The numbering on invoices

If you're on your own. Use a year and a sequence

So say something like INV-2026-001. INV-2026-002. The INV for the type of document. The 2026 so you can file for the year. And numbers that run up.

If you're not issuing too many invoices, three or four sequence digits might be enough. You don't need to start at random. You can just have something simple like this INV-000123. Or use the year in it like 2026-00123.

Using the number in your records means:

• You can look up the history.
• You can see what's still outstanding.
• You can connect payments and credits.
• It makes it easier to discuss with customers.
• And you can trace through your accounting records.

So you'd want to put that whole number close to where you write the date of the invoice and the heading. And it'll help to use it in the file name and when you tell people to pay you. And when you send them reminders by email.

You need to do it in line with the rules where you are too. For example, the European Commission says you need a unique sequential number for a full VAT invoice. And the UK government's field guidance says you need a unique identifying number. But it depends. The difference is the scope. So just because it's convenient doesn't mean it'll work in all cases.

• You need a full number on each invoice.
• And then you need to be able to connect up payments and corrections to that.
• You need to be able to search for customer questions.
• And you need to be able to trace your accounting records back to the file.

Format decision

Don't use the date today and the customer name tomorrow. Start at 001 again next month?

Be aware of local rules first before an annual reset. Use the full number with the year to avoid re-using old references. Store names and so on elsewhere.

Why is there a missing number?

A gap isn't necessarily a violation. But maintain the explanation.

Don't re-use an issued number for another transaction. Don't renumber later invoices to close a gap. Don't add an unrelated transaction.

• Assign from one place.
• Don't re-use issued numbers for other invoices.
• Maintain void / cancelled records.
• Trace unexpected gaps and duplicates.

Keep the number with the transaction

Use it for your PDF filename, email subject line, payment instructions and reminders. Add it to dispute records, credit notes and related purchase orders or estimates where required. Match money received to the invoice and record the payment. Keep the remaining balance if it is a part payment.

Pocket Invoice supports multiple companies and invoice-record searches. For each company, use its own details and keep its name, documents and number sequence consistent. When matching bank payments yourself, use the amount actually received.

• Put the full number at the top.
• Request customers to use it when paying.
• Keep the same reference in related emails/corrections.
• Manage each company's numbering independently.

Test the rule: a month of orders

Simple if you need only one sequence: INV-0001, INV-0002 and INV-0003. If you need to separate numbers by company or invoicing channel, keep a record of it, e.g. WEB-0042. Use each company's numbers with its own details. Don't let several invoicing systems give the same number.

Say September brings three invoices: INV-2026-041, 042 and 043. The first is paid in two parts. The second requires correction. The third is not yet due. Record payments and corrections using the original number. Do not renumber because of a change in status.

Carry on from 044 in October. Say you group by year, and it's allowed by local rules, then maybe next year can start at INV-2027-001. Or group by month: put year and month in the full number. Have a clear record for every full number.

Keep the customer's purchase order number separate

The customer's PO-884 is a PO (purchase order) number. INV-2026-014 is your invoice number. Separate field for each. With labels.

Several deliveries and several invoices per order. No, don't use PO-884 for every invoice. The date of issue is when it was issued. Bank reference to find money. Project number for a job. Reference each to one another, but keep them separate.

Three deliveries from one purchase: a single PO reference. Three invoices with separate numbers. Separate.

Draft versus issued invoice

What distinguishes them? When do you assign a formal number?

Give every discarded test draft a final number and it's harder to follow records. Renumber after a file goes to the customer and you have different versions.

A draft, not issued, you can change. For an issued invoice, accounting entry or tax record, use your system's correction process. Use the applicable rules for correction. Maybe a correction document? Credit note? Etc?

Maintain the original. Maintain links between versions. Maintain notification records.

Duplicate issued number

First look to see if either of the files has made it to the customer approval process or your accounting books. Changing the file name alone is not enough. The PDF number and the customer copy may still be a duplicate. Use the appropriate correction procedure to identify the voided and replacement documents. Inform the payer.

Document why. When was it corrected? What is the new number? Who was informed? Ensure a link to the original. Even if it's a change to contact info, explain what changed. Someone looking back should be able to track what happened without relying on your memory or a history of emails.

What can you get from a register?

A small business might record the number, customer, issue date, sent date, due date, currency, amount, money received and status. Register the invoice when you make it. Capture the sent date. Adjust the balance as money comes in. Sort and filter. Which invoices are due but not paid? Customer history? New invoices this month?

If more than one person is invoicing, set up how you will allocate the numbers. Multiple copied spreadsheets or lists mean it's very easy to apply the same number to multiple transactions.

Before a software change, document the numbers issued, statuses and outstanding amounts. Look to see how the new system continues or distinguishes the sequence. See if you can keep any imported numbers. Search and export a typical invoice to test. Examine the number in the PDF and the payment record. Retain previously issued files. Creating a number does not guarantee full history, full imports or a numbering system that works for you.

Use the full number for your filename and payment reference

Don't use only the last 3 digits. INV-2025-018 and INV-2026-018 are different references which the customer can tell apart at year end. Use the full number in reminders so that they can search for it, rather than remembering an amount.

E.g. “Please quote INV-2026-044 when transferring payment so we can match it.” Don't also ask for internal project, customer and file-version numbers. If multiple invoices are paid in one transfer, request an allocation. Also check what is settled and what balance remains.