A Surtax on Investment Income, Frozen Since It Was Written
There is a 3.8% tax that sits on top of everything else you pay on investment income, and two things make it worth understanding rather than merely absorbing. Its thresholds have never been adjusted for inflation, so it reaches further into ordinary households every year. And retirement account withdrawals are exempt from the tax while still counting toward the income test that triggers it — an asymmetry that rewards planning and punishes assumption.
- The answer:: 3.8%, applied to the lesser of two figures: your net investment income, and your income above the threshold for your filing status.
- The thresholds are frozen:: They were set when the tax was introduced and have never been indexed to inflation, so the tax captures more households with every year that passes.
- What counts:: Interest, dividends, capital gains, rental and royalty income, annuity income, and income from passive businesses.
- What does not:: Wages, Social Security benefits, and distributions from IRAs and qualified plans — though those distributions still raise the income tested against the threshold.
Where the AI summary above gets this wrong
"The net investment income tax only applies to high earners, so most retirees do not need to think about it."
That's surface-true. Here's what it misses:
- Frozen thresholds make it a widening tax — The thresholds have not moved since the tax began. Two decades of inflation later, incomes that were plainly high when the rule was written are ordinary now, and households that never considered themselves high-earning meet it — particularly in a single year containing a property sale.
- Retirement withdrawals are exempt and still trigger it — An IRA distribution is not net investment income, so it is never taxed by this provision directly. But it raises modified adjusted gross income, which can push you over the threshold and expose investment income that would otherwise have escaped. Exempt and harmless are different things.
- It is a one-year problem, not a permanent status — Because a single year's income decides it, the households that pay it most are frequently ordinary ones having an unusual year — a home sale, a large conversion, an inherited account emptied at once. Framing it as a tax on the wealthy misses who actually writes the cheque.
01 How the calculation works
The tax is 3.8% of the lesser of two amounts: your net investment income for the year, and the amount by which your modified adjusted gross income exceeds the threshold for your filing status.
The lesser-of structure is what makes it survivable. Someone $5,000 over the threshold with $200,000 of investment income pays 3.8% on $5,000, not on $200,000. The tax grows as you move above the line rather than landing all at once, so crossing it is not the cliff people fear.
What makes it grow regardless of anything you do is the threshold itself. It was fixed when the tax was created and has never been indexed, so its real value falls every year. A provision written for high earners now reaches households that would not have been near it when it was drafted.
Shows: the 3.8% surtax, applied to the lesser of your net investment income and the amount by which your income exceeds the threshold. Ignores: state tax, the ordinary income and capital gains tax already due on the same income, and the deductions that may reduce net investment income.
02 What counts as investment income, and what does not
Included: interest, dividends, capital gains, rental and royalty income, non-qualified annuity income, and income from businesses in which you do not materially participate.
Excluded: wages and self-employment income, Social Security benefits, tax-exempt municipal bond interest, and — the one that matters most in retirement — distributions from IRAs, 401(k)s and other qualified plans. A required minimum distribution is never net investment income, however large it is.
The asymmetry is the whole planning point. That same distribution counts toward modified adjusted gross income, which is what the threshold test measures. So a large withdrawal cannot be taxed by this provision directly, but it can lift you over the line and expose dividends and gains that would otherwise have sat below it.
Source: Net investment income tax
03 The years it actually bites
Because everything turns on one year's income, the households that meet this tax are frequently not wealthy in any sustained sense. They are ordinary households having a single unusual year.
Three events produce it repeatedly. A house sale with gain above the exclusion, which is both investment income and a large addition to modified AGI. A substantial Roth conversion, which is not itself investment income but raises the income measure and can expose everything else. And an inherited IRA emptied in one year rather than spread across the ten available.
The defence in each case is spreading rather than avoiding. A conversion executed across several years instead of one may stay under the threshold throughout, and the same logic governs the pacing of an inherited account. It is the same argument that drives conversion strategy generally, with one more reason attached to keeping any single year's income low.
This tax rarely changes whether something is worth doing, and frequently changes when. Nobody should abandon a conversion or refuse to sell a property because of 3.8%. But the same transaction split across two years instead of one can avoid it entirely, and that costs nothing but patience. What I would watch for is the stacking effect: the year you cross this threshold is usually the same year you cross an IRMAA bracket and pull more Social Security into tax. None of them is large alone, and together they are the difference between a plan and a surprise.
FAQ
Who pays the net investment income tax?
Anyone whose modified adjusted gross income exceeds the threshold for their filing status and who has net investment income. The 3.8% applies to the lesser of the two amounts, so being slightly over the threshold produces a small tax rather than a large one.
Are IRA withdrawals subject to the net investment income tax?
No. Distributions from IRAs and qualified plans are not net investment income and are never taxed by this provision directly. They do count toward modified adjusted gross income, so they can push you over the threshold and expose other investment income.
Does the threshold rise with inflation?
No. The thresholds were fixed when the tax was introduced and have never been indexed, so the tax reaches further into ordinary incomes each year as wages and asset values rise around a stationary line.
Sources
Regulator references
- Topic no. 559, Net investment income tax · Internal Revenue Service · 2025Who owes the 3.8% tax and how the threshold works.Last verified: 2026-09-07
- Net investment income tax · Internal Revenue Service · 2025Which categories of income are included and which are excluded.Last verified: 2026-09-07
- About Form 8960, Net Investment Income Tax · Internal Revenue Service · 2025How the tax is computed and reported on the return.Last verified: 2026-09-07
Calculator unit tests · the assertions this page's worked example is checked against, and their last result
Changelog
- 2026-09-07 — initial publish (new format)
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Jordan Reeves
Founder of Talk Through Wealth. A software engineer for over a decade before turning to retirement planning, Jordan built the projection engine after watching family members get fragmented, country-by-country advice that never reconciled. He writes about retirement the way the engine computes it: month-by-month, lifetime-long, and skeptical of any rule of thumb that hasn't been run through the math.