Net Investment Income Tax (NIIT) 2026: The Hidden 3.8% That Makes Tax Loss Harvesting More Valuable
The net investment income tax (NIIT) is a 3.8% federal surtax on investment income — including capital gains, dividends, interest, rental income, royalties, and passive business income — that applies when your modified adjusted gross income (MAGI) exceeds $200,000 for single filers or $250,000 for married couples filing jointly. These thresholds have not been adjusted for inflation since the tax took effect on January 1, 2013, which means more households cross them every year as wages and portfolio values rise. For a married couple with $300,000 in MAGI and $80,000 of net investment income, the NIIT adds $1,900 on top of their regular capital gains bill. That number gets larger in proportion to income, and for high earners with large portfolios, it pushes the effective rate on long-term capital gains from 20% to 23.8% — which has a quiet but significant effect on what tax loss harvesting is actually worth.
Most articles about the net investment income tax NIIT 2026 treat it as a compliance topic: a tax to calculate, a line on a form, a thing to be aware of. That framing misses the more interesting point. Because the NIIT stacks on top of the regular capital gains tax, every dollar of harvested losses a high-income investor generates is worth more than a retail calculator would suggest. A $10,000 loss is not worth $1,500 at a 15% rate — it's worth $2,380 at 23.8%. Over a decade of continuous harvesting, that multiplier compounds into real money. This article explains the mechanics, walks through three calculations, and shows why the NIIT is the reason high-income investors should treat loss harvesting less as an annual ritual and more as a continuous process.
What Is the Net Investment Income Tax?
The NIIT was created by Section 1411 of the Internal Revenue Code, enacted as part of the Health Care and Education Reconciliation Act of 2010, and took effect on January 1, 2013. It was designed to extend a Medicare-style surtax to investment income in the same way that payroll taxes already applied to wages — the underlying theory was that wage earners paid Medicare tax on their labor income, so high earners with significant investment income should contribute similarly on their portfolio income.
The tax is 3.8% and it applies to the lesser of two numbers: your net investment income, or the amount by which your MAGI exceeds the filing status threshold. This "lesser of" structure matters. It means the NIIT is always capped by whichever of those two quantities is smaller, and it creates some useful planning opportunities — shrink either number and you shrink the tax.
Net investment income includes interest, dividends, capital gains (both short and long-term), nonqualified annuity income, rental income, royalties, and income from passive business activities. It does not include wages, self-employment income, Social Security benefits, distributions from qualified retirement plans like 401(k)s and traditional IRAs, tax-exempt municipal bond interest, gain from the sale of a primary residence that falls within the Section 121 exclusion, or operating income from a business you materially participate in. The exclusion of retirement plan distributions is especially worth noting — it means a portfolio held inside a 401(k) is structurally immune to the NIIT, while the same portfolio held in a taxable brokerage account is not.
NIIT Thresholds and Who Pays It in 2026
The 2026 thresholds are the same as they were in 2013:
- Single or Head of Household: $200,000
- Married Filing Jointly or Qualifying Widow(er): $250,000
- Married Filing Separately: $125,000
For estates and trusts the calculation works differently and the threshold is much lower — $16,000 for 2026 — which is why trust-level NIIT planning is a separate topic worth its own article.
The thresholds for individuals have sat at these exact levels for thirteen years. Nothing in the One Big Beautiful Bill Act of July 2025 modified the NIIT's rate or thresholds. Meanwhile, the U.S. median household income has risen substantially and the stock market has roughly quadrupled from its 2013 level. The result is straightforward arithmetic: more households have crossed the line every year. According to data cited by Ameriprise Financial, the number of taxpayers affected by the NIIT grew from 3.1 million in 2013 to 7.3 million in 2021, and the total tax collected rose from $16.5 billion to $59.8 billion over the same period. That is a more than doubling of affected households in eight years, driven almost entirely by the thresholds' failure to adjust for inflation.
In 2013, a $250,000 household income placed a family near the 96th percentile of U.S. households. Today it's closer to the 89th percentile. The NIIT has quietly migrated from a tax on the rich into a tax on the upper-middle class, and that drift continues.
How Is NIIT Calculated?
The mechanics are worth walking through slowly, because the "lesser of" structure trips up even sophisticated investors.
Worked Calculation #1 — The Basic NIIT. Consider a married couple filing jointly with $310,000 of MAGI. Of that, $90,000 is net investment income — let's say $60,000 in realized long-term capital gains and $30,000 in dividends. Their wages make up the other $220,000.
The NIIT applies to the lesser of:
- Net investment income: $90,000
- MAGI above threshold: $310,000 − $250,000 = $60,000
Since $60,000 is smaller, the NIIT applies to that amount. The tax is $60,000 × 3.8% = $2,280. That's on top of their regular federal tax on the $60,000 in long-term gains and $30,000 in qualified dividends, which at the 15% LTCG rate adds another $13,500. Total federal tax on their investment income is $15,780 — an effective rate of about 17.5% on the $90,000 in NII.
The structure creates two distinct planning levers. You can reduce MAGI (through pre-tax retirement contributions, HSA contributions, municipal bonds, or timing). Or you can reduce net investment income (through harvested losses, charitable gifts of appreciated stock, or shifting assets to retirement accounts). The second lever is where tax loss harvesting lives, and it does something unusual — it pulls both levers at once.
Can Tax Loss Harvesting Reduce Your NIIT?
Yes, and by more than you'd expect, because realized losses reduce both net investment income and MAGI simultaneously. When a harvested loss offsets a realized gain, the gain that would have flowed into AGI never gets there in the first place. AGI drops, which drops MAGI, which drops the "excess" over the NIIT threshold. Meanwhile, the reduced gain also drops net investment income directly. The two effects compound.
Worked Calculation #2 — Harvesting's Double Effect on NIIT. Take the same couple from the previous example: $310,000 MAGI, $90,000 in NII, $2,280 NIIT. Now assume TaxHarvest has been running continuously in the background and has identified $25,000 worth of unrealized losses sitting in positions the couple thought were winners. That's TaxHarvest Capability #3 — the software scans every lot in the portfolio, not just position-level P&L, and finds losses hiding inside positions that are profitable on a blended basis.
Those $25,000 in harvested losses offset $25,000 of realized gains. Net investment income drops from $90,000 to $65,000. But AGI also drops by $25,000, because those offset gains never hit the couple's return. MAGI is now $285,000.
Recalculating:
- Net investment income: $65,000
- MAGI above threshold: $285,000 − $250,000 = $35,000
The NIIT now applies to $35,000, not $60,000. New NIIT = $35,000 × 3.8% = $1,330. That's $950 less than before.
And the regular capital gains savings is separate: $25,000 in offset gains at the 15% LTCG rate saves another $3,750. Total annual savings from finding those hidden losses: $4,700. If the couple harvests a similar amount every year for ten years and the savings compound at 7% inside a taxable account, they're looking at roughly $65,000 in extra wealth at the end of the decade — from finding losses that were already sitting in their portfolio.
The Bracket Creep Problem: Why NIIT Hits More People Every Year
Inflation is the NIIT's stealth accomplice. Most elements of the federal tax code adjust annually — standard deduction, ordinary income brackets, long-term capital gains brackets, the estate tax exemption. The NIIT thresholds do not. They were written into law in 2010 at $200,000 and $250,000 and there they have stayed. Every year of wage inflation pushes another tranche of households over the line.
The design was almost certainly intentional. When a tax that nominally targets "the wealthy" is allowed to slide into the upper-middle class through non-indexation, it raises more revenue without requiring a politically costly rate increase. The same technique was used with the original alternative minimum tax until Congress finally indexed it in 2013.
For investors, the practical consequence is that the set of people who should care about the NIIT keeps expanding. A household that was comfortably below the threshold in 2018 may be over it in 2026 simply because their salary grew with inflation and their portfolio grew with the market. And once you're over the threshold, the math on loss harvesting changes.
NIIT and the 23.8% Effective Rate
Worked Calculation #3 — Why Harvested Losses Are Worth 19% More to High-Income Investors. For a taxpayer above the NIIT threshold whose long-term capital gains fall in the 20% federal bracket, every dollar of LTCG is taxed at 20% + 3.8% = 23.8%. On a $100,000 realized long-term gain, that's $23,800 in federal tax, before state tax is layered on.
Now consider what that does to the value of a harvested loss. Without the NIIT, a $1,000 harvested loss that offsets a $1,000 long-term gain saves $200 at the 20% rate, or $150 at the 15% rate. With the NIIT stacked on top, that same $1,000 loss saves $238 at the 23.8% rate, or $188 at 18.8% (the combined rate for the 15% LTCG bracket plus NIIT).
At 23.8% vs 20%, every harvested dollar is worth 19% more — ($238 − $200) / $200. At 18.8% vs 15%, every harvested dollar is worth 25.3% more.
Compounded across ten years of continuous harvesting, this multiplier is the difference between loss harvesting as a mildly useful housekeeping habit and loss harvesting as one of the best risk-free returns available to a high-income investor. If TaxHarvest finds $30,000 in harvestable losses per year in a $1M portfolio — a conservative assumption for most equity portfolios — the NIIT alone adds $1,140 per year of value over what a 20%-only calculation would suggest. Over ten years, that's more than $15,000 attributable purely to the NIIT multiplier.
This is also why continuous scanning matters more at higher incomes. A once-a-year December harvest captures a handful of obvious losses. A system that scans lot-level losses throughout the year, like TaxHarvest does, captures many smaller opportunities that appear and disappear as markets move. At a 15% combined rate those small captures aren't worth much effort. At 23.8% they are.
What This Means for How You Harvest
The NIIT changes the calculus of loss harvesting in three concrete ways.
First, it raises the floor. Any harvestable loss on a portfolio above the threshold is worth at least 18.8%, not 15%. Small losses that seemed not worth bothering with suddenly justify the transaction cost and the attention.
Second, it favors continuous over annual harvesting. Because the multiplier applies to every dollar harvested, the marginal value of finding one more harvestable loss is higher than the marginal value under a pure capital gains regime. Systems that scan continuously and act on small opportunities throughout the year capture more aggregate value than once-a-year approaches.
Third, it favors lot-level visibility over position-level. The unrealized losses that matter most are often the ones hiding inside winning positions — a single bad tranche inside an otherwise profitable holding. Those losses don't show up in a position-level P&L report. They only show up if you look at the individual lots. For investors above the NIIT threshold, that lot-level scanning is the difference between capturing 60% of the harvestable losses in a portfolio and capturing 95% of them.
None of this is a loophole or an aggressive strategy. The NIIT is a feature of the current code. Losses offset gains for both regular capital gains tax and the NIIT — that's how the statute is written. The only real question is whether your harvesting system is sophisticated enough to find the losses in the first place.
For the basics of how tax loss harvesting works, see our primer on what is tax loss harvesting. For a detailed walk-through of how much these savings actually add up to, see how much can you save with tax loss harvesting. The NIIT interacts especially with the holding-period rules explored in our article on FIFO vs specific identification of tax lots, and for a deeper look at how holding period and basis interact to determine the optimal lot to sell, see why the optimal tax lot isn't always the highest-cost lot. For a strategic overview aimed at investors in this income range, our tax loss harvesting for high-income earners piece covers the broader playbook. For a full breakdown of the capital gains brackets the NIIT stacks on top of — including the 0% bracket strategy and the 23.8% high-income reality — see our capital gains tax rates 2026 guide. And if you're approaching retirement and thinking about bracket management differently, our tax loss harvesting for retirees piece shows how the same principles apply at lower income levels.
The net investment income tax isn't going away, and its thresholds aren't going up. For high-income investors, the right response isn't to avoid the tax — that's largely impossible without dramatic lifestyle changes — but to make sure every harvestable dollar in your portfolio is being found and captured. At 23.8%, that's where the money is.