US shares in a UK ISA: withholding tax and the W-8BEN, explained
A Stocks and Shares ISA shelters your investments from UK tax, but it does not remove US tax. Dividends from US shares still have US withholding tax taken at source - 15% if your broker holds a W-8BEN form for you, or 30% without one. The good news: gains on US shares are generally untaxed in an ISA, and your broker usually handles the W-8BEN for you.
Does an ISA protect US shares from all tax?
An ISA removes UK tax on your dividends and gains - but it can't override a foreign government's tax. Dividends paid by US companies have US withholding tax deducted at source before the money ever reaches your account, ISA or not.
So a US company paying a $100 dividend into your ISA doesn't land as $100: with the right paperwork, $15 is withheld in the US and $85 arrives. The ISA then means there's no further UK tax on top - but that first US slice is unavoidable.
What is the W-8BEN form, and do I need to fill it in?
The W-8BEN is a US tax form that declares you're a UK (non-US) resident. Under the long-standing UK-US tax treaty, having one on file cuts US dividend withholding from the default 30% down to 15%. Without it, the full 30% is taken.
In practice most UK brokers get you to complete it in a couple of clicks when you open a US-trading account, and it lasts about three years before it needs renewing. It is administrative, not something to agonise over - but it is worth checking your broker has one.
$85 arrives$15 withheld
The treaty rate. Most platforms ask you to sign one when you open the account.
$70 arrives$30 withheld
The default rate for a non-resident with no form.
Is a pension treated differently from an ISA?
Yes, and this is the one place an ISA comes off worse. The US-UK tax treaty recognises a UK pension - a SIPP included - as a qualifying pension scheme, so US dividends can be paid into one with no US withholding tax at all. Not 15%. Nothing.
An ISA is not a pension in the treaty's eyes, so 15% with a W-8BEN is the floor rather than a starting point. On £1,000 of US dividends across a year that is £150 handed to the US from the ISA and £0 from the pension, on identical shares.
The paperwork sits with the provider rather than with you. Where a SIPP offers US shares, the pension scheme handles its own certification with the US, so there is no W-8BEN for you to fill in - which is also why the difference is easy to miss.
That is a fact about withholding tax, not a verdict on either account. They do different jobs: pension money is locked away until at least age 55 (rising to 57) and is taxed as income when it comes out, while ISA money is reachable at any age and comes out untaxed. Withholding tax is one line in a much longer comparison, and the account types page below sets out the rest of it.
What about capital gains, and funds?
Capital gains on US shares are generally not taxed for a UK-resident, non-US investor, and an ISA removes UK capital gains tax too - so if a US share you hold rises in value, that growth is normally free of both US and UK tax inside the wrapper.
With a fund you don't file a W-8BEN yourself. Most world and US trackers available to UK investors are Irish-domiciled, and the 15% withholding is applied inside the fund under the US-Ireland treaty. The W-8BEN only comes up when you hold US shares directly.
A worked example
Say a US share in your ISA pays a $100 dividend. With a W-8BEN on file, $15 is withheld in the US and $85 lands in your ISA, with no further UK tax to pay. Without a W-8BEN it would be $30 withheld, leaving $70. Either way, if the share later rises in value, that growth is normally free of both US and UK tax. Scale that up and the form is worth 15% of your US dividends every year: on £1,000 of dividends it is the difference between £850 and £700 reaching the account. The same £1,000 inside a pension arrives whole.
Common questions
Do I pay US tax on US shares held in a UK ISA?
On dividends, yes: the US takes withholding tax at source - 15% with a W-8BEN, 30% without - because a UK ISA only shelters UK tax, not US tax. Capital gains, though, are generally untaxed for a UK-resident investor, and the ISA removes UK tax on them too.
What is a W-8BEN form and how do I get one?
It's a US tax form confirming you're a non-US resident, which cuts US dividend withholding from 30% to 15% under the UK-US treaty. UK brokers usually prompt you to complete it when you open a US-trading account, and it lasts about three years.
Do US-focused funds need a W-8BEN?
No - you don't file one yourself for a fund. An Irish-domiciled ETF (which is what most UK-available US and world trackers are) has the 15% withholding applied inside the fund. The W-8BEN only matters when you hold US shares directly.
Is US withholding tax really 0% in a SIPP but 15% in an ISA?
Yes. The US-UK treaty recognises a UK pension, including a SIPP, as a qualifying pension scheme, so US dividends can reach it with nothing withheld. An ISA is not a pension in the treaty's eyes, so 15% with a W-8BEN is the lowest rate available to it. The trade-off is access: pension money is locked until at least age 55 and is taxed as income when it comes out.
How much is a W-8BEN actually worth?
15% of your US dividends, every year it keeps paying. Without the form the US withholds 30% instead of 15% - on £1,000 of dividends that is £300 gone rather than £150. Inside an ISA the difference is not recoverable later, because there is no UK tax on the dividend to set the US tax against.
Where to next
- ISA rules
- the basics
- the one global tracker approach
- the glossary
- account types compared
- cash ISA vs stocks and shares ISA
- what the ex-dividend date means
Where these figures come from
The rules and allowances on this page come from the official sources below. Tax rules change, so check the source for the current position.
- IRS: About Form W-8BEN the treaty-rate certification form
- gov.uk: Individual Savings Accounts (ISAs) what a UK ISA does and does not shelter
- IRS: Tax Treaty Tables the withholding rates each treaty sets, including for pension schemes
- gov.uk: Double taxation treaties - residents with UK income the UK side of the same agreement
General information to help you understand investing, not advice about your situation. Figures are illustrative and the rules can change - always check gov.uk or your provider for the latest.
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Last reviewed 3 August 2026. We re-check the rules and figures on this page when they change.