Households earning well above the median but far below the ultra-wealthy bracket are increasingly paying a tax that Congress originally aimed at high earners. The Net Investment Income Tax, set at 3.8 percent, kicks in once modified adjusted gross income tops $200,000 for single filers or $250,000 for joint filers. Those dollar thresholds have not moved since the tax took effect on January 1, 2013, and the IRS has confirmed they carry no inflation adjustment. Thirteen years of rising prices have quietly expanded the tax’s reach, pulling in filers whose real purchasing power has barely changed.
Frozen thresholds and rising prices create a widening mismatch
The statutory text of Section 1411 hard-codes three threshold amounts: $250,000 for joint filers or surviving spouses, $200,000 for other individuals, and $125,000 for married taxpayers filing separately. Unlike the standard deduction or ordinary income tax brackets, which the IRS recalibrates each year using chained CPI, these NIIT thresholds sit in the statute with no indexing mechanism at all.
The IRS itself states plainly in its Q&A guidance that “these threshold amounts are not indexed for inflation.” That single sentence explains why the tax now reaches deeper into the income distribution than it did in 2013. The Congressional Budget Office has separately identified the NIIT thresholds of $200,000 and $250,000 as textbook examples of nominal-dollar tax parameters that lose real value over time, according to a CBO discussion of inflation’s effects on the federal tax code.
The practical result is straightforward. A joint-filing couple needed roughly $250,000 in modified adjusted gross income to trigger the surtax in 2013. If that threshold had been indexed to consumer prices, it would be meaningfully higher by mid-2026. Instead, the line remains at $250,000, so a household whose nominal income grew only enough to keep pace with inflation can now owe a tax it would not have faced when the law was written. The same dynamic applies to single filers at the $200,000 mark and to married-filing-separately returns at $125,000.
How the 3.8 percent calculation works on Form 8960
The tax itself is calculated on IRS Form 8960 and applies at a flat 3.8 percent. It does not hit all investment income, only the lesser of two amounts: total net investment income for the year, or the excess of modified adjusted gross income above the applicable threshold. That formula, spelled out in Treasury regulations, means a filer whose MAGI barely exceeds the threshold pays the surtax on just the overshoot, not on the full investment portfolio.
Net investment income for NIIT purposes includes interest, dividends, capital gains, rental and royalty income, and income from passive business activities, reduced by properly allocable deductions. It generally does not include wages, self-employment earnings, Social Security benefits, or distributions from qualified retirement plans. The tax applies once overall modified adjusted gross income crosses the fixed dollar threshold and there is at least some net investment income in the base.
Consider a married couple filing jointly with $260,000 of modified adjusted gross income, of which $40,000 is net investment income. Their excess MAGI over the $250,000 threshold is $10,000. Because the NIIT base is the lesser of that $10,000 excess or the $40,000 of investment income, only $10,000 is subject to the 3.8 percent rate, resulting in a $380 surtax. If the same couple instead had $20,000 of excess MAGI and only $5,000 of net investment income, the tax would apply to $5,000, producing a $190 liability.
The IRS summarizes these mechanics and the filing requirements on its NIIT overview, which also notes that the tax can affect individuals, estates, and trusts. For individuals, the key triggers are crossing the statutory income threshold and having positive net investment income; for estates and trusts, a much lower threshold applies, so even modest investment portfolios inside a trust can face the 3.8 percent levy.
Bracket creep without a bracket
Because the NIIT thresholds are set in statute, they will remain fixed until Congress acts to change them. In the meantime, rising nominal incomes driven by inflation will keep pulling additional households into the surtax, even if their real after-tax standard of living does not improve. That pattern mirrors traditional “bracket creep” in an unindexed income tax, but here it operates through a single static dollar line rather than through multiple rate brackets.
For taxpayers near the thresholds, the implications are twofold. First, investment decisions that significantly increase interest, dividends, or realized capital gains can push modified adjusted gross income above the line, triggering the surtax for the first time. Second, even without portfolio changes, regular cost-of-living raises can gradually move a household from just under to just over the fixed amount. The NIIT therefore functions as an additional marginal tax that appears only after inflation has done its quiet work.
Absent legislative revisions, that pattern is likely to persist. The 3.8 percent rate may look modest on paper, but when layered on top of existing income and capital gains taxes, it raises the effective burden on investment income for a growing slice of upper-middle-income filers whose real earnings have not kept pace with the tax code’s frozen thresholds.
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David M. Keller is a finance writer based in Columbus, Ohio, covering personal finance and consumer-focused economic topics. He earned his degree in journalism from Ohio University and began his career reporting on local business and economic trends for a regional media outlet. Since then, he has contributed to a variety of online publications, focusing on clear, practical coverage of topics such as cost of living, debt, and everyday financial decision-making.
