Compliance references in this article (HMRC, VAT audit trail) are based on UK tax rules. Your local tax authority will have its own requirements — the general principle of sequential numbering applies universally.
Every VAT invoice must have a unique, sequential invoice number. Even if you are not VAT-registered, a consistent numbering system keeps your records tidy and makes it easy to track outstanding invoices.
Keep it sequential
Invoice numbers must go up — you cannot skip numbers or reuse them. HMRC does not require a specific format, but the sequence must be unbroken. If you void an invoice, note it in your records rather than deleting it.
Common formats
- Simple sequence: 001, 002, 003 — clean and easy
- Year prefix: 2025-001, 2025-002 — useful for separating financial years
- Client prefix: ACME-001, ACME-002 — helpful if you invoice the same clients regularly
- Date-based: 20250601-001 — shows the date at a glance
Start from a number that does not reveal your invoice volume
Starting your invoice numbers at 001 tells new clients you are new in business. Many businesses start at 100, 1000, or use a date-based prefix to avoid this. It is a small thing but it matters for perceived credibility.
Never reuse numbers
If you send an invoice to the wrong client or make an error, void it with a credit note — do not delete it and reuse the number. HMRC requires a complete audit trail.
BillSend automatically assigns sequential invoice numbers for you, so you never have to think about this.
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