Clearquire 4 min read

How to number invoices

Every invoice needs a number you have never used before. Pick one format, never reuse a sent number to fix a mistake, and keep the ledger somewhere you control.

An invoice number has one purpose: it identifies that invoice and no other. The client's accounts team files by it, pays against it, and asks about it. Your bookkeeping matches payments to it. The rules are short, and most of the problems come from breaking them to tidy up a mistake.

This is general practice, not tax or legal advice. In the UK, GOV.UK says an invoice must carry a unique identification number. VAT invoices have extra requirements, and other countries have their own. Check the rule where you are registered.

The rules

  • Unique. No two invoices share a number, ever, across all clients and all years.
  • Sequential. Numbers go up. A gap or a number out of order is what an auditor asks about, because it looks like a missing or deleted invoice.
  • Never reused. If you sent a wrong invoice, do not issue a corrected one under the same number. Issue a credit note or a new invoice, and keep both.
  • Issued in order of date. Invoice 0042 should not be dated before 0041.

That is the whole standard. Everything else is a format choice.

Pick a format

Choose one and keep it. Changing format mid-year creates the "is this the same sequence?" question you were trying to avoid.

  • Plain counter: 0001, 0002, 0003. Simplest. Pad with zeros so files sort correctly.
  • Year prefix: 2026-001, 2026-002. Resets each year, which makes the volume visible and keeps the counter short. The most common choice for freelancers.
  • Client prefix: ACME-2026-001. Useful if you want to see at a glance who an invoice belongs to. Keep one counter per client, or one global counter with the prefix as a label. Either is fine as long as the whole string is unique.

Avoid encoding the amount, the project, or anything that changes. The number is a label, not a description. A separate reference or PO field exists for the client's own identifiers.

Start the sequence

Start at 1, or at 1 within the year. Some people start at a higher number so the first client does not see 0001. That is cosmetic and harmless, but it does not make the sequence less auditable, because the gap is before the first invoice, not between two.

If you already have invoices under an older system, continue from the last number rather than starting again. Two invoices numbered 0001 in your records is the problem the rules exist to prevent.

When you make a mistake

  • Wrong amount, already sent. Issue a credit note that cancels the wrong invoice (referencing its number), then a new invoice with the next number. Or, if the client has not paid and agrees, cancel the first and issue the next number. Record what happened against the old number.
  • Typo in the number itself. If the client already has it, treat the sent number as used. Skip to the next and note why in your records.
  • Draft never sent. If a number was only ever on a draft on your own machine, you can reuse it. Once it has left your inbox, it is used.

Write down each exception. An explanation beside a gap is routine; an unexplained gap is a question.

Keep the counter where you control it

The last number issued has to live somewhere that survives a change of tool.

A spreadsheet with one row per invoice (number, date, client, amount, paid date) is enough. The filename of each PDF can carry the number too: 2026-014 Acme Ltd.pdf sorts itself.

Clearquire has an invoice number field and a separate PO or reference field. Because invoices are not stored on a server, the sequence is yours to keep. If you turn on Keep draft on this device, the last form, including its number, is still in that browser next time, which is a convenient reminder but not a ledger. The spreadsheet is the ledger.

The rest of the invoice is covered in what to include on a freelance invoice.

Checklist

  • One format, chosen once, used for every invoice.
  • Numbers unique and increasing, with dates in the same order.
  • No number reused after it has been sent, for any reason.
  • Credit notes and reissues reference the original number.
  • A ledger of issued numbers that lives outside any single tool.
  • PDF filenames carry the number.