Net Investment Income Tax Explained: The 3.8% Surtax Most High Earners Forget Until Filing Day

Background: IRC §1411 and the ACA Connection

The Net Investment Income Tax came from the Health Care and Education Reconciliation Act of 2010, the companion bill to the Affordable Care Act. It was codified as Internal Revenue Code §1411 and took effect for tax years beginning after December 31, 2012. The original idea was to fund Medicare expansion by adding a surtax on investment income earned by higher-income individuals, trusts, and estates.

The mechanics live in Treasury Regulation §1.1411, and the tax is reported on Form 8960. One thing that surprises people: the thresholds that trigger NIIT have never been indexed for inflation. The $200,000 single and $250,000 married-filing-jointly numbers from 2013 are still the same numbers in 2026. Wage growth and portfolio appreciation have done the rest of the work.

The 3.8% Rate — How It Actually Applies

The NIIT is a flat 3.8% surtax. It applies to the lesser of two amounts: your net investment income for the year, or the amount by which your modified adjusted gross income exceeds the applicable threshold.

That “lesser of” language matters. If your MAGI is $260,000 (married filing jointly) and your net investment income is $80,000, the tax applies to $10,000 — the amount you exceed the $250K threshold, not your full investment income. The bill is $380. Flip it: if your MAGI is $500,000 and your NII is $40,000, the tax applies to the full $40,000 because the threshold excess ($250K) is larger than NII. The bill is $1,520.

This is on top of your regular income tax, capital gains tax, and qualified dividend tax. For someone in the 20% long-term capital gains bracket, NIIT pushes the effective federal rate on those gains to 23.8%.

Income Thresholds: $200K Single, $250K MFJ, $125K MFS

The MAGI thresholds under IRC §1411(b) are:

• Single or Head of Household: $200,000
• Married Filing Jointly or Qualifying Surviving Spouse: $250,000
• Married Filing Separately: $125,000
• Trusts and Estates: the dollar amount at which the highest trust tax bracket begins (around $15,200 for 2026, indexed annually)

The trust threshold is the one that catches people off guard. A trust earning $20,000 of dividend income can owe NIIT on almost all of it, while a married couple earning the same amount owes nothing. This is why distributing trust income to beneficiaries before year-end is one of the most common NIIT planning moves we see.

MAGI for NIIT purposes is generally regular AGI plus certain foreign earned income exclusions. For most domestic taxpayers, MAGI equals AGI.

What Counts as Net Investment Income

Net investment income includes most income earned passively from capital, then reduced by allocable expenses. The main categories under §1411(c):

• Interest — taxable interest from bank accounts, corporate bonds, CDs, money market funds (municipal bond interest is excluded)
• Dividends — ordinary and qualified dividends from stocks, mutual funds, and ETFs
• Capital gains — short-term and long-term gains from stocks, bonds, mutual funds, and the sale of investment property (the portion of a home sale that exceeds the §121 exclusion also counts)
• Rental and royalty income — unless it rises to the level of a trade or business in which you materially participate
• Non-qualified annuity distributions — the earnings portion
• Income from passive activities — K-1 income from partnerships and S corporations where you do not materially participate

Allowable deductions include investment interest expense, state income tax allocable to investment income, and investment advisory fees — though the deductibility of advisory fees was significantly limited by the Tax Cuts and Jobs Act for regular tax and remains complicated for NIIT purposes. See IRS Publication 550 for the full treatment of investment income and expenses.

What's Exempt from NIIT

Plenty of income that feels like “investment-adjacent” actually escapes the surtax. The list of exclusions is just as important as the list of inclusions:

• Wages and self-employment income — already subject to Medicare tax and the 0.9% Additional Medicare Tax above the same thresholds, so Congress chose not to double up
• Distributions from qualified retirement plans — 401(k), 403(b), traditional IRA, Roth IRA, pension, and similar plans (this is huge for retirees with large IRAs)
• Active business income — income from a trade or business in which you materially participate, as long as the business is not trading in financial instruments
• Tax-exempt interest — municipal bond interest
• Gain on the sale of an active business interest — to the extent attributable to assets used in an active trade or business
• Section 121 home sale exclusion — the first $250K (single) or $500K (MFJ) of gain on a primary residence sale is excluded for both regular tax and NIIT
• Veterans’ benefits, Social Security, and unemployment compensation

The retirement plan exclusion is worth a second mention. A retired client pulling $150K from an IRA and earning $30K in dividends owes NIIT only on the dividends (and only if MAGI exceeds the threshold). The IRA distribution itself — even though it includes years of compounded investment growth — is not investment income for NIIT purposes.

Real Estate Professionals — The §469 Carve-Out

Rental income is one of the most common NIIT exposures and also one of the most negotiable. Under IRC §469(c)(7), a taxpayer who qualifies as a real estate professional and materially participates in the rental activity can treat that rental as non-passive. When the rental is non-passive, the income drops out of the NIIT base entirely.

To qualify as a real estate professional, you must satisfy two tests in the same year:

1. More than 50% of your personal services performed in any trade or business during the year are performed in real property trades or businesses in which you materially participate, and
2. You perform more than 750 hours of services during the year in real property trades or businesses in which you materially participate.

For married couples, only one spouse needs to meet both tests. The hours of the other spouse can count toward material participation of specific properties once the threshold spouse qualifies as a real estate professional overall.

This is not a casual designation. The IRS audits real estate professional claims aggressively, and the Tax Court has thrown out the status for taxpayers who couldn’t produce contemporaneous time logs. If you are claiming it, keep a calendar. We work with several real estate agents and business owners who have real estate portfolios where this status saves $20K-$50K per year in combined NIIT and ordinary tax.

Material Participation Test for Pass-Through Businesses

If you own an S corporation, partnership, or LLC interest, your K-1 income is either passive (subject to NIIT) or non-passive (excluded). The dividing line is material participation, governed by Treas. Reg. §1.469-5T. You satisfy material participation if you meet any one of seven tests, the most common being:

• You participate more than 500 hours in the activity during the year
• Your participation constitutes substantially all of the participation in the activity by everyone involved
• You participate more than 100 hours and no one else participates more than you
• You materially participated in the activity for any 5 of the prior 10 years

For a closely held S corp where the owner runs the business day-to-day, material participation is obvious and the K-1 income is non-passive. For a silent investor in someone else’s LLC, the income is passive and full NIIT applies. The interesting cases are in the middle — a part-owner who occasionally works in the business, an inherited interest, or an absentee owner who still signs checks.

Two notes that matter. First, distributions from S corporations that represent return of basis are not investment income. Second, the gain on the sale of an S corp or partnership interest is subject to NIIT only to the extent of the gain attributable to passive assets, not active business assets. The Form 8960 instructions walk through the working interest and active business asset adjustments.

Form 8960 Computation Walkthrough

Form 8960 has three sections. Section 1 is investment income (lines 1–8). Section 2 is investment expenses and adjustments (lines 9–11). Section 3 is the tax computation (lines 12–17).

Walking through a sample for a married couple with $300K MAGI, $40K of dividends, $20K of capital gains, and $5K of state tax allocable to investment income:

• Line 1 – Taxable interest: $0
• Line 2 – Ordinary dividends: $40,000
• Line 5a – Net capital gain: $20,000
• Line 8 – Total investment income: $60,000
• Line 9c – State income tax allocation: $5,000
• Line 11 – Total deductions: $5,000
• Line 12 – Net investment income: $55,000
• Line 13 – MAGI: $300,000
• Line 14 – Threshold: $250,000
• Line 15 – MAGI over threshold: $50,000
• Line 16 – Lesser of line 12 or line 15: $50,000
• Line 17 – NIIT (3.8% × $50,000): $1,900

That $1,900 flows to Schedule 2, line 12 of Form 1040 and is added to the total tax.

One trap to know: IRS Notice 2014-7 excludes certain Medicaid waiver payments from gross income for both regular tax and NIIT. It comes up rarely, but for clients with disabled family members who receive home-care provider income, it can be relevant.