• GOV.UK, Invoices: what they must include
• HMRC, VAT guide (VAT Notice 700), sections 16.3, 18.2.3 and 19.8
• HMRC, Record keeping (VAT Notice 700/21), section 4.1
• European Commission, Invoicing
• European Commission, Explanatory notes on the VAT invoicing rules (2011, informal guidance)
• Revenue, What information is required on a VAT invoice?
• ATO, Tax invoices and Adjustment notes
• business.gov.au, How to invoice
• IRS, What kind of records should I keep

How Clodou helps

Clodou is an AI back office for freelancers and small teams: you describe the invoice in a sentence, and it numbers, totals and formats it. Here is exactly how its numbering works:

• Two series per workspace. Invoices are numbered INV-0001, INV-0002 and on; quotes DEV-0001, DEV-0002 and on. Each workspace has its own series, and a number cannot be used twice in the same workspace.
• One continuous sequence. There is no yearly reset, no custom prefix and no choice of starting number. If you need a year-based series, Clodou cannot produce one today. If you move from another tool whose numbers also start with INV-, ask your accountant how to keep the two sequences apart, because Clodou starts at INV-0001.
• How the next number is chosen. A new invoice takes the highest invoice number in the workspace plus one, whether it comes from the chat, a converted quote, unbilled hours or a retainer draft. Numbers follow the order invoices are created, not their issue dates.
• Deleted drafts. Clodou deletes only drafts, after an owner or admin approves. If you delete the most recent draft, its number is free again and the next invoice takes it. If you delete an older draft, its number stays unused and leaves a gap, so note why.
• Quotes to invoices. "Turn quote DEV-0002 into an invoice" creates a draft invoice with the next INV number and the line "Based on quote DEV-0002" in its notes.
• Checking the sequence. The Documents page exports your list as CSV with the number, type, status, client, dates and amounts, ready to sort in a spreadsheet.

Clodou does not issue credit notes, and it has no void or cancelled status: a discount line can lower an invoice but never bring it to zero or below. To cancel or reduce an invoice you already sent, issue the credit note with your accountant or accounting software and keep it with the original, which stays in Clodou's list. Clodou also does not check your numbering against any country's VAT or GST rules.

Every new workspace starts with a 14-day Team trial, no card needed. After that, the Free plan includes 20 AI requests and 3 quotes or invoices a month; Solo is $29 and Team $79 a month per workspace.