Invoice numbering looks like a formatting preference. It is closer to a legal obligation, and it is one of the first things an auditor examines.
#Why the sequence matters
Tax authorities in most jurisdictions require invoice numbers to run in an unbroken series. The reasoning is straightforward: if numbers can be skipped, revenue can be omitted. A gap in the sequence is the simplest possible indicator that an invoice was raised and then removed from the records.
This is why "I deleted that one because the client cancelled" is an unsatisfying answer during an audit, even when it is entirely true. The correct handling is to keep the invoice and issue a credit note against it, leaving both in the record.
#Four schemes that work
Simple sequential. 1001, 1002, 1003. A single running series, never reset. The simplest to defend and the hardest to get wrong. Start at a number above 1 if you would rather not advertise that a client is your third ever.
Year-prefixed. 2026-001, 2026-002. Resets each January. Makes year-end reconciliation straightforward and tells you at a glance which tax year an invoice belongs to. The most common scheme among small businesses.
Year and month. 2026-08-014. Useful if you issue high volumes, since it narrows the search when reconciling a bank statement line.
Client-coded. ACME-007, NRTH-012. A separate series per client. Legitimate in most jurisdictions provided each series is itself unbroken, and useful if you invoice a handful of large accounts. Adds bookkeeping overhead with many small clients.
Pick one and never change mid-year. Changing scheme creates an apparent discontinuity that has to be explained.
#What to avoid
Dates as invoice numbers. 20260814 breaks the moment you issue two invoices on one day, and the workarounds are ugly.
Random or non-sequential numbers. Some businesses use them deliberately to obscure volume. Most tax authorities take a dim view, and it removes your own ability to spot a missing invoice.
Restarting per project. Creates multiple overlapping series with no clear ordering and makes reconciliation genuinely painful.
Reusing a number. Two documents with the same identifier is the one thing worse than a gap.
#Handling mistakes correctly
Wrong amount or details, not yet paid. Issue a credit note for the full value of the original, referencing its number, then raise a fresh invoice with the next number in the series. Three documents, all retained.
Client cancels before payment. Same approach — credit note against the original. Do not delete.
Duplicate issued by accident. Credit one of them. Note the reason in your records.
Genuine gap already exists. Do not attempt to backfill. Write a short internal memo recording what happened and when you discovered it, and keep it with your records. An explained gap is a minor administrative issue; an unexplained one invites questions.
#Credit notes need their own sequence
Credit notes should have their own unbroken series, distinct from invoices — CN-001, CN-002, or 2026-CN-001. Each should reference the invoice number it relates to, so the two can be matched.
#Practical mechanics
Keep a register. A single spreadsheet or ledger with number, date, client, amount, status and, for any credited invoice, the credit note reference. This takes seconds per invoice and makes an audit trivial.
Never let two systems issue numbers. If your accounting software and a separate invoicing tool both allocate numbers, you will eventually get a duplicate. Choose one authority.
Diarise the year-end reset if you use a year-prefixed scheme, so January's first invoice does not continue December's sequence.
The invoice generator remembers your last number in your browser's local storage, so the sequence continues automatically between sessions without anything being uploaded.
#Retention
Most jurisdictions require invoices to be kept for five to seven years, and increasingly accept electronic copies provided they are complete, legible and unaltered. Keep the PDF you actually sent, not just the underlying data — if a dispute arises, what matters is the document the client received.
Store them in a structured folder by year, named with the invoice number and client, so that finding 2026-014 takes seconds rather than a search through an email archive.
Frequently asked questions
Can I start my invoice numbers at 1000?
Yes. There is no requirement to start at 1, and starting higher is common. The requirement is that the series is unbroken from wherever it begins.
What if I skip a number by accident?
Do not try to backfill it. Record a short internal note explaining the gap and when you noticed it, and continue the sequence. An explained gap is a minor issue; an unexplained one attracts scrutiny.
Do credit notes use the same numbering as invoices?
They should have their own separate, unbroken series and reference the invoice they relate to. Mixing them into the invoice sequence makes reconciliation harder and can confuse an auditor.