The Net Investment Income Tax: Who Owes the Extra 3.8%, and on What

Since 2013, certain individuals, estates, and trusts with investment income above set thresholds have owed an additional 3.8% federal tax on top of their regular income tax bill. Known as the Net Investment Income Tax (NIIT), this surtax was enacted as part of the Health Care and Education Reconciliation Act of 2010 to help fund the Affordable Care Act. Despite being over a decade old, it remains widely misunderstood, partly because the income thresholds that trigger it have never changed.

Who Is Subject to the NIIT

The NIIT applies to individuals whose Modified Adjusted Gross Income (MAGI) exceeds specific statutory thresholds: $200,000 for single filers and heads of household, $250,000 for married couples filing jointly, and $125,000 for married individuals filing separately, according to the IRS. Unlike ordinary tax brackets, these figures are fixed in the tax code and are not indexed for inflation, meaning a growing number of taxpayers can become subject to the tax simply because wages and investment portfolios rise over time. Estates and trusts face NIIT as well, though their threshold is far lower and tied to the top trust tax bracket, adjusted annually.

What Counts as Net Investment Income

The IRS defines net investment income broadly. It generally includes interest, dividends, capital gains, rental and royalty income, non-qualified annuity income, and income from businesses considered passive activities for the taxpayer. Certain gains from the sale of a partnership interest or S-corporation stock can also be included, depending on how the underlying business income was taxed.

• Interest, dividends, and capital gains from taxable brokerage accounts
• Rental and royalty income (unless earned in the ordinary course of a non-passive trade or business)
• Income from passive business activities or trading in financial instruments and commodities
• Net gains from the sale of investment property, including a portion of gain on the sale of a business interest in some cases

What Is Generally Excluded

Wages, self-employment income, unemployment compensation, Social Security benefits, and most distributions from qualified retirement plans such as 401(k)s and IRAs are excluded from net investment income, per IRS guidance. Income from a trade or business in which the taxpayer materially participates is also generally excluded, which is why the distinction between active and passive involvement matters so much for business owners and real estate investors.

How the Tax Is Actually Calculated

The NIIT is not simply 3.8% of all investment income. Instead, it applies to the lesser of two amounts: the taxpayer's total net investment income, or the amount by which MAGI exceeds the applicable threshold. This distinction matters. A taxpayer with $260,000 in MAGI, of which $80,000 is investment income, and who files jointly (threshold $250,000), would owe 3.8% on $10,000 — the excess over the threshold — not the full $80,000, because the excess is the smaller figure. Taxpayers calculate and report this on IRS Form 8960, filed alongside Form 1040.

Why It Matters for Planning

Because the thresholds are fixed rather than inflation-adjusted, taxpayers who once sat comfortably below the line can be pushed over it through ordinary wage growth, bonus income, or a single large capital gain, such as the sale of a business, investment property, or concentrated stock position. Timing income recognition, harvesting losses, using tax-advantaged retirement accounts, and considering the tax treatment of rental real estate activity are all areas where the NIIT can influence decisions. Charitable giving strategies, including donating appreciated securities, can also reduce recognized capital gains that would otherwise count toward net investment income.

A Note on Scope

The NIIT is separate from the Additional Medicare Tax, a related 0.9% surtax on wages and self-employment income above similar thresholds, and the two are sometimes confused. They apply to different categories of income and are calculated on different IRS forms, though both stem from the same 2010 legislation and share the goal of raising revenue from higher earners.