How to number invoices
A simple invoice numbering system: sequential, unique and never reused. What to do about gaps, prefixes and estimates.
An invoice number is the name of the invoice. Your customer quotes it when they pay, you quote it when you follow up, and your accountant uses it to match payments to sales. A good numbering system has three properties: every number is unique, the numbers run in order, and no number is ever reused.
The simplest system that works
Start at a number and add one each time: 1001, 1002, 1003. That is all most businesses need. It is easy to explain, impossible to get out of order, and a missing number is obvious at a glance.
You do not have to start at 1. Starting at 1001, or continuing from where your previous system stopped, is common and avoids advertising that an invoice is your first.
Other formats
- Year prefix: 2026-001, 2026-002. The count restarts each year. Handy if you think in financial years, but it adds a rule to remember every January.
- Client prefix: ACME-014. Each customer has their own sequence. Readable, but harder to keep unique across the whole business.
- Date based: 20260212-01. Sorts well, but is long to read aloud on the phone.
Whichever you choose, keep it short enough to say out loud and stick to it. Changing format partway through a year causes more confusion than any format does.
Rules worth keeping
- Never reuse a number, even for an invoice you cancelled. Two documents with one number cannot be told apart later.
- Avoid gaps where you can. A missing number looks like a missing invoice. If you cancel one, keep a record of it marked as void.
- Do not change a number after sending. Your customer has already filed it under the old one.
- Number in the order you issue. A higher number should mean a later invoice.
Some countries set their own rules for invoice numbering, such as requiring an unbroken sequence. If you are registered for a sales tax or VAT, check the rules that apply to you.
Estimates need their own sequence
Give estimates and quotes a separate run of numbers from invoices. Not every estimate becomes an invoice, so sharing one sequence leaves holes in your invoice numbers. When an estimate is accepted, the invoice takes the next invoice number, and you note which estimate it came from. More on this in invoice, estimate or quote.
Let the software count
Most numbering mistakes happen when numbers are typed by hand: a duplicate here, a skipped one there. Any invoicing tool, Tidy Invoicer included, should hand out the next number for you and refuse to use one twice. If yours does not, a simple list of issued numbers kept in one place is the next best thing.
This guide is general information, not legal or tax advice. Invoicing rules differ between countries, so check what applies where you do business. Updated .