Invoice Today

    How to Number Invoices

    Give every invoice a unique number that follows on from the last one, and never reuse a number. That's the core rule. Within it, you can choose a format that suits how you work.

    Here are four simple systems, how to start, and what to do when things go wrong.

    Why invoice numbers matter

    • They let you and your customer refer to a specific invoice quickly.
    • They make gaps or duplicates easy to spot in your records.
    • VAT-registered businesses in the UK must use a unique, sequential number on VAT invoices. Check GOV.UK for the detail.

    Four numbering systems

    1. Simple sequence

    0001, 0002, 0003. Easy to follow and hard to get wrong. Leading zeros keep numbers sorting in order.

    2. Prefix and sequence

    INV-0001, INV-0002. A prefix makes it obvious the number is an invoice.

    3. Year and sequence

    2026-001, 2026-002. Helpful for seeing the year at a glance. Either restart the sequence each year or carry it on; just stay consistent.

    4. Client code and sequence

    ACME-001, ACME-002. Useful when you invoice a few clients often, but keep an overall log so numbers stay unique across your business.

    Example formats

    FormatFirst invoiceNext invoice
    Simple00010002
    PrefixINV-0001INV-0002
    Year2026-0012026-002
    ClientSAMPLE-001SAMPLE-002

    Starting out

    You don't have to start at 1. Many people start at 001 or 1001. Pick a format, write it down, and stick to it.

    Invoice Today doesn't store your invoices, so it can't track the last number used. Keep a simple log, such as a spreadsheet with invoice number, date, client, amount and paid date.

    Fixing mistakes

    • If an invoice is wrong but not yet sent, correct it and keep the same number.
    • If it's been sent, the usual approach is to issue a credit note against it and a new invoice with the next number, rather than editing it. Ask your accountant how they'd like this handled.
    • Don't delete numbers from your log; mark cancelled invoices as cancelled.

    Frequently asked questions

    Yes. Prefixes such as INV or a client code are common. What matters is that each number is unique and the sequence is clear.

    You can, as long as the year is part of the number so each invoice stays unique, for example 2026-001 and 2027-001.

    In the invoice number field on the create invoice page. It appears at the top right of the PDF.

    Related

    This is general information, not tax or legal advice. Check your own requirements with an accountant or HMRC.