The only rule that matters is that each invoice number is unique and the sequence does not hide a gap. Bookkeeping rules in a lot of countries expect a series you can walk from one bill to the next without a missing page. The statute is not the same in Norway, the UK, and the US, so do not treat this as a ruling for your filing. Treat it as the habit that survives an accountant asking 'where is 015?'
Everything else is format. Sequential, year-prefixed, client-coded, date-based. Pick one and keep it. The invoice number generator will preview the next few numbers. It will not decide whether your country treats a void as a document you must retain. That decision is local. The discipline is not.
What number to start at
Start at 1 if you like honesty. Start at 1001 if you do not want a client to infer they are your first invoice. I have never seen a payment delayed because the number was low. I have seen confusion because someone jumped from 12 to 200 to 'look busy' and then could not explain the jump.
Write the starting number in the same place you keep your invoice files. If you use software, let it own the counter. The failure mode of a handmade series is two people issuing 2026-018 on the same afternoon. One sequence, one owner.
Simple sequential numbers
1, 2, 3, or INV-001, INV-002, INV-003. This works when you are one person, one business, and the volume is low enough that you will not forget the last number. It stops working when a second brand appears, or when you want the year visible without opening the file, or when you restart emotionally every January and the counter is already at 640.
Example. You send INV-014 on 2 March and INV-015 on 18 March. A voided draft is still INV-016 if you issued a number to it. The next real bill is INV-017. If you skip 016 because the draft embarrassed you, you now have a hole. Keep a one-line note: 'INV-016 void, not sent, wrong client.' That note is the whole legal hygiene for a small series.
Year-prefixed numbers
INV-2026-014 is where most businesses end up, because the year is doing useful work and the counter stays short. You can restart at 001 each January. You should still not reuse a number inside the year, and you should not have two 014s because one was a quote. Quotes get their own series if you number them at all. Invoices stay invoices.
Example. Last invoice of 2025 is INV-2025-088. First of 2026 is INV-2026-001. In the folder, both exist. An accountant can see the break was a year, not a missing eighty-nine invoices. That is the point of the prefix. It makes the reset legible.
Client-coded numbers
ACME-2026-004 is useful once you have enough clients that scanning a single list is slow, or when a client demands their own sequence for their accounts system. It is overhead below that. You now have a counter per client, and a new client means a new counter you must remember to start at 001.
Use a short code you will not change when they rebrand. If Acme becomes Northwind, do not rename old numbers. Old numbers are history. New invoices can use the new code, and you write down the date of the switch. Example: ACME-2026-004 was the last under the old code. NORTH-2026-001 starts after the letter that confirms the legal name change.
Date-based numbers
20260318 looks tidy until you invoice twice on 18 March. Then you invent 20260318b. A cancelled day leaves a date that never existed as an invoice, which is fine for a diary and confusing for a sequence, because there is no sequence. People adopt this format because the date is already in their head. The invoice already has a date field. Let the number be a counter.
If a client portal forces a date-like reference, put the date in their reference field and keep your own sequential number as the invoice number. Two identifiers is normal. One identifier doing two jobs is how duplicates happen.
Voids, credit notes, and corrections
Never reuse a number you have issued, even if the PDF never left your laptop and you wish it had not. Mark it void. If the client has it and the amount is wrong, do not overwrite the file in place. Issue a credit note that points at the original number, then a new invoice with the next number. Some regimes allow a corrected invoice with the same number if it is clearly marked and the original is retained. Ask your accountant which of those your country expects. Do not invent a third method called 'I'll just fix the PDF.'
Example. INV-2026-020 went out for $4,000. The work was $3,600. You issue CN-2026-003 against INV-2026-020, or a credit named in the same series if your accountant wants one sequence, and then INV-2026-021 for $3,600. Both documents stay in the folder. The client can see you did not disappear an invoice.
More than one currency or company
Multi-currency does not require multi-numbering. A euro invoice and a dollar invoice can share INV-2026-030 and INV-2026-031. The currency is a field. Splitting sequences by currency feels organised and then breaks the moment one client pays in a second currency.
Multi-entity is different. Two limited companies are two sequences, even if you own both and they share a laptop. A sole trader and a company you later form should break at the incorporation date, with the last sole-trader number written down. Mixing them is a gift to nobody at tax time.
Changing systems without making a mess
Switch at year end. Finish December on the old series. Start January on the new one. Email yourself the handover: 'Last old number INV-1044 on 18 December 2026. First new number INV-2027-001 on 6 January 2027.' That email is your audit note.
Mid-year switches cause problems because reports, bank references, and client memories now span two dialects. If you already switched in March, do not switch again in June to undo it. Finish the year in the new dialect and document the March date. A second change is how three sequences appear in one folder.
Worked examples of each format
Simple sequential, starting wherever you like. You send three invoices in June: INV-014 on 2 June for a logo, INV-015 on 9 June for a website deposit, INV-016 on 18 June for the balance. You cancel the logo because the client pulled the project before you sent the PDF. INV-014 stays in the folder marked void. The next invoice is INV-017, not a recycled 014. A stranger can read the folder and see that nothing was deleted to hide a sale.
Year-prefixed, which is where most people end up once a second year starts. INV-2026-001 in January, INV-2026-042 in June. On 6 January 2027 the next number is INV-2027-001, not INV-2026-043 and not INV-1043 from an older series you half-abandoned. The year in the number matches the year on the invoice. Bank references get shorter to quote, and a December invoice cannot be confused with a January one that reused a bare integer.
Client-coded, only after the client list is long enough that a single counter is hard to scan. ACME-2026-004 is Acme's fourth invoice in 2026. NORTH-2026-001 is a different client's first. You still keep a master list, because 'fourth for Acme' does not tell you what you issued across the whole business on 9 June. If you have eight clients, the code is overhead. If you have forty, the code is how you find their file. Do not invent a code you will change when their marketing team renames the brand.
Date-based, shown so you can see the collision. You invoice Acme and North on 18 June. Both want to be 20260618. You add a suffix and now you have 20260618 and 20260618-2, which is a sequence pretending not to be one. Skip the format. Put 18 June 2026 in the date field, where it belongs, and put INV-2026-042 in the number field.
Estimates, credit notes, and the series you should not mix
An estimate is not an invoice. Give it its own series, EST-2026-010, or a clear prefix on the same counter if your accountant prefers one list. When the estimate becomes an invoice, the invoice gets the next invoice number. Do not turn EST-2026-010 into the invoice by changing the word on the PDF and keeping the number. You will then have two documents, a quote and a bill, wearing one identifier, and a payment reference that could mean either.
Credit notes point backward. CN-2026-003 should name the invoice it reduces, in the body, every time. If your accountant wants credits inside the invoice series, the credit still gets the next number, INV-2026-021, and the document says it credits INV-2026-020. What you must not do is edit INV-2026-020 down from $4,000 to $3,600 and hope the client's copy updates itself. It will not. Their copy is the one their bookkeeper already entered.
What to write down so you can explain the series later
Keep a one-page note next to the numbers. The format. The last number issued. The date you switched formats, if you did. Any void, with a reason in a few words. You do not need software for this at low volume. A spreadsheet with columns for number, date, client, amount, and status (issued, void, credited, paid) is enough, and it is what you will wish you had if a question arrives two years later. The invoice number generator can propose the next token. It cannot remember that you voided one unless you keep the list.
If two people can issue invoices, agree who owns the next number. A shared inbox with two laptops and no list produces INV-2026-018 twice. I have seen that exact collision in support threads: both looked correct, both got paid, and the bookkeeping then had two different jobs under one reference. The fix is boring. One person issues, or the tool issues, and the other person does not type a number from memory.
What a clean year actually looks like
Here is one freelancer's 2026, small enough to follow and large enough to break a sloppy system. January opens on INV-2026-001, a £900 brand workshop. February is 002 and 003. In March she drafts 004, spots a typo before sending, and voids 004 in the spreadsheet with the note 'never sent, wrong client name.' The next real invoice is 005. In June, 012 goes out for £4,000 and should have been £3,600. She does not edit the PDF the client already has. She issues CN-2026-001 against 012, then INV-2026-013 for £3,600. Both stay in the folder. In September a second company, the limited company she just formed, starts at LTD-2026-001. The sole trader series stops at INV-2026-020 on the day before incorporation. She writes that handover in one email to herself.
Nothing in that year required a clever format. It required a next number, a void she did not reuse, a credit that pointed backward, and a hard stop when the legal entity changed. If your year looks like that, you are done designing. If your year has two invoices called 20260918, you are not done, and the fix is to pick the year-prefixed counter and start it at the next January, or at the next number if you document the mid-year switch once and then leave it alone.
The formats, in one line each. Sequential: INV-015. Year-prefixed: INV-2026-015. Client-coded: ACME-2026-004. Date-based: 20260618, which you should not use as the invoice number. Put the real date in the date field, the amount in the total, and the next unused number in the corner. Then stop redesigning it. The guide to writing the rest of the invoice covers the lines around that number. The first freelance invoice is the wider checklist if you are still on invoice three.