NIIT Net Investment Income Tax: The 3.8 Percent Surtax

Key Takeaways

• NIIT net investment income tax imposes a 3.8 percent federal surtax on the lesser of net investment income or the amount by which MAGI exceeds $200,000 single or $250,000 joint, thresholds not indexed for inflation since 2013.
• The tax pushes the true top federal rate on long-term capital gains to 23.8 percent and on short-term gains or interest to 40.8 percent before state tax; many investors budget only the headline rates and are surprised by the final bill.
• Non-grantor trusts face a much lower threshold than individuals (roughly $15,200 of undistributed net investment income in 2026), making trust distributions to beneficiaries an important planning tool.
• State income tax allocable to investment income is deductible on Form 8960 even when the SALT deduction is capped at $10,000 on Schedule A, these are separate computations and the Form 8960 deduction is routinely missed.

What It Is

Section 1411 imposes a tax equal to 3.8 percent of the smaller of two numbers. The first is net investment income for the year. The second is the amount by which modified adjusted gross income (MAGI) exceeds the statutory threshold: $200,000 single, $250,000 married filing jointly, $125,000 married filing separately. The thresholds are not indexed for inflation, so bracket creep has been steady since 2013.

Net investment income includes interest, dividends, capital gains, rental and royalty income, non-qualified annuity income, and income from passive businesses or trading in financial instruments. It does not include wages, active business income, qualified plan distributions, tax-exempt bond interest, or gain on the sale of an active business interest.

The Intuition

Most top-bracket U.S. investors overlook NIIT until they see the final tax bill, because it is layered on top of rates they already budgeted for. A taxpayer in the 20 percent long-term capital gain bracket actually faces 23.8 percent once NIIT is included. On short-term gains or ordinary interest, the same taxpayer runs 40.8 percent federal before state tax. The surtax also catches trust income above roughly $15,200 of undistributed earnings, because the trust threshold in §1411(a)(2)(B) is the top-bracket dollar amount, far lower than the individual thresholds.

The design is unusual in one respect. NIIT is not an investment-only tax. It is effectively a MAGI-gated surtax, so a taxpayer with large wages and modest investment income pays NIIT on all of the investment income, while a retiree with the same investment income and no wages may pay none.

How It Works

The tax is computed on Form 8960 in three stages. First, sum gross investment income. Second, subtract properly allocable expenses, including investment interest, state income tax attributable to investment income, and investment advisory fees (the 2017 Act did not suspend these at the §1411 level, only at the regular §67 level). Third, apply the lesser-of test against MAGI over threshold.

NIIT base = min( net investment income,
                 MAGI - threshold )

NIIT       = 0.038 * NIIT base

MAGI for this purpose is AGI with foreign earned income exclusion added back (§911). A retiree whose investment income exceeds the MAGI excess pays NIIT only on the smaller MAGI gap. A wage earner whose MAGI gap dwarfs investment income pays NIIT on the full investment income.

Treatment of trading gains follows §1411(c)(2) and Treasury Regulation 1.1411-5. A trader who does not elect §475 reports capital gains that remain investment income. A §475 trader has ordinary income but still inside the NIIT base, unless the activity rises to a trade or business and the taxpayer materially participates. That carve-out sits in §1411(c)(1)(A)(ii) and has been litigated repeatedly.

Worked Example

A married couple files jointly with MAGI of $400,000. That includes $280,000 of wages, $60,000 of qualified dividends, $40,000 of long-term capital gains, and $20,000 of interest. Investment expenses properly allocable are $5,000.

Net investment income: $60,000 plus $40,000 plus $20,000 minus $5,000 equals $115,000. MAGI excess over threshold: $400,000 minus $250,000 equals $150,000. NIIT base: the lesser, which is $115,000. NIIT: $115,000 multiplied by 3.8 percent equals $4,370.

Their total federal take on the $40,000 of long-term gain is 20 percent regular plus 3.8 percent NIIT, or $9,520 on that slice. Their interest and short-term items compound even more steeply at 40.8 percent.

Common Mistakes

1. Forgetting the trust threshold. A non-grantor trust pays NIIT once undistributed net investment income exceeds the top-bracket threshold (around $15,200 for 2026). Distributable net income carried out to beneficiaries shifts the tax base to them, which is often a planning win.

2. Mis-allocating state tax. State income tax attributable to investment income is deductible on Form 8960 even when it was capped out at the $10,000 SALT limit on Schedule A. The two computations are separate, and many preparers miss the §1411 deduction.

3. Assuming active business sale is exempt. Gain on the sale of an S corp or partnership interest is excluded from NIIT only to the extent attributable to assets used in a trade or business in which the owner materially participates. The "§1411 netting rule" requires a look-through calculation that often leaves part of the gain subject to NIIT.

4. Overlooking rental income character. Rental income is investment income unless the taxpayer is a real estate professional meeting §469(c)(7). The NIIT analysis and the passive-loss analysis are distinct, and taxpayers sometimes conflate the two.

5. Double-counting PTP distributions. Distributions from publicly traded partnerships that represent return of capital are not investment income for §1411 purposes, but brokers sometimes report gross distributions on 1099-DIV, leading to an inflated NIIT base.

Frequently Asked Questions

Q: What is the NIIT net investment income tax in simple terms? It is a 3.8 percent federal surtax applied to your investment income, dividends, interest, capital gains, and rental income, if your total income exceeds $200,000 single or $250,000 married. It is computed separately from regular income tax on Form 8960 and sits on top of the regular capital gains or ordinary income tax rate.

Q: How does NIIT affect investment decisions? It changes the true after-tax cost of realizing investment income at high income levels. Strategies like loss harvesting, QBI deductions, and active real estate participation have extra value because they reduce the MAGI or investment income base that the 3.8 percent surtax is applied to.

Q: What is a real-world example of NIIT calculation? A married couple has $400,000 MAGI including $115,000 of net investment income. MAGI exceeds the $250,000 threshold by $150,000. NIIT applies to the lesser of $115,000 (investment income) and $150,000 (MAGI excess), so the base is $115,000. NIIT owed: $115,000 multiplied by 3.8 percent equals $4,370.

Q: How can high-income investors reduce their NIIT exposure? Reduce MAGI below the threshold through retirement account contributions or business losses, shift portfolio income into Roth accounts where withdrawals are excluded from MAGI, qualify rental activities as a real estate professional to remove rental income from the NIIT base, and take eligible state income tax deductions on Form 8960 even when the SALT cap limits the Schedule A deduction.

Q: How is NIIT different from the regular capital gains tax? The regular long-term capital gains tax (0, 15, or 20 percent) applies based on taxable income brackets, independent of whether the taxpayer is above any MAGI threshold. NIIT is triggered only when MAGI exceeds the statutory threshold and is computed on the lesser of investment income or the MAGI excess. A retiree with $100,000 of dividends and low wages might pay the 15 percent capital gains rate but owe no NIIT, while a wage earner with the same dividends and high wages pays 3.8 percent NIIT on the full dividend amount.

Sources

1. Cornell Legal Information Institute. "26 U.S. Code Section 1411, Imposition of tax." https://www.law.cornell.edu/uscode/text/26/1411
2. Internal Revenue Service. "Topic No. 559, Net Investment Income Tax." https://www.irs.gov/taxtopics/tc559
3. Internal Revenue Service. "Instructions for Form 8960, Net Investment Income Tax." https://www.irs.gov/instructions/i8960
4. Joint Committee on Taxation. "Technical Explanation of the Revenue Provisions of the Reconciliation Act, JCX-18-10." https://www.jct.gov/publications/2010/jcx-18-10/

Disclaimer

This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.