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
Check the client behind the invoice
Numbering keeps your books tidy, but it does not tell you if a client will pay. RecoupIQ runs a forensic check on any UK company so you know who you are invoicing.
When should you verify a UK company?
Public records show financial distress weeks before credit rating agencies update. Select your situation to see the specific legal risks and what to verify before funds or work leave your hands:
Client has gone quiet after work was delivered
Trigger: Your invoice is 7 to 30 days overdue. Emails are bouncing or promises to pay keep slipping.
High Court winding-up petitions are presented 14 to 30 days before public registers show liquidation. Once a winding-up order is made, trade creditors recover an average of £0.02 on the pound.
Verify live London Gazette winding-up notices, active strike-off proposals, and debenture registrations before you lose statutory priority.
Search any UK company to verify live risk
Enter any company name or 8-digit Companies House number to see live status, balance sheet deficit, and adverse notices: