Can invoice numbers contain letters?

Yes. Prefixes such as INV- or a client code are widely used and perfectly acceptable as long as the numeric part still increments.

It is better not to. If a gap happens, record why, so you can answer the question later.

Should I restart numbering each year?

Only if the year is part of the number. Otherwise you will end up with duplicate identifiers across years.

What number should my first invoice be?

0001 is honest and fine. If you prefer not to advertise that it is your first, starting at 1001 is a common choice.

Do credit notes use the same sequence?

Use a separate sequence with its own prefix so credit notes and invoices never collide.

Guide

Invoice Numbering Systems

Q: How do I fix a mistake on a sent invoice?

Issue a credit note against the original number, then issue a corrected invoice with the next number. Do not silently edit and resend.

An invoice number is the document's identity. It has to be unique, it has to follow a sequence you can explain, and it has to stay stable once the invoice has been sent.

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How should invoices be numbered?

Use a unique, sequential number for every invoice, with no gaps you cannot account for and no reused values. A simple padded counter such as 0001, 0002, 0003 is sufficient; prefixes and years are optional additions.

Sequence matters more than format. Auditors and clients both use the sequence to confirm nothing is missing, which is why deleting an invoice and reusing its number is the one thing to avoid.

What is a good invoice number format?

Common workable formats are 0001, INV-0001, 2026-0001 and INV-2026-0001. Add a client or project code only if you genuinely need to sort by it.

Avoid formats that embed information that can change, such as a client's old trading name, and avoid dates as the whole number — two invoices issued the same day would collide.

Formats and what they are good for

Format	Example	Suits
Plain sequential	0001	Low volume, single-entity invoicing
Prefixed sequential	INV-0001	Distinguishing invoices from quotes and credit notes
Year plus sequence	2026-0001	Businesses that reconcile by financial year
Prefix, year, sequence	INV-2026-0001	Higher volume with year-based filing
Client code plus sequence	ACME-0007	A handful of long-term clients, per-client records
Project code plus sequence	ALDER-0003	Contractors invoicing per site or job

Rules worth following

One number per invoice, never reused, even if the invoice was wrong
Numbers increase — the next invoice is always higher than the last
Pad the counter (0001, not 1) so files sort correctly and look established
Keep the format fixed; change it only at a clean boundary such as a new year
If you void an invoice, keep the number and mark it voided
Correct a sent invoice with a credit note and a new invoice, not by editing the original

Restarting each year

Restarting the counter annually is fine as long as the year is part of the number — 2026-0001 and 2027-0001 are distinct, while 0001 twice is not. If you restart, restart on a consistent boundary and keep the padding width the same so old and new files sort predictably.

Gaps, mistakes and credit notes

Gaps in a sequence are not a crime, but they do attract questions, so keep a note of why one exists. If you discover an error after sending, issue a credit note referencing the original invoice number, then issue a corrected invoice with the next number in the sequence. That leaves an audit trail rather than a hole.

Credit notes should have their own sequence and prefix — CN-0001 — so they are never confused with invoices in either system.

How the generator handles numbering

You control the number, so it can match whatever system your records already use. When you duplicate a saved draft, the generator increments the trailing counter for you, which is the common case for monthly retainers. Nothing is numbered automatically behind your back.

Frequently asked questions

Published 18 September 2026. Last updated 18 September 2026. Written by the Free Invoice Generator team. General invoicing information, not tax or legal advice.

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