NIIT: The 3.8% Tax Most Retirees Don’t Know About

Retirees spend hours optimizing their federal income tax bracket. Then they discover that a separate 3.8% surtaxhas been quietly eating into their investment returns for years — and they never planned for it. The Net Investment Income Tax (NIIT) is one of the most misunderstood taxes in the code, and for retirees with investment portfolios, rental properties, or even moderate capital gains, it can add thousands of dollars to the annual tax bill.

What Is the NIIT?

The Net Investment Income Tax was established under IRC §1411 as part of the Affordable Care Act in 2013. It imposes a flat 3.8% surtax on the net investment income of individuals, estates, and trusts whose modified adjusted gross income (MAGI) exceeds certain thresholds.[1]

The word “surtax” is important: the NIIT is levied in addition toregular income tax and capital gains tax. It is not a replacement — it stacks on top. A long-term capital gain that would otherwise be taxed at 15% becomes effectively 18.8% once NIIT applies. For taxpayers in the top capital gains bracket (20%), the combined rate reaches 23.8%.

NIIT Thresholds: Frozen Since 2013

The NIIT applies when your MAGI exceeds these filing-status-specific thresholds:[2]

• $200,000— Single
• $250,000— Married Filing Jointly (MFJ)
• $125,000— Married Filing Separately (MFS)
• $200,000— Head of Household (HoH)

Critically, these thresholds have never been indexed for inflation. When the NIIT was enacted in 2013, $250,000 of household income represented a significantly higher standard of living than it does in 2026. Thirteen years of cumulative inflation means the tax now captures a much broader population than originally intended. The One Big Beautiful Bill Act (2025) did not adjust these thresholds, so they remain frozen heading into the 2026 tax year.[1]

What Counts as Net Investment Income?

The IRS defines net investment income broadly to include most “passive” income streams. Understanding what is — and is not — NII is essential to managing the tax:[4]

Income Included in NII

• Interest income (savings, bonds, CDs)
• Dividend income (qualified and non-qualified)
• Capital gains (short-term and long-term, including home sales above the exclusion)
• Rental and royalty income
• Income from passive business activities
• Gains from trading in financial instruments or commodities
• Income from annuities (non-qualified)

Income Excluded from NII

• Wages, salaries, and self-employment income
• Social Security benefits
• Distributions from IRAs, 401(k)s, and other qualified retirement plans
• Income from an active trade or business (material participation)
• Tax-exempt interest (e.g., municipal bond interest)
• Veterans' benefits and Alaska Permanent Fund dividends

This distinction creates a critical nuance: a Roth conversion or traditional IRA distribution is not net investment income, but it doesincrease your MAGI — which can push existing investment income above the NIIT threshold.

How NIIT Is Calculated

The NIIT equals 3.8% of the lesser of:[2]

1. Your net investment income, or
2. The amount by which your MAGI exceeds the threshold for your filing status

Example (Married Filing Jointly):A retired couple has $280,000 in MAGI and $60,000 in net investment income. Their MAGI exceeds the $250,000 threshold by $30,000. Since $30,000 is less than their $60,000 of NII, the NIIT applies to $30,000. The tax: $30,000 × 3.8% = $1,140.

Example (Single):A single retiree has $230,000 in MAGI and $50,000 in net investment income. Their MAGI exceeds $200,000 by $30,000. The NIIT applies to $30,000: $30,000 × 3.8% = $1,140. If that same retiree had only $20,000 in NII, the tax would apply to $20,000 instead (the lesser amount): $20,000 × 3.8% = $760.

How NIIT Stacks With Other Taxes

The NIIT does not operate in isolation. It stacks on top of existing federal taxes, creating combined rates that surprise many retirees:[3]

• Long-term capital gains (15% bracket) + NIIT: 15% + 3.8% = 18.8%
• Long-term capital gains (20% bracket) + NIIT: 20% + 3.8% = 23.8%
• Qualified dividends (15%) + NIIT: 15% + 3.8% = 18.8%
• Interest income (32% bracket) + NIIT: 32% + 3.8% = 35.8%

Add state income taxes to these figures and a retiree in a high-tax state like California (13.3% top rate) could face an all-in rate exceeding 37% on capital gains. Even in a no-income-tax state, the combined federal rate of 23.8% on capital gains is substantially higher than the 15% or 20% rate retirees typically plan around.

The Roth Conversion NIIT Trap

This is one of the most frequently overlooked interactions in retirement tax planning. When you convert a traditional IRA to a Roth IRA, the conversion amount is added to your MAGI as ordinary income. However, the conversion itself is notnet investment income — it is a retirement plan distribution.[5]

Here's the trap: if you already have investment income sitting below the NIIT threshold, a large Roth conversion can push your total MAGI above $200,000 (single) or $250,000 (MFJ) — suddenly making that existing investment income subject to the 3.8% surtax.

Example:A married couple has $180,000 in pension and Social Security income plus $60,000 in qualified dividends and capital gains. Total MAGI: $240,000 — below the $250,000 threshold, so no NIIT. They decide to do a $50,000 Roth conversion, pushing MAGI to $290,000. Now the NIIT applies to the lesser of $60,000 (their NII) or $40,000 (MAGI over threshold). The Roth conversion costs them an additional $1,520in NIIT (3.8% × $40,000) — on top of the income tax on the conversion itself.

This does not mean Roth conversions are bad — reducing future RMDs and investment income over a 20-30 year retirement can save far more than the one-time NIIT cost. But it means the conversion must be sized carefully. The optimal conversion amount often targets the NIIT threshold as a ceiling.

Strategies to Manage the NIIT

While you cannot eliminate the NIIT entirely if you have significant investment income and high MAGI, several strategies can reduce exposure:

1. Shift Income Into Tax-Advantaged Accounts

Investment income earned inside a Roth IRA, traditional IRA, or 401(k) is not included in net investment income. By holding high-yield bonds, REITs, and actively traded positions inside tax-advantaged accounts, you reduce the NII that appears on your return. This asset location strategy can be worth thousands per year.

2. Harvest Capital Losses

Capital losses offset capital gains dollar-for-dollar when calculating net investment income. Systematically harvesting losses in taxable accounts during market downturns reduces the NII component of the NIIT calculation. Up to $3,000 in net losses can also offset ordinary income per year, with the remainder carried forward.

3. Use Municipal Bonds for Fixed Income

Municipal bond interest is excluded from both net investment income and MAGI for NIIT purposes. Replacing taxable bonds with munis in a taxable brokerage account directly reduces NII. The tax-equivalent yield of munis is particularly attractive for taxpayers subject to both NIIT and high marginal income tax rates.

4. Time Income to Stay Below the Threshold

If your MAGI is near the NIIT threshold, consider deferring capital gains realizations, bunching deductions, or splitting income across tax years. Even shifting $10,000 of income from one year to the next can save $380 in NIIT. Over a multi-decade retirement, these annual savings compound significantly.

5. Size Roth Conversions Carefully

As described above, Roth conversions increase MAGI without increasing NII — but they can still trigger NIIT on existing investment income. The optimal approach is to convert up to the NIIT threshold (accounting for all other income sources) and stop. Multi-year conversion strategies that spread conversions over the “gap years” between retirement and Social Security claiming are particularly effective.

6. Consider Qualified Opportunity Zone Investments

Capital gains invested in Qualified Opportunity Zone (QOZ) funds can defer and partially reduce capital gains recognition. While the original basis step-up provisions have expired, gains held in QOZ investments for 10+ years may be excluded from income entirely — including for NIIT purposes. This is a more complex strategy suited to taxpayers with large one-time gains.

NIIT and Other Hidden Retirement Taxes

The NIIT does not exist in a vacuum. It interacts with the other stealth taxes that affect retirees:

• Social Security Tax Torpedo: NIIT and the torpedo use different income measures (MAGI vs. provisional income), but a large capital gain can trigger both simultaneously.
• IRMAA: IRMAA also uses MAGI, so the same income that triggers NIIT may also push you into a higher IRMAA tier two years later.
• Capital Gains Bump Zone:Capital gains that trigger NIIT at the top end may also be bumping other income through the 0%/15% boundary at the bottom end — a double hit.
• ACA Subsidy Cliff: For pre-Medicare retirees, the same MAGI that triggers NIIT could also push you over the ACA 400% FPL cliff, compounding the cost of that additional dollar of income.

Modeling these interactions together is the only way to find your true marginal tax rate. A dollar of investment income near the NIIT threshold may carry an effective rate far beyond 3.8% when all interactions are considered.

The Bottom Line

The NIIT is a permanent feature of the tax code that grows more relevant every year as inflation pushes more retirees above the frozen thresholds. At 3.8%, it may seem modest — but when stacked with capital gains taxes, IRMAA, and the Social Security torpedo, it can push effective rates well above 40%.

The most effective defense is proactive planning: know your MAGI relative to the threshold, manage asset location across taxable and tax-advantaged accounts, and size Roth conversions to avoid inadvertently triggering NIIT on existing investment income. Use the calculators below to model your specific situation.