The Net Investment Income Tax (NIIT) is a 3.8% surtax that catches a lot of people by surprise the first year it applies to them — usually after a big capital gain, a strong dividend year, or a jump in rental income pushes their total income over a fixed threshold. Unlike most tax brackets, the NIIT thresholds have never been adjusted for inflation, so more taxpayers cross into it every year without actually getting “richer” in real terms.
Who Owes the 3.8% NIIT
The NIIT applies once your modified adjusted gross income (MAGI) exceeds:
• $200,000 for single filers and head of household
• $250,000 for married filing jointly and qualifying surviving spouses
• $125,000 for married filing separately
These thresholds have stayed fixed since the tax took effect in 2013. The tax applies to individuals, estates, and trusts with net investment income above these amounts.
What Counts as Net Investment Income
Net investment income generally includes interest, dividends, capital gains, rental and royalty income, and non-qualified annuity income. It does not include wages, most self-employment income, Social Security benefits, or distributions from qualified retirement plans like 401(k)s and IRAs. Income from an active trade or business you materially participate in is also generally excluded.
How the Tax Is Actually Calculated
The 3.8% applies to the lesser of two numbers: your net investment income for the year, or the amount by which your MAGI exceeds the threshold for your filing status. This matters because someone with a large MAGI but modest investment income may owe less NIIT than the headline rate suggests — the tax is capped by whichever number is smaller.
For example, a single filer with $220,000 MAGI and $15,000 of net investment income would owe 3.8% on the smaller number: the $15,000 of investment income (since the $20,000 excess over the threshold is larger), for a NIIT bill of $570.
Where This Intersects With Other Tax Optimization Moves
Because NIIT is triggered by investment income specifically, strategies that already reduce capital gains exposure also reduce NIIT exposure. Tax-loss harvesting, covered in our guide to tax-loss harvesting and the wash-sale rule, can offset gains that would otherwise count toward net investment income. Understanding which income falls into which capital gains bracket, covered in our capital gains tax brackets guide, is also a prerequisite for estimating whether you’re close to an NIIT threshold in a given year.
Form 8960 Is Where This Gets Reported
Taxpayers who owe NIIT report and calculate it on Form 8960, filed with their federal return. Because the tax depends on MAGI (not taxable income), it’s worth recalculating mid-year if you’re near a threshold and considering a large capital gain, Roth conversion, or asset sale — each of those raises MAGI and can trigger or increase NIIT liability.
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