How to Number Invoices: Four Numbering Systems and When to Use Each

Open the free invoice generatorDownload the invoice number log (PDF)

What the rules actually require

Two official sources set the floor. The UK government's page on what an invoice must include starts its list with "a unique identification number". For VAT-registered businesses, HMRC's VAT guide (Notice 700) is more specific: a VAT invoice must show "a sequential number based on one or more series which uniquely identifies the document".

That phrase "one or more series" matters. It means you are not forced into one long count for the whole business; a series per year or per client is fine as long as each series runs in order and no number appears twice. Other countries' tax offices have their own wording, so if you charge VAT or GST outside the UK, check yours, but unique and in order is the safe rule everywhere.

Even where nothing is legally required, a clear series helps you. A missing number in the sequence is a quick way to spot an invoice you forgot to send, and a client asking about "the March one" can be answered in seconds.

Four numbering systems compared

Running count (INV-0001). One series that never restarts. It is the easiest to keep and the hardest to get wrong. Leading zeros keep the numbers the same width so they sort properly in a folder or spreadsheet. It does reveal roughly how many invoices you have sent; if that bothers you, start at a higher number such as INV-1001.

By year (2026-001). The year and a count that restarts each January. You can see at a glance which tax year an invoice belongs to. Give the count enough digits for a busy year.

By client (ACME-001). A short client code and that client's own count. Useful when a few large clients each receive many invoices and want to refer to them easily. You need a list of codes, and the series are separate, so keep a register of every number across all clients.

By project (P118-01). The project or job number and a stage count. Good for long jobs billed in stages, because every invoice for one job sorts together. Combine it with a register, or numbers from different projects get hard to track.

Whichever you choose, write the format down, because the system you meant to use and the one you actually used drift apart after a busy month.

Step by step: set up and keep the series

1. Choose a system

If you are unsure, use a running count. You can switch at the start of a year, as long as no new number repeats an old one.

2. Write the format down

Prefix (INV, a year, a client code), separator (a hyphen is easiest to read) and how many digits. The numbering plan with this guide has a box for it.

3. Set the first number

In the free invoice generator, type your first number into the Invoice number field. It starts at INV-0001 if you have never invoiced; if you have, start one after your last number.

4. Let it count up

Press New invoice for the next one. The generator adds one to the last number in the field and keeps its leading zeros, so INV-0009 becomes INV-0010, 2026-014 becomes 2026-015, and ACME-2026-07 becomes ACME-2026-08. Your number is remembered in that browser, so use the same browser and device for invoicing, or note the last number in your register.

5. Log every number

Keep one list with each number, date, client, amount and status. A spreadsheet works; so does the printable log with this guide.

6. Fix mistakes with a credit note

Do not delete a sent invoice or reuse its number. Issue a credit note that refers to the original invoice's number and date and cancels some or all of it, then send a corrected invoice with the next number. HMRC's notice lists the identifying number of the original invoice among the details a credit note should show, and the habit is good practice anywhere.

Common numbering mistakes

1. Restarting the count by accident. Invoicing from a new laptop or phone and starting again at 0001. Keep the last number in your register, or invoice from one device.
2. Dates as numbers. Using the date alone (20261011) as the number breaks the moment you send two invoices on one day.
3. Reusing a number after a cancelled job. The number was issued, so it stays used. Mark it cancelled or credited in the register.
4. Mixing quotes and invoices in one series. Give estimates their own prefix (EST-0001) so an accepted quote and its invoice are never confused.
5. Too few digits. INV-99 to INV-100 sorts badly in folders. Four digits with leading zeros lasts most small businesses for years.

Your first invoice, and moving to software later

There is no rule that your first invoice must be number 1. Many people start at 1001 or with the year, which is fine as long as the series continues from there.

If you later move to invoicing software, carry the series on rather than starting again. In Fynl, invoices are numbered automatically, sent as a link the client pays online through your own Stripe account, and chased with reminders before and after the due date, so the register keeps itself. The Starter plan is $49 a month.

For what else belongs on the invoice, read how to make an invoice for freelance work; for the due date, which payment terms to use. Every free tool is on one page.

Questions people also ask

Sources

• HMRC — VAT guide (VAT Notice 700), section 16.3 Information required on a VAT invoice
• GOV.UK — Invoices: what they must include