The net investment income tax (NIIT, IRC Section 1411) adds 3.8% on top of capital gains tax once your income passes a threshold that has not changed since 2013. Many retirees never pay it until the year they sell a rental, land or a business. Then one sale pushes them over. Here is how it is calculated and what can change it.
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The rule
The tax is 3.8% of the smaller of (a) your net investment income or (b) the amount your modified adjusted gross income exceeds the threshold. It is figured on Form 8960.
| Filing status | MAGI threshold (not indexed for inflation) |
|---|---|
| Married filing jointly, qualifying surviving spouse | $250,000 |
| Single, head of household | $200,000 |
| Married filing separately | $125,000 |
Counts as net investment income: capital gains on rental, land and investment property, rents, interest (including interest on a seller-financed note), dividends, and income from passive businesses.
Generally does not count: wages, Social Security, IRA and 401(k) distributions (though they raise MAGI), gain excluded on a home sale under Section 121, and gain on business assets in a trade or business where you materially participate. Qualifying real estate professionals may be able to treat rental income as non-investment income (Treas. Reg. 1.1411-4(g)(7)).
Worked example
A married couple has $150,000 of other income and sells a rental with a $900,000 long-term gain.
| All in one year | Spread evenly over 10 years | |
|---|---|---|
| Gain reported per year | $900,000 | $90,000 |
| MAGI | $1,050,000 | $240,000 |
| Excess over $250,000 | $800,000 | $0 |
| NIIT | 3.8% × $800,000 = $30,400 | $0 a year, if other income stays the same |
Two cautions. Interest on a seller-financed note is itself investment income and raises MAGI. And if your income stays above the threshold every year, spreading the gain does not avoid the tax. It just pays it later. The result depends entirely on your income in each future year.
See the combined bill, NIIT included, with the capital gains tax calculator. For how spreading a sale works, see Form 6252 and the installment sale.
Before a sale
See whether your sale crosses the 3.8% line: one page from purchase price to gain, recapture, NIIT and state tax. Free from a 501(c)(3); nothing to buy.
The Retirement Literacy Foundation is a 501(c)(3) public charity (EIN 41-5062266). General education, not tax advice. We don’t sell financial products. Email only; unsubscribe any time.
Audit trail
Reviewed for the Retirement Literacy Foundation, a 501(c)(3) education nonprofit. Prepared by Hans Goldstein, retirement educator; IRS Special Enrollment Examination Parts 1, 2 and 3 passed, enrollment pending; California licensed insurance professional #4273294. Informational only. Not tax, legal or investment advice. Every figure below cites its source so your CPA or attorney can check it.
Figures are 2026 federal amounts and 2025 California rates unless stated. Examples are illustrative, rounded, and assume no other income; your numbers will differ. Last reviewed September 28, 2026.
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