NIIT: Who Pays the 3.8% Investment Income Tax

A household earning $300,000 with $80,000 in dividends and capital gains doesn’t just face a federal income tax bill — it faces an additional $3,040 from a tax most people underestimate until they see their Form 1040. The net investment income tax (NIIT) — a 3.8% surtax introduced by the Health Care and Education Reconciliation Act of 2010 and effective since January 2013 — applies on top of regular federal income tax and capital gains tax. It doesn’t replace either. It stacks.

For households in the $150k–$500k+ income range, NIIT is one of the most consequential line items that rarely gets modeled until the tax return is done. The thresholds have never been indexed for inflation. That single design feature has pulled millions of additional taxpayers into scope since 2013 — and will continue to do so as incomes rise.

Key Numbers at a Glance

Sources: IRS TC559, IRS Publication 505 (April 2026), IRS Statistics of Income via CRS report IF11820, IRS Revenue Procedure 2025-32.

What NIIT Is — and What It Isn’t

The net investment income tax is a 3.8% surtax on the lesser of a taxpayer’s net investment income (NII) or the amount by which their modified adjusted gross income (MAGI) exceeds the applicable threshold. That lesser-of rule matters enormously in practice, and many high earners miscalculate their exposure by applying 3.8% to their entire investment portfolio without running the comparison.

MAGI, for most taxpayers without foreign earned income, equals adjusted gross income (AGI). The IRS defines what counts as net investment income on Form 8960. Included: interest, dividends, capital gains from stocks and mutual funds, rental and royalty income, income from passive business activities, and gains from the sale of investment real estate. The FICA and Medicare surtax structure is a separate layer — NIIT is not a payroll tax and has no wage base ceiling.

Excluded from NII: wages, salaries, bonuses, Social Security benefits, unemployment compensation, distributions from qualified retirement plans such as 401(k)s and 403(b)s, tax-exempt interest (municipal bonds), and income from businesses in which the taxpayer materially participates. That last exclusion is significant — an S corporation owner or limited partnership investor who qualifies as materially participating keeps that business income outside the NII calculation.

The home sale exclusion also applies. Gain from a primary residence sale excluded under IRC §121 — up to $250,000 for single filers or $500,000 for married filing jointly — is excluded from NII as well. Only the portion of gain exceeding the §121 exclusion enters the NIIT calculation.

Who Actually Gets Hit

IRS Statistics of Income data, as analyzed by the Congressional Research Service, show that 7.3 million taxpayers paid NIIT in tax year 2021 — more than double the 3.1 million subject to it in 2013, the first year the tax was in effect. That growth isn’t primarily a function of people becoming wealthier. It’s a function of fixed thresholds in an inflationary environment. The $200,000 single and $250,000 MFJ thresholds have never been indexed. In 2013 dollars, a $200,000 income threshold would be worth approximately $265,000 today — meaning the real threshold has declined by roughly a quarter over a decade.

The taxpayer concentration tells a clear story. According to the same CRS data, those earning between $200,000 and $500,000 accounted for 69.6% of all taxpayers subject to NIIT in 2019. The bulk of the population paying this tax is not the ultra-wealthy — it is the household earning $225,000 in W-2 income who also collected $40,000 in dividends from a taxable brokerage account. That’s exactly the $150k+ household this analysis is built around.

What the data also shows — and most coverage misses — is that a high-income household can face NIIT exposure even when its investment income is modest, if wage income pushes MAGI well above the threshold. A single filer earning $280,000 in salary with $25,000 in capital gains owes NIIT not on $25,000, but on the lesser of $25,000 (NII) or $80,000 (the MAGI excess above $200,000). The lesser figure here is $25,000, so the NIIT bill is $950. Wages do not create NIIT liability directly, but they determine how much of the threshold headroom is consumed — leaving investment income fully exposed.

The Threshold Mechanics: Four Scenarios

Applying the lesser-of rule across realistic household income profiles shows how differently NIIT lands depending on income mix. The table below models four household types using 2026 parameters. All figures assume MAGI equals AGI (no foreign income), and no above-the-line deductions are applied to MAGI for NIIT purposes — they would reduce AGI and MAGI equally in these scenarios.

Source: Finluxy calculations based on IRS Publication 505 (2026) NIIT mechanics. NIIT = 3.8% × lesser of NII or MAGI excess above IRS threshold. Thresholds: $200,000 single; $250,000 MFJ. Assumes MAGI = AGI.

Scenario 3 is the counterintuitive one. A married couple with modest W-2 income and a large one-time capital gain — say, from selling a vacation property or a business stake — hits the NIIT threshold hard. Their MAGI excess ($100,000) is less than their total NII ($150,000), so only $100,000 faces the 3.8% rate. But that’s still $3,800 that arrives on top of the regular capital gains tax. For a full breakdown of how these $300k household income tax rates vary by state, the total picture is materially different from the federal-only figure.

How NIIT Stacks on Capital Gains

Long-term capital gains are taxed at preferential federal rates — 0%, 15%, or 20% depending on taxable income — and NIIT adds a fixed 3.8% on top of whichever rate applies. For a household in the 15% long-term gains bracket, the combined federal rate on those gains becomes 18.8%. For a household in the 20% bracket, it becomes 23.8%. Short-term gains, taxed as ordinary income at rates up to 37%, face a combined rate of up to 40.8% when NIIT applies.

The 2026 long-term capital gains rate thresholds, as released by the IRS under Revenue Procedure 2025-32, place the 20% rate at income above approximately $545,500 for single filers and $613,700 for married filing jointly (Tax Foundation, 2026). Below those levels — which covers the substantial majority of households in the $150k–$500k range — the marginal capital gains rate is 15%. That means NIIT adds roughly a quarter of additional tax burden on top of the base capital gains rate for this income segment. Understanding the relationship between your effective and marginal tax rate is essential before modeling any large asset sale.

Qualified dividends follow the same rate schedule as long-term capital gains. Non-qualified dividends (from most bond funds, REITs, and short-term holdings) are taxed as ordinary income and also subject to NIIT if thresholds are crossed. For a $250k MFJ household with $40,000 in qualified dividends, NIIT adds $1,520 to a tax bill that already includes $6,000 in capital gains tax at the 15% rate — a 25.3% increase in the total bill on that income alone.

Rental Income and Passive Business Income

Rental income is passive income by IRS default, which places it squarely inside NII. A rental property producing $36,000 in net rental income annually adds to both MAGI and NII. If that income pushes a single filer’s MAGI from $185,000 to $221,000, the NIIT applies to the lesser of $36,000 (NII) or $21,000 (MAGI excess). The bill: $798. Not catastrophic — but a persistent annual cost that compounds across a multi-property portfolio. The interaction between state income tax and your federal math adds another layer on top of that figure in high-rate states.

Material participation is the escape hatch. Taxpayers who meet the IRS material participation tests — most commonly, spending more than 500 hours per year in the activity — can reclassify business income from passive to active. Active income is excluded from NII. This is why real estate professionals who qualify under IRC §469 often have structurally lower NIIT exposure than passive investors holding comparable assets. The qualification threshold is strict: it requires both hours-tracking documentation and a formal determination that the real estate professional’s primary occupation is real estate activities. A high-earning physician who owns two rental properties and spends 120 hours managing them does not meet the standard.

Finluxy Effective Total Tax Rate: Three Household Models

The Finluxy Effective Total Tax Rate captures total federal income tax + FICA (Federal Insurance Contributions Act) + NIIT + state income tax as a percentage of gross household income. It is the all-in rate — not just the federal income tax rate, not just the marginal tax rate. Models below assume wage income only for FICA purposes, California state income tax at a blended effective rate of approximately 9.3% for the $300k MFJ scenario (Tax Foundation, 2026 state data), no state income tax for Texas/Florida/Nevada, and standard deduction applied at 2026 IRS figures ($16,100 single; $32,200 MFJ). Pre-tax contributions are not deducted to show gross exposure before planning.

Sources: IRS Revenue Procedure 2025-32 (2026 brackets, standard deductions); IRS TC751 (SS wage base $184,500; Medicare rates); IRS Publication 505 (2026) (Additional Medicare Tax thresholds; NIIT thresholds); Tax Foundation (2026 state income tax rates, California). Federal income tax estimates calculated by applying 2026 brackets progressively to taxable income. California effective rates are approximations based on Tax Foundation marginal rate data; actual figures depend on California-specific deductions and credits. The Finluxy Effective Total Tax Rate equals total federal income tax + FICA (employee share) + NIIT + state income tax ÷ gross household income × 100.

The California single filer at $250,000 faces a Finluxy Effective Total Tax Rate of 37.9% before any pre-tax planning. Moving to Texas or Florida saves the same MFJ $300k household 8.7 percentage points — roughly $26,100 annually in state tax that simply doesn’t exist. For a full breakdown of how dual income at $300k changes the tax math, the picture shifts substantially when two earners each stay below the $250k MFJ NIIT threshold individually but exceed it jointly.

The Overlooked Mechanic: How Wage Income Consumes Threshold Headroom

Standard coverage of NIIT focuses on whether a household “has enough investment income to be subject to the tax.” That framing misses the more important dynamic: the MAGI threshold operates as a combined trigger, not a separate investment-income trigger. Every dollar of wages and salary counts toward MAGI. A household with $230,000 in wages and zero investment income has $30,000 in threshold overhang sitting ready to apply 3.8% to the first $30,000 of NII it accumulates — whether from a brokerage account dividend, a rental property, or a single stock sale.

This means W-2 households at the $200k–$300k income range cannot plan around NIIT the way a lower-income investor might — by simply ensuring their investment income stays small. Their wages have already consumed the threshold. For earners between $150k and $500k, NIIT exposure on investment income is effectively automatic once wages clear the threshold. The tax-planning question shifts from “will I owe NIIT?” to “how much NII can I generate before I hit the lesser-of cap?”

Reduction Strategies That Actually Move the Number

Three mechanics genuinely reduce NIIT exposure. Each works differently depending on income profile.

Pre-tax retirement contributions reduce MAGI. A 401(k) contribution reduces AGI directly. If a single filer earning $210,000 contributes the 2026 maximum (confirm annually at IRS.gov — the 2025 limit was $23,500), AGI drops below $200,000, potentially eliminating NIIT exposure entirely if investment income is modest. The mechanics of how pre-tax deductions reduce your tax bill apply directly here — NIIT exposure shrinks with every dollar of above-the-line reduction. HSA contributions work the same way.

Tax-exempt interest is excluded from both AGI and NII. Municipal bond interest doesn’t appear in MAGI and doesn’t appear in net investment income. For a high-income household already above the NIIT threshold on wages alone, shifting taxable bond interest to municipal bonds reduces NII directly. The trade-off is yield — munis typically carry lower pre-tax yields — so the math requires comparing after-tax yields at your specific effective rate.

Tax-loss harvesting reduces NII in the year of realization. Capital gains are included in NII; capital losses offset them. A household holding a $50,000 unrealized gain and a $20,000 unrealized loss can sell both in the same tax year, reducing NII to $30,000 from $50,000 — saving $760 in NIIT. The federal tax bracket effect and NIIT effect compound here: the same loss offsets ordinary-rate short-term gains or preferential-rate long-term gains depending on the character of the positions sold. This intersects directly with AMT exposure at the $300k–$750k range — the alternative minimum tax — since preference items and adjustments interact with tax-loss timing strategies.

Practical Context for the $150k+ Household

A household earning $175,000 in wages is below the NIIT threshold and owes nothing. At $210,000 in wages with a $25,000 brokerage account yielding dividends and short-term gains, the NIIT bill is $380. These are not catastrophic sums. But NIIT interacts with every other tax layer, and the compounding matters most in years with large one-time income events — a business sale, a real estate disposition, an equity compensation payout.

A $500k+ income household faces NIIT almost certainly on all investment income, since wages alone push MAGI well above either threshold. At that level, the question is portfolio structure: how much of the portfolio generates NII in taxable accounts versus tax-deferred accounts, and whether passive income sources can be converted to active income through material participation. For a full-layer view of what a $500k income tax rate looks like with every component broken out, NIIT typically contributes 1–2 percentage points to the all-in effective rate depending on investment income mix.

For married filing jointly households near the $250,000 threshold, the marginal decision to generate an additional $10,000 of investment income carries a 3.8% federal surtax in addition to whatever ordinary or preferential rate applies. On qualified dividends at 15%, that’s an effective 18.8% federal rate on incremental investment income — well above what many households model when projecting after-tax returns. Anyone modeling retirement drawdown, Roth conversion ladders, or taxable account withdrawal sequencing should incorporate the NIIT exposure threshold before finalizing those projections. The tax gap between married filing jointly and single is also relevant here — MFS filers face the harshest NIIT threshold at $125,000, making filing status selection a NIIT-material decision for couples with divergent income levels. Understanding your total exposure across every federal layer starts with seeing where the $200k income tax bill sits before adding state and investment income surtaxes.