UK vs US Capital Gains Tax Estimator

Potentially yes, but the Foreign Tax Credit usually prevents true double taxation. As a US citizen, you report the gain to the IRS on your Form 1040. You also report it to HMRC. The US generally allows a credit for UK CGT paid, which offsets your US federal capital gains tax on the same gain. However, differences in how each country measures the gain — cost basis, reliefs, currency movement — can mean residual tax arises in one jurisdiction.

The US Section 121 exclusion ($250,000 for single filers, $500,000 for married filing jointly) can apply to a US citizen's principal residence even if it is outside the US, provided they meet the 2-of-5-years ownership and use test. The UK's equivalent, Principal Private Residence Relief, operates separately and under different rules. Both can apply to the same sale, but they are calculated independently.

Several reasons. The UK and US may measure the gain differently — the US uses a USD cost basis, so currency movement since purchase can create a US gain even when the GBP gain is the same or smaller. The UK annual exempt amount does not apply for US purposes. The 3.8% NIIT cannot be offset by the FTC. And different timing of when tax is paid can affect the credit calculation.

The October 2024 UK Budget unified CGT rates for most assets. From October 30 2024, the rates became 18% (basic rate) and 24% (higher rate) for residential property and other assets. The annual exempt amount was cut to £3,000. This estimator uses the current 2025/26 rates.

Yes — timing can make a meaningful difference. The tax year mismatch (UK April–April vs US calendar year), your income level in the year of disposal, the USD/GBP rate, and whether you have FTC carryforwards from prior years all affect the combined bill. Planning a disposal 6–12 months ahead gives time to optimise.