Give every invoice a unique number that follows on from the last one, such as 001, 002, 003. You can add a prefix, the year or a client code, but the sequence must never repeat. Here’s how to pick a scheme, handle cancelled invoices and stay on the right side of tax rules.
An invoice number is the one reference you, your client and your accountant all share. It appears on bank transfer references, in email threads and in your bookkeeping, and it’s how everyone finds the right document months later.
Many tax authorities also expect invoice numbers to be unique and sequential, because an unbroken run makes it easy to see that every sale has been recorded. A messy or repeated numbering system won’t necessarily get you in trouble, but it invites questions you’d rather not answer during a tax check.
The easiest scheme is a plain sequence: your first invoice is 1, the next is 2, and so on. Padding with zeros, such as 0001, keeps the numbers the same length so they sort properly in folders and spreadsheets.
You don’t have to start at 1. Plenty of new businesses start at 100 or 1001 so their first clients don’t see “Invoice 1”. That’s generally fine as long as you then carry on in order without skipping or reusing numbers.
Number invoices in the order you issue them, not the order the work happened. If you finish a job on Monday but invoice it on Friday after another client’s bill, the Friday invoice simply takes the later number. Trying to backdate numbers to match the work only creates confusion, and the invoice date already records when it was issued.
Once you send more than a handful of invoices, a little structure helps. A prefix identifies your business or document type, the year shows when it was issued at a glance, and a client code groups work by customer. Keep it short: the number should be easy to read out over the phone or type into a bank reference.
Plain sequence: 0001, 0002, 0003 Prefix: INV-0001, INV-0002, INV-0003 Year and sequence: 2026-001, 2026-002, 2026-003 Prefix, year and sequence: INV-2026-001, INV-2026-002 Year and month: 2026-10-001, 2026-10-002 Client code: ACME-001, ACME-002, BLUE-001 Client code with global sequence: ACME-0145, BLUE-0146
Year-based schemes are popular because they reset neatly and make annual bookkeeping easy. If you use one, make sure the year part changes so a number can never repeat, and think about whether your tax year matches the calendar year. In India, for example, the financial year runs from April to March, so a scheme like 2026-27/001 often fits better.
Client-based schemes are handy if you bill a few regular clients, but a separate sequence per client can make it harder to prove you haven’t missed an invoice overall. A safe compromise is to keep one global running number and add the client code as a label, as in the last example above.
Whatever you pick, write it down and stick with it. Switching schemes mid-year is allowed in many places, but it’s easier on your accountant if changes happen at the start of a new year.
Never delete an invoice and reuse its number. If you made a mistake before sending, you can usually mark that number as void, keep a copy for your records, and issue the corrected invoice under the next number.
If the invoice has already been sent, and especially if it included tax, the usual approach is to issue a credit note that cancels all or part of it, then raise a new invoice if needed. Credit notes often have their own sequence, such as CN-001. Rules on credit notes vary, so check with your accountant if tax has already been reported.
A gap in your sequence, such as jumping from 041 to 043, isn’t automatically a problem, but you should be able to explain it. A voided invoice kept on file explains it instantly; an unexplained gap could look like an unrecorded sale.
Some tax systems set specific rules. In the UK, HMRC expects each VAT invoice to carry a unique number that follows on from the last. EU VAT rules generally call for a sequential number that uniquely identifies the invoice. India’s GST rules are stricter: the invoice number can be up to 16 characters, may use letters, numbers, hyphens and slashes, and must be unique for each financial year. These details can change, so confirm the current rules with your tax authority or accountant.
BillFlow has an invoice number field where you can type any scheme you like, from 0001 to INV-2026-001. Keep a simple log or spreadsheet of the numbers you’ve used, including any voided ones, so you always know the next number in line. Because your business details are remembered on your device, creating the next invoice is mostly a matter of updating the number, client and line items.
Usually yes, as long as each number stays unique, which is why year-based schemes include the year. India’s GST rules specifically require uniqueness within each financial year. Check with your accountant if you’re unsure.
Yes. Each business or trading entity should keep its own invoice sequence, so its records are complete and easy to check on their own.
It’s good practice to give it a reference, but many businesses use a separate sequence such as PF-001, because a proforma isn’t a final tax invoice. The real invoice then gets the next number in your main sequence.
It’s best not to. Random numbers make it hard to show that every sale is recorded, and several tax systems expect a sequence. A simple running number is safer and easier to manage.
Contact the client, cancel one of the duplicates with a credit note or a void note, and reissue it under a new number. Keep a record of what happened for your books.