How to number
your invoices

An invoice number is the single reference both you and your customer use for an invoice: to pay it, query it, match it to a bank payment and find it again when HMRC asks. The rules are short, and a good format makes them easy to keep.

What the rules say

GOV.UK lists a unique identification number as the first detail every invoice must include. For a VAT invoice, VAT Notice 700 is more specific: it must carry a sequential number based on one or more series which uniquely identifies the document. Nothing prescribes the format. What matters is that no number is used twice and each series only counts upwards.

Common formats

The invoice number generator builds any of these and lists your next numbers.

Choosing a starting number

You can start anywhere. Some businesses start at 1000 so a new customer's first invoice does not read as number one. Whatever you choose, keep counting up from there.

Allow enough digits for growth. A number that outgrows its padding still works, but 0999 followed by 1000 sorts oddly in a spreadsheet.

More than one sequence

A business with several shops, or separate trades, can run more than one series, as VAT Notice 700 allows. Give each series its own prefix, for example LEEDS-0001 and YORK-0001, so no number can appear in both.

Restarting each year

Restarting the count each year is fine when the year is part of the number, because INV-2027-0001 can never be confused with INV-2026-0001. Restarting a plain sequence would repeat numbers you have already used, which is exactly what the rules forbid.

When an invoice goes wrong

Do not delete it, reuse its number or leave a gap. Keep the original, issue a credit note to cancel or reduce it, and raise a new invoice with the next number. The sequence then tells the full story, and there is no missing number to explain. The guide on how to cancel an invoice covers the steps.

Numbers and payments

Ask customers to quote the invoice number when they pay. It turns matching a payment to an invoice into a glance at your bank statement rather than a puzzle, and it is the reference a remittance advice will carry.

Numbering across quotes, invoices and credit notes

Give each kind of document its own prefix and sequence: QUO for quotes, INV for invoices, CN for credit notes. A quote does not need to follow the invoice sequence, and keeping them apart means a declined quote never leaves a hole in your invoice numbers.

Numbers when you change software

Moving to new invoicing software is the most common moment a sequence breaks. Before you send the first invoice from the new system, set its next number to follow on from the last one you issued in the old system, and keep the old records. If the new software cannot continue your format, start a new series with a new prefix rather than repeating numbers, and note the change in your records so the switch is easy to explain.

Keeping the record

Your numbered invoices are part of the business records you must keep: at least 5 years after the 31 January submission deadline for a sole trader's tax year, and at least 6 years for VAT records. A complete, unbroken sequence makes those records easy to check, which is the point of numbering them in the first place.

Picking a format: a decision table

Whatever you choose, keep the prefix the same length and the digits the same width, so every number sorts correctly in a spreadsheet and in the customer's system.

How long a sequence lasts before the padding runs out

• 3 digits (001 to 999)about 4 years
• 4 digits (0001 to 9999)about 41 years
• 5 digits (00001 to 99999)over 400 years

Numbers across documents

Invoices are not the only numbered documents. Give each its own prefix and sequence so none interferes with another.

A credit note must carry its own identifying number and refer to the invoice it corrects. A proforma invoice is not a demand for payment, so keeping it out of your invoice sequence avoids gaps when a proforma never turns into an order. Quotes are covered in quote vs estimate.

Worked examples

A sole trader starting out chooses INV-0001. After eighteen months the sequence has reached INV-0371. A customer cancels an order that was invoiced as INV-0368, so the trader issues CN-0004 against it and carries on with INV-0372. The sequence is unbroken and the cancellation is explained.

A bakery with two shops runs LEE-0001 onwards and HGT-0001 onwards. Each shop's sequence is complete, and the prefixes stop any number appearing twice. At the end of each VAT period, the bookkeeper checks both sequences for gaps in minutes.

What the number does after you send it

An invoice number does more work than it looks. Your customer enters it into their accounts as the unique reference for your invoice, and many systems reject a second invoice with the same number from the same supplier as a duplicate, even when it is a genuine new invoice. Their payment run lists invoices by number, and their remittance advice tells you which numbers a payment covers. When you chase a late payment, the number is how both sides find the invoice. When HMRC checks your records, the sequence is how it sees that nothing is missing.

So a number that is clear, unique and in order saves time at every one of those steps, for you and for your customer. It is also the reason never to reuse a number, even for a corrected invoice: the customer's system may treat the replacement as a duplicate and refuse to pay it.

Numbers on corrected invoices

When you correct an invoice with a credit note and reissue it, the replacement takes the next number in your sequence, not the old one. Refer to the original in the description, for example "Replaces INV-0368, cancelled by CN-0004", so the customer can match everything up. The guides on how to correct an invoice and how to cancel an invoice walk through both cases.

Checking your sequence

Once a quarter, sort your invoices by number and look for gaps or duplicates. A gap usually means an invoice was drafted and abandoned; if it was never sent, note why in your records. A duplicate means two invoices share a number; credit one and reissue it with the next number. A clean sequence is part of showing HMRC your records are complete, alongside keeping them for the required time; see how long to keep invoices.

Numbers and e-invoicing

When VAT invoices move to e-invoicing from April 2029, the invoice number becomes a field that software reads, not just a label. A consistent format now makes that move simpler. The e-invoicing hub explains what is changing.

Common questions about invoice numbers

Can my first invoice be number 1? Yes. There is no rule against it. Some businesses prefer to start higher, which is equally fine.

Can I use letters only? Every invoice needs a unique number, and a VAT invoice a sequential one, so include a counting element. Letters work well as a prefix in front of it.

What if I skipped a number by accident? Note in your records that the number was not used and why, then carry on. Do not go back and issue an old number later, because the dates and sequence would no longer line up.

Does the customer's reference replace my invoice number? No. Quote their purchase order or reference as well, but your invoice still needs its own unique number from your sequence.

Should credit notes use the invoice sequence? Give them their own. A credit note must carry its own identifying number, and a separate CN series keeps your invoice sequence free of gaps. Each credit note then refers to the invoice number it corrects.

Does the invoice number need to appear on the payment? Ask customers to quote it as the payment reference. It is not a legal requirement, but it turns matching payments to invoices into a glance at your bank statement.