How to Write a Professional Invoice: Step-by-Step Guide

The Invoice That Cost 45 Days

You deliver the project. The client says nothing for two weeks. Then you follow up, and they reply: "We can't process this — your VAT number is missing." That is not an accident. That is a standard stall tactic — and it works because their accounts payable policy gives them grounds to hold payment until the document is compliant. A missing field costs you 30 to 45 additional days on your payment cycle.

The fix is knowing exactly what goes on every invoice before you hit send.

Section 1: The 16 Mandatory EU Invoice Fields

EU VAT Directive Article 226 specifies what a full VAT invoice must contain. Every field below is legally required for VAT-registered transactions. Non-VAT invoices can omit the VAT-specific fields, but everything else still applies as good practice.

1. Your full legal name or company name — exactly as registered with your tax authority
2. Your address — registered business address, not a P.O. box
3. Your VAT number — required if you are VAT registered (format varies by country, e.g. DE123456789)
4. Client's full legal name and address — as it appears in their business registration
5. Client's VAT number — required for B2B cross-border supplies where reverse charge applies
6. Unique sequential invoice number — must be part of a continuous sequence with no gaps
7. Invoice date — the date you issue the document
8. Supply date — the date goods were delivered or services were completed (often different from invoice date)
9. Description of goods or services — specific enough to identify what was supplied
10. Unit price and quantity — e.g. "8 hours @ €95/hr" or "1 logo design"
11. Net amount (excl. VAT) — the subtotal before tax
12. VAT rate applied — e.g. 21%, 20%, 19%, or the statement "Reverse charge — VAT to be accounted for by the recipient"
13. VAT amount — the actual euro figure of VAT charged
14. Gross total (incl. VAT) — the amount the client must pay
15. Payment terms and due date — e.g. "Net 30 — due by 3 March 2026"
16. Your bank details — IBAN and BIC/SWIFT code for international payments

Section 2: Invoice Numbering — Why Sequential Matters

Your invoice numbers are not just labels — they are an audit trail. Tax authorities in multiple EU countries can and do disallow VAT input claims when invoice numbers appear out of sequence, duplicated, or missing.

The rules are simple:

• Every invoice number must be unique
• Numbers must follow a continuous, ascending sequence
• You must never reuse a number, even for a voided invoice
• Gaps in the sequence invite scrutiny

Common formats that work:

Pick one format and use it consistently. The format itself is not legally mandated — the uniqueness and sequence are.

Section 3: Payment Terms — The Numbers That Move Money

Payment terms directly influence when you get paid. The data is stark: invoices with a due date of 7 days are paid approximately three times faster than invoices marked Net 30. Clients are not malicious — they prioritise the most urgent payables first.

The most common terms:

Add a late payment clause. EU Directive 2011/7/EU entitles you to charge statutory interest on overdue B2B invoices — currently the ECB reference rate plus 8 percentage points. State this on the invoice. Most clients pay faster when they see it.

Section 4: Common Invoice Mistakes to Avoid

Write Every Invoice Correctly, Every Time

Checking 16 fields manually on every invoice is a mistake waiting to happen. Use the free invoice generator on this site: it applies the correct fields automatically, calculates VAT at the right rate for your country, and generates a sequential invoice number so your records stay clean.

EU VAT Directive 2006/112/EC, Article 226. Late payment interest rates: European Central Bank reference rate + 8 pp per Directive 2011/7/EU.