An invoice number is a small thing that carries a lot of weight. Clients quote it on the payment, bookkeepers file by it, tax authorities expect the sequence to be continuous, and you search by it when a question arrives a year later. A scheme that works for invoice number three should still work for invoice number three thousand.
This guide sets out the conventions that hold up, the choices that are matters of taste, and the mistakes that cause real trouble. It ends with how numbering works in Simple Invoice so you can set it up once and stop thinking about it.
The three rules that are not optional
Everything else in this guide is preference. These three are what auditors, clients' systems and your own future self depend on.
- Unique. No two invoices from you ever share a number. Client systems reject duplicates, and a duplicate in your books looks like a hidden invoice.
- Sequential. Numbers go up in the order invoices are issued. Many tax authorities require an unbroken sequence; all of them dislike numbers that jump around.
- Permanent. Once an invoice is sent, its number does not change. If the invoice is wrong, void it and issue a new one with the next number rather than editing the old one.
Choose a pattern
A pattern is a prefix, optional date parts and a counter. Simpler is better; every extra element is something to keep consistent.
INV-0001— a prefix and a padded counter. Enough for most freelancers, and it never needs a decision.INV-2026-0001— adds the year. Useful when you file by tax year, and it makes an annual reset unambiguous.INV-2026-09-0001— adds the month. Only worth it if you issue many invoices a month and want them to sort by period.2026-0001— no prefix. Fine if you only issue invoices; a prefix helps once quotes and credit notes exist alongside them.
Zero padding and why it matters
Pad the counter with leading zeros so numbers sort correctly as text: INV-0009 comes before INV-0010 in every file browser and spreadsheet, whereas INV-9 sorts after INV-10. Four digits covers ten thousand invoices, which is more than most sole traders issue in a career. Padding should never truncate — when the counter passes 9999 it simply becomes 10000.
Gaps, voids and mistakes
Gaps happen: you create an invoice and the deal falls through, or you spot an error after sending. The answer is never to reuse the number. Keep the invoice, set its status to void and let the next invoice take the next number. A void invoice in the sequence is an explanation; a missing number is a question.
If a sent invoice contains an error, the clean approach is to void it and issue a corrected invoice with a new number that references the original. Some jurisdictions expect a credit note that cancels the first invoice instead — your accountant will know which applies.
Drafts that were never sent are the one case where deleting is harmless, provided your tool did not already assign the number to a document the client saw.
What number to start with
Start at one. Some people start at a higher number so the first invoice does not advertise a new business; if you do, pick it once and never lower the counter afterwards. Moving from another tool or from spreadsheets, continue the existing sequence rather than starting again — set the counter to the next unused number.
Resetting the counter each year
Resetting is optional and only safe when the year is part of the pattern, otherwise 0001 would recur. If you include {YYYY}, resetting on 1 January gives you a tidy INV-2027-0001. If you do not, keep counting: a continuous counter across years is equally valid and simpler.
In Simple Invoice, type the number on the current invoice itself. Keep the sequence in your own records; the editor does not maintain a multi-invoice counter.
Quotes, pro forma invoices and credit notes
Documents that are not invoices should not consume invoice numbers. Give quotes their own prefix and counter (Q-0001) and credit notes theirs (CN-0001), or a reference that points to the invoice they correct. A pro forma invoice is a quote in invoice clothing; number it as a quote and issue the real invoice with the next invoice number once the deal is confirmed. Invoice vs receipt vs quote explains the differences.
How numbering works in Simple Invoice
Type the number on the invoice. Use a clear prefix, year if useful, and a padded sequence such as INV-0001; check your own register for the next unused value.
The editor keeps one invoice and does not advance a multi-document counter or check an invoice list for duplicates. Record every issued number outside the app before replacing the current document.
Keep cancelled invoice numbers in your external register and mark them void there, so the sequence remains explainable. How to make an invoice shows where numbering fits in the workflow.
Questions
Can invoice numbers contain letters?
Yes. A prefix such as INV- is letters, and it is common. The requirement is that the sequence is unique and, in most places, continuous — not that it is purely numeric.
Should I number invoices per client?
Usually not. One continuous sequence for all clients is what tax rules expect and is simpler to maintain. If you want to see the client at a glance, put their name or code in the reference field instead.
I deleted an invoice and now there is a gap. What should I do?
If it was never sent, note the gap in your records and move on. If it was sent, recreate it as a void invoice with the same number so the sequence has an explanation. Going forward, void rather than delete.