Invoice Number Generator

An invoice number has one job: to be unique, so that you, your client and your accountant can refer to one document without ambiguity. The generator builds each number from up to four parts in a fixed order: an optional prefix (such as INV), an optional client code, an optional date (the year, or year and month) and a sequence number, joined by the separator you choose.

The sequence is padded with leading zeros to at least three digits, or to the length of your starting number if that is longer, so numbers sort correctly as text: 001, 002 … 010 rather than 1, 10, 2. With the prefix INV, client code ACME, the year, a dash separator and a start of 1, the first three numbers are INV-ACME-2026-001, INV-ACME-2026-002 and INV-ACME-2026-003.

Start at 1 only if you are comfortable with a client seeing that they are your first. Many freelancers start at 100 or 1000 for that reason; the generator pads to match. Whatever you choose, never reuse or skip numbers once issued, and never renumber a sent invoice, because gaps and duplicates are the first thing an auditor or a client's accounts-payable system flags.

The generator runs entirely in your browser and stores nothing. It is a way to pick a scheme, not a ledger: the number you send has to be recorded somewhere that increments it for the next invoice. For the reasoning behind each scheme, and what to do when you have already made a mess of the sequence, see invoice numbering best practices.