How to number your invoices

Why invoice numbers matter

The number is how everyone finds one invoice among hundreds. Your client's accounts team uses it to match the payment, you use it to see which invoices are still open, and a tax inspector uses an unbroken sequence to check that no sales are missing from your books.

That last point is why the rules focus on two things: each number must be unique (no two invoices share it) and the numbers should follow a sequence you can explain. A gap with no explanation looks like a missing invoice, and a missing invoice looks like hidden income.

Common numbering formats

Pick one format and stick with it. Any of these works as long as each number is unique and the order is clear.

A few practical points:

• Pad with zeros (0047, not 47) so the numbers sort correctly in a folder or spreadsheet.
• Do not start at 0001 if you would rather not look new. Starting at 1001 or 2026-100 is fine, as long as you count up from there.
• Year-based numbers restart each January, which is allowed because the year part keeps every number unique. The example beside this article, 2026-047, is the 47th invoice of 2026.
• Client codes make each client's history easy to read, but you should still be able to list all invoices in issue order. If you have more than a handful of clients, a single running sequence is simpler.

What the rules say

Rules differ by country, so check your own tax authority. Two examples we have checked:

• UK. gov.uk says every invoice should have a unique identification number. For VAT-registered businesses, HMRC's VAT guide (Notice 700) says a full VAT invoice must show "a sequential number based on one or more series which uniquely identifies the document".
• EU. The EU VAT Directive (2006/112/EC, Article 226) has the same wording for VAT invoices: a sequential number, based on one or more series, which uniquely identifies the invoice. Each country writes this into its own law, and some add details.

Note the phrase "one or more series". You are allowed to run separate series, for example one per shop or one per year, as long as each series counts up on its own and no number appears twice.

Outside the UK and EU, rules vary, but a unique number on every invoice is expected almost everywhere, and it is what accountants and clients assume.

Gaps, voided invoices and mistakes

Things go wrong. Here is how to handle the usual cases without breaking your sequence:

• You made a mistake on an invoice you have not sent. Correct it before sending. The number stays the same.
• You sent an invoice with a mistake. Do not delete it or quietly reuse its number. Issue a correction: usually a credit note that cancels the wrong invoice (referring to its number), then a new invoice with the next number in your sequence.
• A job was cancelled after you invoiced. Same approach: cancel it with a credit note and keep both documents in your records.
• A number was skipped by accident (your software jumped, or you typed 0049 after 0047). Do not go back and fill the gap with a later invoice. Write a short note in your records saying the number was never used and why.
• You voided an invoice before sending it. Keep it, marked "void", in your files rather than deleting it.

The aim is that anyone looking at your list can account for every number. Your accountant may have a preferred method for corrections, so ask them once and do it the same way every time.

Where the number goes, and how clients use it

Put the invoice number near the top, next to the invoice date, where the client's accounts team looks first. Then ask the client to use it when they pay: "Please quote 2026-047 as the payment reference." When the money arrives, the reference on your bank statement tells you exactly which invoice it settles, even if one client has three open invoices with similar amounts.

Use the number everywhere else too:

• in the subject line when you email the invoice ("Invoice 2026-047 from Liffey Pixel Studio")
• in every reminder, so the client can find it in their system in seconds
• on the receipt or payment confirmation, next to the receipt's own number
• in your file names (2026-047 Greenway Outdoor.pdf), so your folder sorts in order

Keep the number separate from the client's own reference. Larger clients often give you a purchase order (PO) number or project code, like "GW-12" in the example. That goes in its own field. It identifies their order, while your invoice number identifies your bill.

How numbering works in Duebolt

Duebolt counts up for you, and each document type has its own series: invoices start at INV-0001, receipts at RCT-0001, estimates at EST-0001, quotes at QTE-0001 and proforma invoices at PRO-0001. Making a quote never uses up an invoice number.

Prefer your own format? Type it into the number field once, for example 2026-047 or GRW-015, and the next document of that type continues from it: 2026-048, GRW-016. Numbers ending in digits count up; your prefix stays as you typed it.

Making your first invoice? The steps are in how to make an invoice. For receipts and their own series, see the receipt generator.