Invoice Numbering and Credit Notes: Getting Them Right

Every invoice you issue needs its own unique number, and once a number has been used it should never be deleted or reused. If an invoice turns out to be wrong, you correct it with a credit note, not by editing or removing the original. Those two habits, a clean number sequence and credit notes for corrections, are what make a sales record that you, your accountant and HMRC can all follow.

Key takeaways

• A unique identification number is a legal requirement on every invoice.
• Use one consistent, running sequence and never reuse a number.
• Do not delete or overwrite an issued invoice. Cancel it with a credit note.
• Credit notes need their own numbers and should refer to the original invoice.
• Pro forma invoices and quotes are not invoices and sit outside the sequence.

What is an invoice number?

An invoice number is the reference that identifies one invoice and no other. It is how a customer tells you which bill they are paying, how you match a bank receipt to a sale, and how anyone reviewing your books confirms that every sale has been counted once. GOV.UK lists "a unique identification number" first among the things an invoice must contain.

What every invoice must include

According to the GOV.UK guidance on what invoices must include, your invoice must show:

• a unique identification number;
• your company name, address and contact information;
• the name and address of the customer you are invoicing;
• a clear description of what you are charging for;
• the date the goods or service were provided (the supply date);
• the date of the invoice;
• the amount or amounts being charged;
• the VAT amount, if applicable;
• the total amount owed.

Sole traders must also show their own name and any business name they use, plus an address where legal documents can be delivered if they trade under a business name. Limited companies must show the full company name as it appears on the certificate of incorporation. If you choose to put directors' names on the invoice, you must include all of them.

If you and your customer are VAT registered, you must use VAT invoices, which carry more detail. Our guide to VAT invoice requirements covers the extra items.

How to number invoices

The law asks for a unique number. It does not prescribe a format, so you can choose one that suits the business, provided you stick to it. HMRC's VAT guidance describes the number on a VAT invoice as a sequential number based on one or more series, so a running sequence is the safe choice for everyone.

• Plain sequence: 1001, 1002, 1003. Simple and hard to get wrong.
• Year prefix: 2026-001, 2026-002. Useful if you like to see the year at a glance.
• Series prefix: separate series for different parts of the business, such as LON-0001 and MAN-0001. Each series must run in order.

Avoid numbering by customer or by date alone. Two invoices to different customers on the same day would clash, and gaps become impossible to spot. Let your bookkeeping software assign the number automatically where you can. If you are still choosing software, see our bookkeeping software comparison.

Why you never delete or reuse a number

A gap in the sequence raises an obvious question: was there a sale that has not been declared? A duplicate raises the opposite one: has the same sale been counted twice? Either way, someone has to spend time finding out. HMRC's guidance on keeping VAT records says to keep copies of all the sales invoices you issue, even if you cancel them or produce one by mistake. The same discipline serves a business that is not VAT registered, because the general duty to keep accurate business records applies to everyone.

What is a credit note?

A credit note is a document that reduces or cancels an invoice already sent. You issue one when a customer returns goods, when you agree a discount after invoicing, when you overcharged, or when the invoice went to the wrong customer. A sensible credit note shows:

• its own unique number, usually from a separate series such as CN-001;
• the date of issue;
• your details and the customer's details;
• the number and date of the invoice it relates to;
• the reason for the credit;
• the amount credited, with any VAT shown separately.

Invoice, credit note and pro forma compared

Step by step: correcting an invoice

1. Leave the original alone. Do not edit, delete or void it in a way that removes it from your records.
2. Raise a credit note against it, for the full amount if the whole invoice is wrong or for the difference if only part is.
3. Quote the original invoice number and the reason on the credit note.
4. Issue a new invoice with the next number in the sequence if the customer still owes something.
5. Send both documents to the customer so their records match yours.
6. Deal with any payment already made: refund it or allocate it to the new invoice.
7. Check the customer's balance in your software and that the entries agree to the bank. See bank reconciliation explained.

Worked example (illustrative)

"Priya", an illustrative example, is a sole trader who fits kitchens and is not VAT registered. On 5 October 2026 she sends invoice 1047 to a customer for £2,400. Two days later the customer points out that the agreed price was £2,150, because they supplied the worktop themselves.

Priya's first instinct is to open invoice 1047 and change the figure. She does not. She issues credit note CN-012 for £2,400, dated 7 October, stating "Cancels invoice 1047, incorrect price". She then issues invoice 1049 for £2,150, because 1048 has already gone to another customer. Her records now show three linked documents: £2,400, minus £2,400, plus £2,150. Her sales for the job are £2,150 and the sequence has no gaps.

A simpler route would have been a partial credit note for £250 against invoice 1047, leaving £2,150 due. Either is fine. Deleting 1047 is not.

Priya and her figures are invented for illustration and are not a real client.

Common mistakes

• Editing an invoice after it has been sent. Your copy and the customer's copy no longer agree.
• Restarting at 1 each year without a prefix. You end up with two invoices numbered 1.
• Running two systems at once, such as a spreadsheet and an app, each producing its own numbers.
• Giving pro formas real invoice numbers, which leaves gaps when the sale does not go ahead.
• Using a credit note number that matches an invoice number. Keep the two series visibly different.
• Leaving out the supply date when it differs from the invoice date.
• Not keeping copies. Store every invoice and credit note. Our guide to how long to keep business records explains the retention periods.

How we can help

We set up invoice and credit note sequences in your software, tidy up gaps and duplicates in existing records, and keep your sales ledger reconciled each month. Bookkeeping starts from £150 per month on a fixed fee. See our bookkeeping service, view our pricing, or contact us if your invoice numbering needs sorting out.