The short answer
- Give every invoice a unique number that follows on from the previous one. No duplicates, no reused numbers, no going backwards.
- UK VAT invoices need “a sequential number based on one or more series which uniquely identifies the document” (HMRC). Non-VAT invoices in the UK need a unique identification number. The EU VAT Directive uses almost the same wording.
- In the US there’s no federal numbering rule, but the IRS expects invoices among the records that support your income. A clean sequence is simply the easiest way to prove nothing is missing.
- Formats are up to you. INV-2026-0042 is a good default: the year lets you restart at 1 each January without duplicates.
- Never delete or overwrite a sent invoice. Cancel or correct it with a credit note that quotes the original number and date, then issue a new invoice with the next number.
What the rules actually say
United Kingdom. HMRC’s record-keeping notice (VAT Notice 700/21, section 4.1) lists the details a full VAT invoice must show, starting with “a sequential number based on one or more series which uniquely identifies the document”. If you aren’t VAT-registered, GOV.UK’s guidance for all invoices still asks for “a unique identification number”, alongside your details, the customer’s, the supply date and the amounts.
European Union. Article 226 of the VAT Directive (2006/112/EC) requires “a sequential number, based on one or more series, which uniquely identifies the invoice”. Each member state writes this into its own law. France, for example, asks for a number “based on a chronological and continuous sequence”, and its tax guidance allows separate series only when the way the business operates justifies them.
United States. No federal law dictates invoice numbers. The IRS’s guide to record-keeping for small businesses (Publication 583) lists invoices among the documents that show your gross receipts, next to deposit slips and receipt books. Numbering isn’t mandatory, but if an auditor asks how you know every sale was recorded, an unbroken sequence answers the question in seconds.
So whatever your country, the practical rule is the same: unique, in order, no gaps you can’t explain.
Pick a format that will last
| Format | Example | Good for | Watch out for |
|---|---|---|---|
| Plain counter | 1, 2, 3… 142 | The simplest start | Nothing tells the years apart |
| Year + counter | 2026-0001, 2026-0002 | Restarting each year without duplicates | Leave enough digits (4 covers up to 9,999 a year) |
| Prefix + year + counter | INV-2026-0042 | Telling invoices, quotes and credit notes apart | Keep the same prefix year after year |
| Series by location | SHOP-2026-0042, WEB-2026-0017 | Two tills that can’t coordinate | Each series must stay continuous on its own |
Plain counter
Example1, 2, 3… 142
Good forThe simplest start
Watch out forNothing tells the years apart
Year + counter
Example2026-0001, 2026-0002
Good forRestarting each year without duplicates
Watch out forLeave enough digits (4 covers up to 9,999 a year)
Prefix + year + counter
ExampleINV-2026-0042
Good forTelling invoices, quotes and credit notes apart
Watch out forKeep the same prefix year after year
Series by location
ExampleSHOP-2026-0042, WEB-2026-0017
Good forTwo tills that can’t coordinate
Watch out forEach series must stay continuous on its own
The yearly counter is the most common choice, and it works under UK and EU rules because the year prefix keeps every number unique. Resetting every month (2026-10-001, then 2026-11-001) is harder to defend: you’re effectively running twelve series a year with no business reason for them. Don’t.
Two formats to avoid:
- Client names in the number (“SMITH-01”). You end up with one series per client, which makes “sequential” meaningless. The client’s name is already on the invoice.
- Numbers that encode the date and nothing else (“20261006”). Two invoices on the same day collide.
When separate series make sense
“One or more series” is written into both the UK and EU wording, so running more than one sequence is allowed. The test is whether each series is unique and continuous in itself, and whether you can explain why it exists. France’s tax guidance gives useful examples of good reasons that apply anywhere:
- several places that issue invoices, such as a shop and a market stall;
- different customer types with different invoicing rules, such as businesses and consumers;
- several ways of issuing invoices at once, such as paper and electronic, or invoices issued by a third party on your behalf.
Give each series its own prefix. Example: Priya runs a gift shop in Leeds and sells at a Saturday market. She numbers SHOP-2026-0001 onwards at the till and MKT-2026-0001 onwards at the stall. Both run in parallel, and neither ever skips a number.
What doesn’t work is opening a new series to hide a problem in the old one. If the main sequence has a gap, explain the gap.
Mistakes that create gaps or duplicates
- Deleting an invoice that went wrong. A common one. The number vanishes and your sequence has a hole. See the next section for the right fix.
- Numbering drafts. If your tool assigns a number when you start typing, every abandoned draft leaves a gap. Number at the moment you issue.
- Backdating. An invoice dated 3 October with a higher number than one dated 8 October breaks the order.
- Two tools, two counters. A spreadsheet on the laptop and an app on the phone will eventually both produce INV-2026-0043.
- Quotes in the invoice sequence. A quote isn’t an invoice. If it takes an invoice number and the client says no, you’ve got a gap. Number quotes separately (Q-2026-…); our guide to quotes vs invoices explains the handover.
- Restarting at 1 when you switch software. Carry on from the last number instead.
Cancelled or wrong invoices: the credit note method
Once an invoice has been sent, don’t delete it and don’t edit it. Leave it in your records and correct it with a second document.
Credit note. HMRC’s VAT guide (Notice 700, section 18.2) says a VAT credit note must show, among other things, its own identifying number and date of issue, the supplier’s and customer’s details, the amount credited excluding VAT, the VAT rate and amount, and the number and date of the original invoice. Outside VAT, the same structure is simply good practice.
Replacement invoice. After the credit note, issue a new invoice with the correct amount and the next number in your sequence.
The credit note has its own number. You can put it in your invoice sequence or run a separate credit note series (CN-2026-…), as long as that series is continuous too.
Worked example. On 6 October, Priya invoices a café for 40 scented candles at £12 each: INV-2026-0042, £480 plus 20% VAT, £576 in total. The agreed price was £10.50. She doesn’t touch INV-2026-0042. She issues a credit note cancelling it in full, then INV-2026-0047 (0043 to 0046 were used in between) for 40 × £10.50 = £420 plus £84 VAT, £504 in total.
| Description | Qty | Unit price | Total |
|---|---|---|---|
| Full credit of invoice INV-2026-0042 dated 06/10/2026: scented candle | 40 | −£12.00 | −£480.00 |
Credit note cancelling invoice INV-2026-0042 of 06/10/2026 (wrong unit price). Replacement invoice: INV-2026-0047 of 09/10/2026.
- Its own number, in a continuous credit note series.
- The original invoice’s number and date, quoted exactly.
- The amount credited excluding VAT, the VAT rate and the VAT amount.
- A pointer to the replacement invoice, which takes the next free number.
What about an invoice you numbered but never sent? If the number is already in your records, don’t erase it. Mark it “void”, note why, and keep it. That isn’t a rule quoted from HMRC or the IRS, but it’s what lets you explain the number if anyone asks.
Selling to clients in France or the EU
If you invoice EU clients, their tax authorities expect the same thing: a unique, sequential number. France is stricter than most in how it phrases it (“chronological and continuous”), and from 1 September 2027 French small businesses must issue e-invoices to other French businesses. That doesn’t change the numbering rule, but it’s worth knowing if you work with French clients; our guide to France’s e-invoicing reform covers the timeline.
- This month’s numbers run without gaps, series by series
- No number appears twice
- Issue dates follow the order of the numbers
- Every cancelled invoice has a credit note and, if needed, a replacement
- Every credit note quotes the original invoice number and date
- Quotes use their own numbering, separate from invoices
Invoice Corner Café: 40 scented candles at £10.50, VAT 20%
Ready: invoice INV-2026-0047 for Corner Café Ltd, £420.00 plus £84.00 VAT, £504.00 total. Save it?
The number follows your last invoice automatically; quotes keep their own sequence.
Try Binome360 for freeIn Binome360, quotes and invoices each get automatic numbering, so you never have to look up the last number you used. Your business details and legal notes sit in the footer once, and nothing is saved until you confirm. It isn’t a certified e-invoicing platform.
Frequently asked questions
Do invoice numbers have to be sequential?
For UK VAT invoices and invoices under EU VAT rules, yes: the number must be sequential and unique within its series. In the US there’s no federal requirement, but sequential numbers are the easiest way to show your records are complete.
Can I restart invoice numbers every year?
Yes, if the year is part of the number (2026-0001, then 2027-0001). Every number stays unique. Restarting without a year prefix would create duplicates, which is exactly what the rules forbid.
What do I do with the number of a cancelled invoice?
Keep it. Don’t reuse it and don’t delete the invoice. Issue a credit note that quotes the original number and date, then a new invoice with the next number in your sequence.
Do credit notes need their own numbers?
Yes. HMRC requires a VAT credit note to show its own identifying number and date of issue. You can use your invoice sequence or a separate, continuous credit note series.
What number should my first invoice be?
The rules require uniqueness and sequence, not a starting point. INV-2026-0001 is clear and easy to follow. What matters is that you never skip a number after that.
In short
One invoice, one number, in order and never reused. A year prefix lets you restart each January, separate series are fine when your business needs them, and mistakes are fixed with a credit note, never by deleting. First step: open this year’s invoices and check that the numbers run without a gap since January.
Sources
- HM Revenue & Customs, “Record keeping (VAT Notice 700/21)”, section 4.1, last updated 18 March 2024: gov.uk.
- HM Revenue & Customs, “VAT guide (VAT Notice 700)”, section 18.2 (credit notes), last updated 25 June 2026: gov.uk.
- GOV.UK, “Invoicing and taking payment from customers: invoices, what they must include”: gov.uk.
- Council Directive 2006/112/EC on the common system of value added tax, Article 226: eur-lex.europa.eu.
- Internal Revenue Service, Publication 583, “Starting a Business and Keeping Records” (supporting documents for gross receipts): irs.gov.
- Direction générale des finances publiques, BOFiP, BOI-TVA-DECLA-30-20-20-10, § 70 to 130 (French numbering rules and separate series): bofip.impots.gouv.fr.
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