The three main systems
Sequential (1001, 1002, 1003) is the simplest and what most software uses by default. Dated (2026-001, 2026-002) makes it easy to see at a glance what year an invoice belongs to. Prefixed by customer (ACME-001, ACME-002) makes each customer's invoice history easy to find.
Whichever you pick, don't reuse numbers, don't skip numbers randomly (auditors flag it), and don't restart at 1 each month or year unless you use a year prefix.
Why the first number matters
Starting at 1 tells your first customer they're your first customer. Starting at 1001 or 5000 doesn't. It's a small psychological signal but a real one — especially if you're brand new and pitching to bigger clients.
Formats that hold up over time
Good formats scale as your business grows:
- 1001, 1002, 1003 — simple, works forever
- 2026-001, 2026-002 — resets each year with a year prefix
- INV-1001 — clear that it's an invoice number (vs estimate or quote)
- ACME-001 — customer-prefixed, useful if you have a few big clients
What to avoid
A few patterns that cause problems:
- Random numbers (0483, 7291) — hard to spot gaps, easy to duplicate
- Reusing numbers when you delete a draft — creates ambiguity
- Restarting at 1 each year with no year prefix — collisions
- Numbers over 6 digits without a prefix — hard to read aloud
Voiding vs deleting
If you create an invoice by mistake, void it (set the total to zero, keep the number in the sequence) rather than delete. This preserves the audit trail and prevents gaps. Glo Invoice handles this automatically — voided invoices stay in your history with a clear 'void' status.