Line 6a of Form W-8BEN is the single most misread line on the form. It asks for a "Foreign tax identifying number", and the reflex — particularly among people who have dealt with US paperwork before — is to reach for a US number, or to leave it blank because no US number exists. Both cause the same outcome: the withholding agent rejects the form and comes back for a corrected one, usually after the payment has already been held. This is not tax advice, and the IRS instructions for Form W-8BEN are the authority; what follows is what the line is asking for and where people go wrong filling it in.
Line 6a of Form W-8BEN wants the taxpayer identification number issued by your country of tax residence — not a US number. Line 5 is the separate box for a US SSN or ITIN. If your jurisdiction does not legally require you to have one, you check box 6b instead of leaving 6a empty.
What counts as a Foreign Tax Identifying Number?
Whatever number your country of tax residence uses to identify you as a taxpayer. Each jurisdiction has its own, under its own name, and there is no universal format — which is exactly why the form asks for it as free text rather than a validated field. In the UK it is typically the National Insurance number or the Unique Taxpayer Reference; elsewhere it may be a tax file number, a fiscal code, a personal identity number that doubles as a tax ID, or a business registration number for an entity.
Two practical rules. First, use the number for the country you entered on line 3 as your residence — if those two disagree, the form is internally inconsistent. Second, enter it exactly as it is issued, including any letters; withholding agents match against the format the issuing country uses.
How is line 6a different from line 5?
Line 5 is the US taxpayer identification number — an SSN for individuals eligible for one, or an ITIN. Line 6a is the foreign one. They are separate lines because most people filing a W-8BEN legitimately have one and not the other, and a few need both.
Where a US TIN matters most is treaty benefits. Claiming a reduced withholding rate under a tax treaty in Part II generally depends on having the right identifying number on the form, and the instructions set out when a US TIN is required rather than optional. If you are claiming a treaty rate, read that section rather than guessing — the difference between a correct claim and an incorrect one is typically 30% of the payment.
When do I check box 6b instead?
Box 6b is the declaration that you are not legally required to obtain an FTIN from your jurisdiction of residence. It exists because a blank line 6a is ambiguous — it could mean "I don't have one" or "I forgot" — and the checkbox turns it into a statement.
The IRS instructions describe the circumstances: accounts held at US financial institutions generally require an FTIN with limited exceptions, including residents of US territories and residents of jurisdictions on the IRS's published list of those that do not issue tax identifying numbers. The mistake to avoid is checking 6b because you don't know your number. "I have not looked it up" is not the same statement as "my jurisdiction does not require one", and you are signing the form under penalties of perjury.
What is Form W-8BEN actually for?
Its full title is Certificate of Foreign Status of Beneficial Owner for United States Tax Withholding and Reporting, and the job it does is establishing that you are not a US person, so the party paying you knows which withholding rules apply and whether a treaty rate is in play. You give it to the withholding agent or payer, not to the IRS — it never gets filed with a return.
Timing matters more than people expect: it should be in the payer's hands before the income is paid or credited, not after. A form generally remains valid from the date it is signed through the end of the third succeeding calendar year, unless something on it stops being true — a change of address, a change of residence country, a new tax ID — in which case a fresh one is due. Our fuller walkthrough of how to fill Form W-8BEN covers the rest of the lines.
How do I stop retyping this for every contractor?
If you are on the paying side, the W-8BEN is rarely a one-off. Every non-US contractor you engage needs one, they expire on a rolling three-year cycle, and the fields — name, country of citizenship, residence address, tax ID, date of birth — are the same fields you already hold in your contractor records and are already typing into agreements and payment forms.
Filly AI is built for exactly that repetition: a saved profile per person, and any form you upload gets filled from it. You upload the W-8BEN, pick the contractor, review every value before exporting, and send it for signature as a link they open in a browser with no account — which matters when the signer is in another time zone and has no PDF editor. The free plan is 10 forms, 5 clients and 10 fills a month with no card; paid plans start at $19/month. The same profile fills the contractor agreement and the invoice that follow it. If your contractors are a mix of US and non-US, our guide to filling W-9 forms for multiple contractors is the domestic counterpart.
Frequently asked questions
Can I leave line 6a blank if I don't have a foreign tax ID?
Leaving it blank without checking box 6b is the version most likely to come back to you. If you genuinely are not legally required to hold one, check 6b. If you are required to hold one, find it before submitting.
What is the UK foreign tax identifying number for a W-8BEN?
UK residents most commonly give their National Insurance number or their Unique Taxpayer Reference. Which one your payer expects can vary, so if their onboarding guidance names one, follow it.
Do I need an ITIN to file a W-8BEN?
Not for the form itself in every case — but a US TIN can be required in order to claim treaty benefits in Part II. Check the instructions for your specific claim before assuming line 5 can stay empty.
How long is a W-8BEN valid?
Generally from the date of signature through the end of the third succeeding calendar year, unless a change in circumstances makes any information on it incorrect — at which point you owe the payer a new one.
Who do I send the completed W-8BEN to?
The withholding agent or payer who requested it. Do not send it to the IRS.
Is Form W-8BEN the same as W-8BEN-E?
No. W-8BEN is for individuals; W-8BEN-E is the entity version and is considerably longer. Sending the wrong one is a common reason a payment gets held.