Invoice Number Generator
Generate sequential invoice numbers in any format. Free, no signup, nothing stored.
Your next invoice number
INV-1001
Prefix
Letters before the number. Leave blank for numbers only.
Start at
The first number in the run.
Zero padding (4 digits)
Keeps numbers sorting correctly in a spreadsheet.
Separator
Character between each part.
- INV-1001
- INV-1002
- INV-1003
- INV-1004
- INV-1005
- INV-1006
- INV-1007
- INV-1008
- INV-1009
- INV-1010
How invoice numbering works
An invoice number is the unique reference that identifies one invoice in your records. It is how you, your client, and your accountant all point at the same document — and in most countries it is a legal requirement rather than a convention.
The rules are short, and they matter more than the format you pick:
- Unique. No number is ever used twice, including on cancelled invoices.
- Sequential. Numbers go up, one at a time, with no gaps you cannot explain.
- Consistent. Pick a format and keep it. Switching mid-year makes reconciliation painful.
- Not random. Random or date-only numbers collide and make missing invoices invisible.
Common invoice number formats
| Format | Example | Best for |
|---|---|---|
| Prefix + counter | INV-1001 | Most small businesses. Simple, sorts correctly, never resets. |
| Year + counter | 2026-0001 | Businesses that want the counter to reset each financial year. |
| Prefix + year + counter | INV-2026-001 | Mixing invoices and estimates in one filing system. |
| Counter only | 0001 | Sole traders with low volume who want the shortest possible reference. |
Zero-padding matters more than it looks. INV-1001 through INV-1010 sort correctly as text, but INV-1 through INV-10 will sort as 1, 10, 2, 3 in a spreadsheet — which is how numbering mistakes start.
Mistakes that cause real problems
- Reusing a number after cancelling. Two documents with one reference is the single most common source of payment disputes.
- Restarting at 1 each year without a year in the number. You will produce duplicates within twelve months.
- Using the date as the number. Two invoices on the same day collide immediately.
- Numbering by client. Harder to reconcile, and some tax regimes expect one continuous series per business.
Questions people ask
What should my first invoice number be?
Most businesses start at 1001 or 0001 rather than 1. Starting at 1 tells a client they are your very first customer, which can undercut your negotiating position. There is no legal requirement to start at any particular number — only that the sequence is consistent from then on.
Do invoice numbers have to be sequential?
In most jurisdictions, yes. Tax authorities generally require invoice numbers to be unique and sequential so that the record can be audited and missing invoices are visible. Check your local rules, but sequential numbering with no gaps is the safe default everywhere.
Can I reuse an invoice number?
No. Every invoice number must be unique, including for cancelled or voided invoices. If you cancel an invoice, keep the number recorded as cancelled and move on to the next one — do not reassign it.
Should the invoice number include the year?
It is optional and useful if you want the counter to reset each financial year, for example 2026-0001. Keep in mind that if you reset the counter annually you must keep the year in the number, or you will produce duplicates across years.
What if I skip a number by mistake?
Do not go back and fill the gap. Note what happened in your records and carry on with the sequence. Auditors are far more comfortable with a documented gap than with numbers that appear to have been inserted after the fact.
Can I use a different number series per client?
You can, for example ACME-001 and BETA-001, but it makes reconciliation harder and some tax regimes expect one continuous series per business. If you need to identify the client, put it in a reference field on the invoice rather than in the number.
Stop tracking numbers by hand
A generator is useful once. After that, the numbering should just happen. InvoiceFast assigns the next number automatically every time you create an invoice or estimate, so the sequence stays correct without you thinking about it — and the numbers stay in step across your phone and your other devices.
Related reading: what to include on an invoice, how to write an invoice, and invoice payment terms explained.