NIIT and Capital Gains: When the 3.8% Kicks In

A married couple with $300,000 of modified adjusted gross income who sells stock for a $100,000 long-term gain pays the 3.8% net investment income tax (NIIT) on every dollar of that gain — even though their income sits roughly $300,000 below the threshold where the 20% long-term capital gains (LTCG) rate begins. That gap is the entire story of this surtax. The 3.8% kicks in at income levels that, for most $150k+ households, are reached long before the top federal rate on gains does.

The NIIT thresholds have not moved since the tax took effect in 2013. The Internal Revenue Service confirms the trigger points in Topic 559: modified adjusted gross income above $200,000 for single filers, $250,000 for married filing jointly, and $125,000 for married filing separately. No inflation adjustment. Ameriprise, citing IRS data, reports the number of taxpayers hit by the surtax climbed from 3.1 million in 2013 to 7.3 million by 2021 — pure bracket creep, since wages rose and the threshold did not.

The numbers that matter

Source: IRS Topic 559 (NIIT thresholds); IRS Rev. Proc. 2024-40 §3.03 (2025 LTCG brackets), accessed June 2026.

Why the surtax bites before the top rate does

Two separate threshold systems govern what a high earner pays on a gain, and they are badly misaligned. The 0%, 15%, and 20% LTCG brackets key off taxable income and adjust for inflation every year. The 3.8% surtax keys off MAGI and has been frozen at its 2013 level for over a decade. The result is a wide income band — roughly $200,000 to $533,400 for a single filer in 2025 — where a household pays the 15% federal rate on gains plus the 3.8% surtax, for a combined 18.8% federal cost, while never touching the 20% bracket.

Consider the layering. For 2025, IRS Rev. Proc. 2024-40 sets the 0% LTCG rate for taxable income up to $48,350 single and $96,700 married filing jointly, the 15% rate up to $533,400 single and $600,050 married filing jointly, and the 20% rate above those points. Set the NIIT thresholds beside them and the mismatch is stark: a joint filer crosses into NIIT territory at $250,000 of MAGI but does not reach the 20% LTCG rate until $600,050 of taxable income. The surtax arrives first, and for a $150k+ household it usually arrives the moment a meaningful gain is realized.

Source: IRS Rev. Proc. 2024-40 §3.03 (LTCG thresholds); IRS Topic 559 (NIIT). NIIT applies to the lesser of net investment income or MAGI over threshold; the combined rate assumes the gain falls entirely within the band. Accessed June 2026.

One nuance the table compresses: the 3.8% does not apply to the whole gain in every case. The IRS levies NIIT on the lesser of net investment income or the amount by which MAGI exceeds the threshold. A joint filer at $260,000 MAGI with $100,000 of investment income owes the surtax only on the $10,000 of excess MAGI — not the full $100,000. Cross far enough above the threshold and the calculation flips, applying 3.8% to the entire net investment income figure. The deeper mechanics of where each gain lands sit inside the broader capital gains tax guide for $150k+ investors.

What the gain actually costs: three scenarios

Abstract rates obscure the dollars. Three households, each realizing a long-term gain in 2025, show how the surtax and state tax stack on top of the base federal rate. Each assumes the gain falls fully within the stated band and that MAGI exceeds the relevant threshold by more than the investment income, so the full 3.8% applies.

Scenario one: a joint-filing couple in a no-income-tax state (Texas, Florida, Washington on non-gain income, and six others, per Tax Foundation 2025 data) with $350,000 MAGI sells stock bought at a cost basis of $100,000 for $250,000 — a $150,000 gain. They sit in the 15% LTCG band and above the $250,000 NIIT threshold, so federal cost is 18.8%. No state tax on the gain. Total tax: $28,200. Net gain: $121,800.

Scenario two: same numbers, but the couple lives in California, which Tax Foundation data confirms taxes capital gains as ordinary income with a top marginal rate of 13.3%. At this income level their California marginal rate runs near 9.3%. Federal 18.8% plus state 9.3% gives a combined 28.1% on the gain. Total tax: $42,150. Net gain: $107,850.

Scenario three: a high earner above $600,050 taxable income in California, in the 20% federal LTCG band, facing the state’s top 13.3% marginal rate on the gain. Federal 23.8% plus state 13.3% equals 37.1%. On the same $150,000 gain, total tax: $55,650. Net gain: $94,350. The reason California sits at the extreme of every gains scenario gets a fuller treatment in the breakdown of California’s highest-in-nation gains tax.

Source: IRS Topic 559 and Rev. Proc. 2024-40; Tax Foundation, state individual income tax data as of January 2025. State rates are marginal and illustrative; actual California liability depends on total taxable income across the state’s progressive brackets. Finluxy After-Tax Gain Rate = net gain ÷ cost basis × 100.

The Finluxy After-Tax Gain Rate

Pre-tax gain rate flatters every brokerage statement. The Finluxy After-Tax Gain Rate strips the illusion: net gain after all applicable taxes (federal LTCG + NIIT + state) divided by cost basis, expressed as a percentage, set against the pre-tax figure. On a $100,000 cost basis returning a $150,000 gain, the pre-tax gain rate is 150%. The three scenarios above land at 121.8%, 107.9%, and 94.4% respectively.

The spread between those numbers is the tax haircut — pre-tax gain rate minus after-tax gain rate. In the no-tax state, the haircut is 28.2 points. In high-income California, it is 55.6 points. That is more than a third of the gain surrendered, and the single largest driver of the difference between the second and third rows is not the federal 20% rate but California’s progressive state tax climbing toward its ceiling. The 3.8% surtax, by contrast, is a constant 3.8-point drag across all three — small per dollar, but applied to a band of income most $150k+ households now occupy by default.

What most coverage misses

Nearly every explainer frames NIIT as a tax on “high earners,” which invites the assumption that it travels with the 20% LTCG rate. It does not. The data shows the surtax operating almost entirely inside the 15% federal band for $150k+ households. A single filer realizing gains between $200,000 and $533,400 of taxable income — a range that captures most upper-middle and mass-affluent investors selling a meaningful position — pays 15% plus 3.8%, never the 20% rate. The surtax is functionally a 15%-band phenomenon for this group, not a top-bracket one, and planning that treats it as a problem only for the very wealthy mistimes the exposure by hundreds of thousands of dollars.

The frozen threshold compounds this. Because the $200,000 and $250,000 trigger points have not moved since 2013, a 2025 household earning the inflation-adjusted equivalent of a 2013 income that comfortably cleared the surtax now finds itself well inside it. The tax was designed to hit a narrow band of high earners; thirteen years of unindexed thresholds turned it into a near-default cost for anyone realizing investment income above a six-figure salary. The interaction between a one-time gain and these static thresholds is exactly what makes the tax cost of selling early worth modeling before any sale.

Managing the threshold: levers and limits

The surtax is mechanical, which means a handful of inputs control it. Because NIIT applies to the lesser of net investment income or MAGI-over-threshold, two distinct levers exist: reduce MAGI, or reduce net investment income in the year of a large realization.

Spreading a large gain across two tax years can keep MAGI nearer the threshold in each, shrinking the excess-MAGI figure the 3.8% applies to. Realizing gains in a year when wage or business income dips does the same. Tax-loss harvesting reduces net investment income directly — harvested losses offset realized gains dollar for dollar before the surtax is computed, subject to the wash sale rule, which disallows the loss if a substantially identical security is repurchased within 30 days. For households deciding whether a position has crossed from short-term to long-term status, the short-term versus long-term cost gap often dwarfs the surtax: short-term capital gains (STCG) are taxed at ordinary rates up to 37%, and the 3.8% stacks on those too.

State residency is the largest single lever and the least flexible. The difference between scenario one and scenario three above is almost entirely a state-tax artifact. A household weighing a major liquidity event — a business sale, a concentrated stock position — finds the state line worth more than any federal maneuver, which is why the full state-by-state gains comparison matters before timing a sale. Primary-residence gains carry their own carve-out under the home sale capital gains exclusion, which removes up to $250,000 ($500,000 joint) from both the LTCG calculation and the NIIT base.

For the $150k+ household

The practical takeaway for households in this income band is that the 3.8% surtax is not a distant top-bracket concern — it is the baseline federal add-on on nearly any realized gain once a six-figure salary is in place. A dual-income couple at $250,000 of combined wages crosses the NIIT threshold before they sell a single share; every dollar of net investment income from that point carries the surtax. The decision that moves the most money is rarely whether to accept the 3.8% — it is usually unavoidable — but how to keep a large one-time gain from stacking into both a higher LTCG band and a higher state bracket in the same year. Splitting a realization across tax years, harvesting offsetting losses within the wash sale constraint, and weighing the state in which a major sale is recognized each shift the after-tax outcome by tens of thousands of dollars on a six-figure gain, as the scenario table shows. For households contemplating a concentrated-position sale or a business exit, the gap between a 121.8% and a 94.4% after-tax gain rate on identical pre-tax economics is large enough that the modeling earns its keep, and complex events of that size are where a tax professional who can run your specific bracket math pays for itself.