How to use the invoice number generator
- Choose a prefix. Most businesses use
INV. A short brand code such asACMEworks too. - Add a date part. A year makes invoices easy to sort by tax year. A month helps if you send many invoices.
- Set the starting number and digits. Three digits (
001) cover up to 999 invoices per period; pick four if you expect more. - Copy, download or use it. Click Use in Invoice Creator to put the first number on a new invoice, or download the whole list as a CSV.
Already invoicing? Paste your last number into Already have numbers? and the tool shows the next one in the same format.
Invoice number formats that work
There is no single correct format. What matters is that every number is unique and that you can understand your own system a year from now. These are the formats small businesses use most:
| Format | Example | Best for |
|---|---|---|
| Simple sequential | 1001, 1002, 1003 | Low volume, one person |
| Prefix + number | INV-0001 | Most freelancers and small businesses |
| Prefix + year + number | INV-2026-001 | Filing by tax year, restarting each January |
| Year + month + number | INV-202611-01 | Many invoices a month |
| Client code + number | ACME-2026-003 | A few large, repeat clients |
Five rules for invoice numbering
- Never reuse a number. Even if an invoice is cancelled, keep its number retired.
- Count up in order. Sequential numbers make a missing or duplicate invoice obvious.
- Pad with zeros.
INV-007sorts correctly in a spreadsheet;INV-7lands afterINV-10. - Keep it short. Clients type invoice numbers into payment references. Under 15 characters is kind to them.
- Write the rule down. One line in your notes ("INV-YEAR-###, restart in January") saves confusion later.
Common mistakes to avoid
- Starting a new sequence for every client without a client code, which creates duplicates such as two different invoices numbered 001.
- Using only the date, such as 20261110, which breaks the moment you send two invoices on the same day.
- Changing formats mid-year, which makes your records hard to follow at tax time. Change formats in January if you must.
Want the full background? Read our guide on how to create an invoice or see 15 invoice examples.
Frequently asked questions
What is an invoice number?
An invoice number is a unique ID printed on each invoice. It lets you and your client refer to one specific bill, track whether it was paid, and find it quickly in your records.
Does an invoice number have to be sequential?
In the US there is no federal rule that dictates a specific format, but sequential numbers are the safest choice because they make gaps and duplicates easy to spot. In the UK, VAT invoices must carry a unique number that follows on from the previous one.
Can I start my invoice numbers at 1000 instead of 1?
Yes. Many businesses start at 1000 or 1001 so a new business does not look brand new to clients. Just keep counting up from wherever you start and never reuse a number.
Should I include the year in my invoice numbers?
Including the year, as in INV-2026-001, makes invoices easy to sort and file by tax year. If you restart the counter each January, the year prefix keeps every number unique.
What if I skip or duplicate an invoice number by mistake?
Do not reuse the duplicated number. Void or correct the invoice that has the error, note what happened in your records, and continue the sequence. A clear note is usually enough to explain a gap.
Can I use this invoice number generator for free?
Yes. The generator is free, works in your browser, and does not ask you to sign up. You can copy the numbers or download them as a CSV file.
This page is general information, not tax or legal advice. Invoice rules vary by country and state.