Net Investment Income Tax

The Net Investment Income Tax, established under Internal Revenue Code Section 1411 as part of the Affordable Care Act, took effect in 2013 and continues to apply in 2025. It is levied on the lesser of (a) the taxpayer's net investment income or (b) the amount by which their modified adjusted gross income (MAGI) exceeds the applicable threshold: $200,000 for single filers, $250,000 for married filing jointly, $125,000 for married filing separately, and $200,000 for head of household. These thresholds are not indexed for inflation and have remained unchanged since the tax was introduced, meaning more taxpayers become subject to it each year through bracket creep.

Net investment income subject to the NIIT includes dividends, interest, capital gains (both short-term and long-term), rental income, royalties, and passive business income. It does not include wages, self-employment income, Social Security benefits, alimony, tax-exempt interest, or distributions from qualified retirement plans such as 401(k)s and IRAs.

For a high-income investor, the NIIT effectively raises the top rate on long-term capital gains and qualified dividends from 20% to 23.8%, and adds 3.8% on top of ordinary rates for interest, nonqualified dividends, and short-term gains. Understanding NIIT exposure is important for year-end tax planning, particularly when deciding whether to realize large capital gains or harvest losses.

Strategies to manage NIIT liability include contributing to tax-deferred retirement accounts (which reduces MAGI), directing investments into tax-exempt bonds (whose interest is excluded from net investment income), making charitable contributions, and accelerating deductions. Business owners may be able to restructure income from passive to active participation to remove rental or business income from the NIIT base.

The NIIT is calculated on Form 8960 and reported on Form 1040. It is separate from and in addition to regular income tax, self-employment tax, and the additional Medicare tax on wages. High-income investors should incorporate NIIT projections into their quarterly estimated tax calculations to avoid underpayment penalties.

NIIT Calculation: The NIIT is assessed at a flat 3.8% on the lesser of two figures. First, the taxpayer's net investment income (NII) — defined as gross investment income minus investment-related deductions such as investment interest expense, investment advisory fees (though deductibility of advisory fees was eliminated by TCJA for tax years 2018-2025), and allocable portions of state and local taxes. Second, the amount by which MAGI exceeds the threshold. For example, a single filer with $250,000 of MAGI and $70,000 of NII applies the 3.8% rate to the lesser of $70,000 (NII) or $50,000 (MAGI above the $200,000 threshold). The NIIT is $50,000 x 3.8% = $1,900. If NII were only $30,000, the NIIT would be $30,000 x 3.8% = $1,140 — because NII is the lesser amount. The calculation is completed on Form 8960.

Who Pays NIIT: The NIIT applies to U.S. citizens, resident aliens, and certain trusts and estates with investment income above the thresholds. Non-resident aliens are generally not subject to NIIT. Estates and trusts are subject to NIIT at a much lower threshold — the dollar amount at which the highest trust income tax bracket begins ($15,650 in 2025) — making NIIT nearly universal for non-grantor trusts with investment income. High-income investors subject to NIIT often encounter it for the first time in years when they realize large capital gains from a business sale, real estate transaction, or concentrated stock position sale. A year-end estimate of MAGI and NII, with specific attention to whether total MAGI will cross the applicable threshold, allows for proactive planning such as accelerating deductible expenses or harvesting losses to reduce net investment income below the threshold.

Planning Strategies: Several planning techniques can reduce or eliminate NIIT liability. Maximizing pre-tax 401(k), 403(b), HSA, and SEP IRA contributions reduces MAGI directly — every dollar contributed lowers the MAGI figure used to calculate the NIIT threshold excess. Converting passive rental activities to active participation (by qualifying as a real estate professional under IRS rules or materially participating in the activity) removes that income from the NIIT base entirely. Investing in tax-exempt municipal bonds produces interest income that is excluded from net investment income. For business owners, restructuring a passive investment in an S corporation to active participation can convert passive income (NIIT-subject) into non-passive income (exempt from NIIT). Installment sales spread large capital gains over multiple years, potentially keeping MAGI below the NIIT threshold in any single year. Consult a tax professional to identify which strategies apply most effectively to your income composition.