Tax identification numbers are the least glamorous fields on an invoice and among the most consequential. They are how a tax authority connects a document to a registered business, and how your customer proves their right to reclaim tax. Get one wrong and the invoice may be rejected; omit one where it is required and the invoice may not be valid at all.
The confusion is that every country has its own scheme with its own name, and they do not all do the same job. Some identify a business for tax generally; some specifically evidence a registration that permits reclaiming tax. Knowing which kind you are dealing with tells you whether it belongs on the invoice.
The main schemes you will meet
A handful of identifiers account for most international invoicing. They are not equivalents of each other, and the differences matter when deciding what to print.
| Identifier | Where | What it identifies |
|---|---|---|
| VAT number | EU member states, UK and many others | A business registered for VAT, able to charge and reclaim it |
| EIN | United States | A business for federal tax purposes — not a sales tax permit |
| State sales tax permit | US states individually | Authority to collect that state’s sales tax |
| ABN | Australia | A business in dealings with government and other businesses |
| GST registration number | Australia, Canada, India, Singapore and others | A business registered for GST |
| GSTIN | India | A GST-registered business, encoding state and PAN |
| Company registration number | Most jurisdictions | Legal existence, not tax status |
VAT numbers do the most work
In VAT and GST systems the registration number is central rather than decorative. Your own number establishes that you are entitled to charge the tax you are charging. Your customer’s number, where required, establishes their status — and on a cross-border business-to-business supply it is what permits you to invoice without VAT under the reverse charge.
VAT numbers follow national formats, usually a country prefix followed by a fixed pattern of digits and sometimes letters. Format alone tells you very little: a well-formed number can be entirely fictitious, and a number that was valid last year can have been cancelled since.
This is why the EU operates VIES, a validation service linked to every member state’s registry. Checking a customer’s number there before relying on it is the difference between having evidence and having a hopeful assumption. Capture the date and the confirmation reference rather than just the result, because the evidence is what an auditor asks to see.
The United States is the odd one out
US invoices frequently carry no tax identifier at all, and that is normal rather than an omission. There is no VAT, so there is no reclaim chain requiring a number on the document, and invoice content is not federally prescribed.
An EIN identifies a business for federal tax purposes, but it is not a sales tax permit and is not usually printed on invoices — it appears on tax filings and on forms like the W-9 that a customer may request so they can issue information returns. Authority to collect sales tax comes from a state-issued permit, which is separate again.
The practical consequence for a US business invoicing internationally is that a customer may ask for "your tax number" and be surprised when you do not have the kind they mean. Explaining that you are not VAT-registered because the US has no VAT usually resolves it, though some customers will want a W-9 or a tax residency certificate for their own withholding rules.
Which numbers to put on the invoice
Print your own tax registration number whenever you are registered for a tax you are charging. In VAT and GST countries this is required rather than optional, and its absence can make the invoice defective for your customer’s reclaim.
Print your customer’s number when the transaction depends on their status: cross-border business-to-business supplies where the reverse charge applies, and generally where local rules require the customer to be identified. For ordinary domestic sales to consumers it is neither needed nor appropriate.
Print your company registration number where local law or convention expects it — several jurisdictions require registered company details on business documents — but do not confuse it with a tax number. They answer different questions, and a customer’s finance team looking for a VAT number will not accept a company number in its place.
Verifying before you rely
The rule is proportionate to the stake. If nothing depends on the customer’s number, recording it accurately is enough. If your VAT treatment depends on it — which is precisely the reverse charge case — verify it and keep the evidence.
Most VAT and GST jurisdictions publish a lookup service, and the EU’s covers all member states in one place. Verification is quick, free, and the only thing standing between you and personally funding tax you never collected.
Two habits make this durable. Verify at onboarding rather than at invoicing time, so the answer is known before it is urgent. And re-verify long-running customers periodically, because a registration cancelled two years into a relationship will not announce itself on the purchase order.
Common Mistakes
Confusing a company number with a tax number
A company registration number proves legal existence, not tax status. A customer asking for a VAT number needs the tax registration, and the two are not interchangeable.
Treating a well-formed number as a valid one
Format checks catch typos, not fiction or cancellation. Where your tax treatment depends on the number, validate it against the official registry and keep the confirmation.
Putting an EIN on invoices as a VAT equivalent
An EIN is a federal tax identifier, not a VAT registration and not a sales tax permit. It rarely belongs on an invoice, and it will not satisfy a customer looking for VAT evidence.
Omitting your own number where it is required
In VAT and GST systems your registration number is mandatory invoice content. Leaving it off can invalidate the document for your customer’s reclaim and get it returned.
Frequently Asked Questions
Do I need a tax ID to send an invoice?
Not inherently. If you are not registered for any tax — below a VAT threshold, or in a country without VAT — you invoice without one and charge no tax. What you must not do is display a registration number you do not hold, or charge tax you are not registered to charge.
How do I check a customer VAT number?
For EU numbers, use the European Commission’s VIES service, which queries member state registries directly. Other jurisdictions publish their own lookups. Record the date and confirmation reference, not just the outcome.
What is the US equivalent of a VAT number?
There is not a direct one. An EIN identifies a business federally, a state sales tax permit authorises collecting a state’s sales tax, and neither functions as a VAT registration because the US has no VAT.
What if my customer refuses to give their VAT number?
Then you generally cannot treat the supply as a business-to-business one for reverse charge purposes, and you may have to charge VAT as you would to a consumer. Explain that the number is what permits the zero-rating — most refusals are misunderstandings about why you are asking.
Sources & Further Reading
- VIES — VAT number validation service — European Commission
- Employer Identification Number (EIN) — Internal Revenue Service
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