Invoice numbering might seem trivial, but HMRC requires that every invoice has a unique, sequential identifier. A good numbering system also helps you stay organised, track payments, and look professional.
HMRC Requirements
HMRC requires that invoices are numbered sequentially and uniquely. There should be no gaps in the sequence (a gap could suggest a missing or deleted invoice, which raises red flags during an investigation).
Popular Numbering Systems
1. Simple Sequential
The simplest approach — start at 001 and count up:
- INV-001, INV-002, INV-003...
- Pros: Simple, clear, easy to track
- Cons: Reveals how many invoices you have sent (INV-003 tells the client you are new)
Pro Tip: Start Higher
Start your sequence at 1001 instead of 001. This avoids revealing your invoice volume to clients and looks more established. HMRC does not require you to start at 1 — just that the sequence is consistent.
2. Date-Based
Incorporate the year or month into the number:
- 2026-001, 2026-002... (resets each year)
- 202601-01, 202601-02... (year + month + sequence)
- Pros: Easy to find invoices by date, natural organisation
- Cons: Slightly more complex
3. Client-Based
Include a client code:
- ACME-001, ACME-002, WIDG-001...
- Pros: Easy to track invoices per client
- Cons: Can become complex with many clients
4. Hybrid
Combine date and sequence:
- RQ-2026-001 (your initials + year + sequence)
- Pros: Professional, organised, easy to search
- Recommended for most freelancers
Rules to Follow
Be sequential
No gaps — every number must be used in order
Be unique
Never reuse an invoice number, even for different clients
Be consistent
Pick a system and stick with it — do not change mid-year
Keep records
Maintain a log of all invoice numbers issued