The 3.8% Net Investment Income Tax: the surtax that ambushes big-income retirement years
There’s a quiet 3.8% surtax that most retirees never plan for until it shows up on the return: the Net Investment Income Tax. It hits your investment income once your modified adjusted gross income crosses $200,000 (single) or $250,000 (joint) — thresholds that have never been adjusted for inflation since 2013, so they catch more people every year. The real trap for retirees is indirect: a big Roth conversion, RMD, or TSP withdrawal isn’t investment income and is never taxed by the NIIT itself — but it raises your MAGI and can drag your dividends, interest, and capital gains into the surtax. Here’s exactly how it works, and a calculator to see your exposure.
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1. The tax nobody plans for
Enacted in 2013 to help fund the Affordable Care Act, the Net Investment Income Tax (Internal Revenue Code § 1411) is a 3.8% surtax that sits on top of regular income tax and capital gains tax. For a high-income year it pushes the effective federal rate on long-term capital gains to 23.8% (20% + 3.8%). Most people never think about it — until a Roth conversion, a home sale, or a strong market year drops it onto their return. For retirees running deliberate withdrawal and conversion strategies, that surprise is entirely avoidable with a little planning.
2. The “lesser of” rule
The single most important feature of the NIIT is how the base is computed. The 3.8% applies to the lesser of two numbers:
This cuts both ways. If your MAGI is only a little over the line, you pay 3.8% on that small excess even if you have large investment income. But if a one-time event rockets your MAGI far past the threshold, far more of your investment income is exposed. IRS example: a couple with $225,000 of net investment income and MAGI of $300,000 (i.e., $50,000 over the $250,000 line) pays 3.8% on the lesser figure — $50,000 — for a $1,900 NIIT.
3. What counts — and what doesn’t
The inclusions and exclusions are the whole game:
- IS net investment income: interest, dividends, capital gains, rental & royalty income, non-qualified annuity income, passive business income, and home-sale gain above the $250K/$500K exclusion.
- Is NOT: wages and self-employment income, Social Security, VA benefits, tax-exempt municipal bond interest, and — critically — distributions from 401(k)s, IRAs, TSP, and Roth accounts.
Your pension, TSP withdrawals, Social Security, and VA compensation are all outside net investment income. The NIIT only reaches money in a taxable brokerage account — the dividends, interest, and capital gains outside your tax-advantaged wrappers. If most of your wealth is in TSP/IRA and pensions, your NII may be small.
4. The MAGI drag: the retiree trap
Here’s the subtlety that catches people. Retirement-account distributions aren’t net investment income — but they still raise your MAGI, and the Form 8960 MAGI worksheet does not subtract them back out. So even though a Roth conversion, an RMD, or a big TSP withdrawal is never taxed by the NIIT directly, it can lift your MAGI over the threshold and expose your other investment income to the 3.8%.
Example: a single retiree with $40,000 of pension income and $500 of interest converts $120,000 to Roth. The conversion itself owes no NIIT, and MAGI ($160,500) stays under $200,000 — so no NIIT at all. But push the conversion or add capital gains until MAGI clears $200,000, and that $500 of interest (plus any dividends and gains) suddenly gets surtaxed. The lesson: size and time conversions so they don’t spike MAGI past the line in a year you’re also realizing investment income.
5. The unindexed threshold
Unlike tax brackets, the NIIT thresholds are fixed in statute and never indexed for inflation. They’ve been $200,000 / $250,000 / $125,000 since 2013. As wages, pensions, and portfolios grow, the fixed lines quietly ensnare more people every year — the count subject to the tax more than doubled from 3.1 million in 2013 to 7.3 million by 2021. A retiree who felt safely under the line a decade ago may not be today. Also note the widow’s penalty angle: a surviving spouse drops from the $250,000 joint threshold to the $200,000 single one, making the surtax easier to trigger on the same income. See the widow’s penalty.
6. Calculate your NIIT
Enter your filing status, MAGI, and net investment income. The calculator applies the “lesser of” rule.
Your numbers
The NIIT is 3.8% of the lesser of your net investment income or the amount MAGI exceeds the threshold. 2026 thresholds: $200K single, $250K joint, $125K separate — not inflation-indexed. Estimate, not advice.
7. Ways to reduce it
Because the tax is the lesser of two numbers, shrinking either one helps — and controlling MAGI is usually the cleanest:
- Keep MAGI under the threshold. At or below the line, you owe zero NIIT regardless of investment income.
- Time capital gains for lower-income years; harvest losses to offset gains.
- Spread Roth conversions across years to avoid a single MAGI spike.
- Donate appreciated stock or use qualified charitable distributions instead of selling.
- Hold fixed income in municipal bonds — their interest is excluded from MAGI.
- Keep assets in tax-advantaged wrappers — income inside a 401(k)/IRA/TSP/Roth isn’t NII.
8. How it stacks with IRMAA
The NIIT doesn’t operate in isolation. The same income spike that triggers the surtax can also push you into a higher IRMAA Medicare bracket and increase the taxable share of your Social Security — all keying off income in the same year. A large Roth conversion can therefore hit you three ways at once. That’s why conversion and withdrawal planning has to look at NIIT, IRMAA, and Social Security taxation together, not one at a time. See IRMAA explained.
9. Frequently asked questions
What is the Net Investment Income Tax?
The NIIT is a 3.8% surtax on investment income, enacted in 2013 under the Affordable Care Act (Internal Revenue Code Section 1411). It applies to individuals whose modified adjusted gross income (MAGI) exceeds a threshold — $200,000 single or head of household, $250,000 married filing jointly, and $125,000 married filing separately. The tax is 3.8% of the lesser of your net investment income or the amount your MAGI exceeds the threshold. It’s in addition to regular income tax and capital gains tax.
What income is subject to the NIIT?
Net investment income includes interest, dividends, capital gains, rental and royalty income, non-qualified annuity income, and passive business income. It does NOT include wages, self-employment income, Social Security benefits, VA benefits, tax-exempt municipal bond interest, or distributions from qualified retirement plans like 401(k)s, IRAs, TSP, and Roth accounts. Home-sale gain above the Section 121 exclusion ($250,000 single / $500,000 joint) is investment income; gain within the exclusion is not.
Does a Roth conversion or RMD trigger the NIIT?
Not directly — but indirectly, yes. Roth conversions, required minimum distributions, and TSP or IRA withdrawals are not themselves net investment income, so they’re never taxed by the NIIT on their own. However, they DO raise your MAGI, and the MAGI worksheet doesn’t subtract them back out. A large conversion or RMD can push your MAGI over the threshold and expose your other investment income — dividends, interest, capital gains in a taxable account — to the 3.8% surtax that year.
Are the NIIT thresholds adjusted for inflation?
No. The $200,000, $250,000, and $125,000 thresholds were set in statute in 2013 and have never been indexed for inflation. As wages, pensions, and asset values rise over time, more taxpayers cross the fixed lines each year — the number subject to the NIIT more than doubled from 3.1 million in 2013 to 7.3 million by 2021. For retirees, this steadily increases the odds that a big income year triggers the surtax.
How can retirees reduce the NIIT?
The cleanest lever is managing MAGI: if your MAGI stays at or below the threshold, you owe no NIIT no matter how much investment income you have. Strategies include timing capital gains for lower-income years, spreading Roth conversions to avoid a MAGI spike, harvesting investment losses, donating appreciated stock or using qualified charitable distributions, holding fixed income in tax-exempt municipal bonds (their interest isn’t in MAGI), and keeping assets inside tax-advantaged accounts. Because the tax is the lesser of two numbers, shrinking either one helps.