Three schemes that work
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Plain sequential
0001, 0002, 0003. Simplest to keep unique and the easiest to explain. Pad it so the numbers sort correctly as text, which is how most software will sort them.
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Year plus sequence
2026-0001. Groups a year's work together and makes an old number obvious at a glance. Restart the sequence each January and never reuse a whole identifier.
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Client prefix plus sequence
ACME-0007. Useful when a client requires their own reference. Keep the sequence global rather than per client if you ever need to prove no invoice is missing.
What actually causes trouble
- Reusing a number after cancelling an invoice — the cancelled one still exists in somebody's records
- Embedding a date you might change, so the number stops matching the document
- Starting at 1 when you would rather a client did not know they are your first
- Letting two people or two tools issue numbers from the same range
Credit notes and revisions
Do not edit an issued invoice and keep its number. Issue a credit note referencing the original, then a new invoice with the next number. The trail is then readable by an auditor who was not there, which is the entire point of numbering.
Common questions
- Can I start my invoice numbers at 1000?
- Yes. Nothing requires a sequence to start at one, and starting higher is a common and harmless choice.
- Do invoice numbers have to be sequential?
- They have to be unique. Sequence is what makes uniqueness easy to maintain and gaps easy to explain, which is why almost everyone uses it.
- What if I accidentally issue the same number twice?
- Issue a corrected invoice with a fresh number, reference it to the duplicate, and tell the client which one to pay. Leaving two live documents with one name is how a payment gets applied to the wrong job.