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US Net Investment Income Tax calculator
The 3.8% NIIT on investment income above the $200k / $250k thresholds.
About this calculator
What it estimates
The 3.8% Net Investment Income Tax on interest, dividends, capital gains, rental and other passive income once your modified adjusted gross income exceeds the threshold.
Assumptions (US 2025 tax year rates)
- •Thresholds of $200,000 (single) and $250,000 (married filing jointly) — fixed by statute, not indexed
- •Tax = 3.8% × the lesser of net investment income or MAGI above the threshold
- •Investment expenses and allocable deductions not modelled
Estimates only, for orientation. Rates and thresholds change each tax year and your actual liability depends on facts this tool does not ask for. This is not tax advice.
For US–UK filers
Why your real number differs
NIIT is the tax Americans in the UK most often owe unexpectedly: under US domestic law the Foreign Tax Credit cannot offset it, so UK tax on the same dividends or gains does not make it go away. Whether the US–UK treaty changes that answer is a position that must be taken and disclosed deliberately.