Sequential numbering sounds trivial until an accountant, an auditor, or a duplicate payment makes it matter. Here is how to set it up once and never think about it again.
An invoice number does three jobs. It lets you and the client refer to one specific document without ambiguity - on payments, in emails, in disputes. It lets your accountant confirm that the invoice record is complete, because an unbroken sequence shows nothing has been lost or quietly deleted. And in many tax systems it is a legal requirement: VAT regimes in particular generally require sequential, unique numbering.
Everything about choosing a scheme follows from those three jobs. The number must be unique, it should be sequential enough that completeness can be checked, and it should be easy to quote accurately over the phone.
Plain sequence: 1, 2, 3, usually padded to a fixed width - 0001, 0002. This is the simplest scheme, satisfies every requirement, and is the right default for most small businesses.
Year-prefixed: 2026-001, restarting the counter each year. This keeps numbers short, makes the age of any invoice obvious at a glance, and matches the way accounts are reviewed - by year. The mild cost is that "invoice 14" is ambiguous without the year attached.
Client-prefixed: ACME-014, one counter per client. Useful for businesses with a few large clients who quote your numbers back into their own systems, and for keeping one client's history readable. Awkward beyond a dozen active clients, and completeness checks now have to run per prefix.
What all three share: the counter only ever goes up, and the format never changes mid-year. Changing scheme is fine - do it at a year boundary and note where the old sequence ended.
Gaps happen: an invoice gets created and then cancelled, or a draft consumes a number and is abandoned. A gap is not a crisis, but it should have an explanation, because a sequence with unexplained holes is exactly what makes an auditor look harder. The clean practice is to keep the cancelled invoice as a record - marked void, never deleted - so number 0042 exists, shows as cancelled, and the sequence stays accountable.
A wrong invoice that has already been sent is corrected with a credit note and a new invoice, not by editing the original. Once a numbered document has left the building, that number means that document; reusing or rewriting it creates two versions of the truth, and the client's copy is the one you do not control.
Never reuse a number, even from a genuinely voided invoice, and never number backwards to fill a hole. Both save a moment of tidiness now and cost hours of explanation later.
Starting at an inflated number to look established - 1000 instead of 1 - is harmless in most places but pointless: clients do not check, and your accountant now has a sequence that implies 999 missing invoices. If a low number bothers you, a year prefix solves it without the fiction.
Two people issuing invoices from separate spreadsheets is how duplicates actually happen in small businesses. The same number on two different invoices means at least one payment will be misallocated. The fix is a single shared counter - which in practice means issuing invoices from one system rather than from personal files.
Encoding meaning into the number - dates, job codes, amounts - tends to grow until numbers are twenty characters long and still ambiguous. Keep the number for identity, and put everything else in the fields that exist for it.
These pages connect nearby invoice searches, business use cases, and template needs so visitors can find the right starting point faster.
Create 5 invoices every month for free, then upgrade as you grow.