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.
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.
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.
Where to next
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.