Every invoice needs its own number. It lets you and your client talk about the same document, and tax offices in most countries require it. Here is how to pick a format and keep it clean.
The simple rule
Use numbers that go up by one and never repeat. Start wherever you like; you don't have to start at 1.
- INV-0001, INV-0002, INV-0003: the simplest format, and the one this generator uses.
Formats that work
- With the year: 2026-001, 2026-002. Easy to read, and many businesses restart the count each year.
- With a client code: ACME-001, ACME-002. Handy when you bill a few large clients.
- With the document type: INV-0001 for invoices, QUO-0001 for quotes, REC-0001 for receipts. The generator keeps a separate count for each type.
What tax offices expect
Most countries ask for a unique, sequential number. The UK and the EU require a sequence that identifies the invoice. India allows up to 16 characters, unique for the financial year. Gaps are not illegal everywhere, but they can raise questions in an audit, so avoid skipping or deleting numbers.
Made a mistake?
Don't delete the invoice or reuse its number. Mark it as cancelled and issue a new invoice with the next number, or send a credit note for the wrong amount. Keep both in your records.
Let the generator do it
invoice-gen.net numbers each new invoice for you, one higher than the last, with separate counts for quotes, estimates, receipts and proformas. You can type your own number at any time.
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Free, no sign-up needed. Download the PDF or email it to your client.
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