Invoice Number: Format, Rules, and How to Number Invoices (2025)
Updated May 2025 · 7 min read
Every invoice needs a unique invoice number. It's a legal requirement in the UK and EU, it makes tracking payments easier, and it helps your clients find the right invoice quickly. Here's everything you need to know about invoice numbering — including the common mistakes that cause problems with HMRC.
What Is an Invoice Number?
An invoice number is a unique reference code assigned to each invoice you issue. It's used by:
- You — to track which invoices are paid, unpaid, or overdue
- Your client — to reference the correct invoice in payment instructions
- HMRC / tax authorities — to audit your income and verify tax records
- Your bank — as a payment reference for reconciliation
📋 Quick rule
Every invoice number must be unique and sequential. No two invoices should ever share the same number.
Legal Requirements (UK & EU)
| Jurisdiction | Requirement | Applies to |
|---|---|---|
| UK (non-VAT) | Unique sequential number | All businesses |
| UK (VAT registered) | Unique sequential number (must be on VAT invoice) | VAT-registered businesses |
| Netherlands | Volgnummer (sequential number) | All businesses |
| Germany | Rechnungsnummer (sequential) | All businesses |
| EU generally | Unique sequential reference per the EU VAT Directive | VAT-registered businesses |
The format is not legally prescribed — you can use numbers, letters, dashes, or dates — as long as the sequence is unique and continuous.
Invoice Number Formats
There's no mandated format — pick one that works for your business and stick with it. Here are the most common approaches:
Sequential
✅ Simplest. Easy to spot gaps. No year reset needed.
⚠️ Reveals how many invoices you've issued (some prefer to start at 101 or 500).
Year-prefixed
✅ Immediately shows which tax year the invoice belongs to.
⚠️ Resets each year — keep separate sequences for 2024, 2025, etc.
Full date-based
✅ Invoice date embedded in the number. Easy to trace.
⚠️ Long numbers. Sequential padding required per day.
Client-prefixed
✅ Easy to group by client.
⚠️ Harder to maintain a single sequence. Not recommended for HMRC.
Combined
✅ Professional, date-aware, easy to read.
⚠️ Slightly longer than pure sequential.
How to Number Your Invoices
Pick a format and stick to it
You can change format at the start of a new financial year, but not mid-year. Consistency is what HMRC expects.
Start wherever makes sense
001, 100, 1000 — all fine. Starting at a higher number is completely acceptable and doesn't cause any issues.
Never reuse a number
Even if an invoice was cancelled. If you issue INV-047 and then cancel it, the next invoice is still INV-048. INV-047 should be marked as 'void' or 'cancelled'.
Keep a record
A simple spreadsheet or invoicing software tracks all your invoice numbers automatically. You should be able to see every invoice issued in numerical order.
Include it prominently on every invoice
Top right is conventional. It should appear on the PDF, the email subject line, and the bank transfer reference.
Gaps in Your Sequence — Is That a Problem?
Yes, potentially. Gaps in invoice numbering sequences are a red flag for HMRC during a tax investigation. They can imply that invoices were issued and then hidden from your tax records.
⚠️ How gaps happen
- • You issued an invoice then cancelled it without marking it void
- • Your invoicing software skipped a number due to a bug
- • You use multiple invoice series (e.g. one for each client)
- • You deleted an invoice rather than voiding it
What to do:If you have gaps, document them. Mark cancelled invoices as "VOID" in your records (don't delete them). If queried by HMRC, you need to be able to explain every gap.
Common Mistakes and How to Fix Them
❌ Using the same invoice number twice
✅ Fix: Issue a corrected invoice with a new number. Mark the original as 'Amended' and reference the original number. Don't delete either.
❌ Restarting at 1 mid-year
✅ Fix: If you want to reset per year, do it at the start of the new financial year only. Add the year to your format: '2025-001' → '2026-001' next year.
❌ Random or non-sequential numbers
✅ Fix: Start a consistent sequence going forward. For your records, explain that all prior invoices used a different system.
❌ Forgetting the invoice number on the invoice
✅ Fix: Add it — HMRC requires it on all invoices. It's also what clients use to reference payment.
❌ Deleting old invoices
✅ Fix: Never delete — always void/cancel. HMRC may audit up to 6 years back. Missing invoices are a bigger problem than cancelled ones.
FAQ
Can I start my invoice numbers at any number?▾
Yes. There's no legal requirement to start at 001. Many freelancers start at 101 or 500 to avoid appearing inexperienced. Just be consistent from that starting point.
Do I need separate invoice sequences for different clients?▾
No — one unified sequence is simpler and what HMRC prefers. If you want client-specific references, add a prefix (ACME-INV-001) but maintain a single running number underneath.
Can my credit notes use the same numbering sequence as invoices?▾
Best practice is to use a separate sequence for credit notes (e.g. CN-001) so they don't interrupt your invoice sequence. However, some businesses include them in the same sequence — either is acceptable.
What if I use multiple currencies?▾
Use the same invoice sequence regardless of currency. The currency is indicated elsewhere on the invoice.
How does invoice numbering work in invoicing software?▾
Good invoicing software (like Chaser) assigns numbers automatically, maintains the sequence, and prevents duplicates. You can usually set your starting number and format in settings.
Invoice numbering on autopilot
Chaser assigns sequential invoice numbers automatically, maintains your sequence, and sends payment reminders until your client pays — so you focus on the work.
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