A Roth Conversion Isn’t Investment Income. It Can Still Trigger the 3.8% Surtax on the Dividends You Collect That Year

A Roth conversion doesn’t count as investment income. It can still hand you a 3.8% surtax on dividends, interest and capital gains you were going to collect anyway. That is the quiet trap in the net investment income tax, and it catches retirees who move a big slug from a traditional IRA into a Roth in a single year.

What the 3.8% Surtax Actually Taxes

The net investment income tax (call it NII tax or NIIT) is a 3.8% surtax that fires once modified adjusted gross income, your AGI with a few add-backs, crosses $200,000 for single filers and $250,000 for married couples filing jointly. Joy Taylor, editor of The Kiplinger Tax Letter, notes that “The income figures aren’t indexed to inflation, so this tax hits more filers each year.” A threshold set in 2013 dollars is now catching households that never thought of themselves as high earners.

Lesser of Two Numbers, Not Both

Here is the mechanic most conversion checklists skip. Joy Taylor writes that “The tax, which is added to the regular income tax, is due on the smaller of NII or the excess of modified AGI over the income thresholds.”

Translation: the surtax base is capped. If your MAGI runs well over the threshold but you only earned a modest amount of dividends and interest, the 3.8% applies to that smaller investment total. If your investment income is large but your MAGI barely clears the line, the overage sets the ceiling. A Roth conversion inflates MAGI without adding a single dollar of investment income, yet it can still uncork the surtax on the dividends and gains already sitting on your 1099s.

What Counts as Net Investment Income

Joy Taylor’s list is specific: “NII includes what is commonly thought of as investment income: Dividends, capital gains, taxable interest, annuities, royalties and passive rental income. Trade or business income derived through a passive activity is also NII, provided that the business income isn’t already subject to self-employment tax.” The passive test turns on material participation, which is a facts-and-hours question rather than a checkbox.

Retirement account distributions and Roth conversions are not on that list. That is precisely why the trap is easy to miss. The conversion is invisible to the NII definition but fully visible to the MAGI threshold, so it can pull otherwise unrelated dividends and capital gains into the 3.8% net.

No Relief for Filers Living Abroad

Two recent appellate rulings closed a door for U.S. citizens overseas. Joy Taylor reports that “The U.S. foreign tax credit cannot offset the net investment income tax under the U.S.-France income tax treaty or the U.S.-Canada tax treaty, the U.S. Court of Appeals for the Federal Circuit decides in two separate cases, reversing the trial court.” Expats paying French or Canadian tax on the same dividends still owe the 3.8% to the IRS, with no treaty credit to soften it. If you live abroad and are eyeing a conversion, that stacking is real.

What to Watch Before You Convert

Model the MAGI number first and the tax bill second. Stage conversions across two or three years to keep MAGI closer to the threshold, harvest capital losses inside taxable accounts to shrink the NII side of the lesser-of calculation, and time large dividend distributions and mutual fund capital-gain payouts to fall outside your conversion year. Check the 2025 ordinary brackets while you are at it, because the conversion itself is taxed as ordinary income and can push you into the 24% or 32% bracket before the surtax even shows up. Those low-tax years between your last paycheck and your first RMD are usually where conversions pencil out best, and we sized up that window in a free Roth guide here. This is the kind of math worth running with a CPA or fiduciary advisor, on a spreadsheet, before the wire hits your Roth.

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