A Roth Conversion Can Pull Your Dividends Into the 3.8%…

A large traditional IRA-to-Roth conversion is not itself investment income, but the income it adds to a retiree's modified adjusted gross income can push dividends and capital gains collected that year into the 3.8% net investment income tax.

A Roth conversion is one of the cleanest moves in retirement planning: you pay tax now on money leaving a traditional IRA, and everything it earns afterward comes out untaxed. What conversion guides rarely spell out is that the tax bill for the year can land in two places at once — on the converted balance, and on dividends and capital gains the retiree assumed had nothing to do with the transaction.

The culprit is the 3.8% net investment income tax, usually shortened to NIIT. As 24/7 Wall St lays out, a conversion is not investment income and is never itself subject to the surtax. It can still be the reason the surtax applies to everything else in the portfolio that year.

Two separate tests, and a conversion only fails one of them

The surtax works off a pair of tests, and the tax is charged on the smaller of the two results. The first test asks how much net investment income you had: interest, dividends, capital gains, rents, royalties, annuity income and passive business income. The second asks how far your modified adjusted gross income sits above a fixed threshold.

A Roth conversion is ordinary income. It goes on the front of the return the same way a pension or an IRA withdrawal does, and it flows straight into modified adjusted gross income. It never enters the first bucket. But because it inflates the second, it can turn a year in which no surtax would have been due into one in which the whole of your dividend and gain income is exposed.

That is the mechanical oddity worth sitting with. The converted dollars themselves escape the 3.8%. The dividends you collected quietly all year, on which you had already budgeted a qualified-dividend rate, do not. The surtax is layered on top of whatever rate those dividends already carried.

Why the thresholds are the thing to plan around

The NIIT thresholds are not indexed to inflation the way tax brackets are. They were written into law at fixed dollar amounts and have stayed there, which means every year of nominal portfolio growth, every year of rising dividend payouts, pushes more households toward them without any change in behavior on the household's part.

For most retirees living off Social Security, a modest pension and portfolio income, ordinary years stay comfortably below the line. A conversion year does not. Moving a six-figure traditional IRA balance in one go can add more income in twelve months than the household normally reports in several years combined. The threshold is cleared in a single step, and the surtax attaches to the investment income sitting underneath it.

The same logic explains why the damage is concentrated. It is a one-year event. Convert over multiple years in smaller slices and the income never spikes far enough above the threshold for the surtax to bite hard — or at all, if each year's conversion is sized to land beneath it.

The knock-on costs that arrive with the same spike

The 3.8% surtax is rarely the only consequence of a large conversion year, and retirees who model only the income tax on the converted amount tend to be surprised twice. A higher modified adjusted gross income can also affect the share of Social Security benefits included in taxable income and the Medicare premium surcharges assessed on higher-income beneficiaries, which are calculated with a lag off a prior year's return.

8% surtax is rarely the only consequence of a large conversion year, and retirees who model only the income tax on the converted amount tend to be surprised twice.

None of those are the surtax, and they operate under their own rules and their own thresholds. But they share a trigger. Anything that lifts reportable income in a single year — a conversion, a property sale, a lump-sum distribution, the realization of a long-held position — sets several meters running at once.

Ways to keep the surtax from attaching

The planning responses are unglamorous and mostly come down to controlling when income lands.

Split the conversion across calendar years. The most direct fix. Size each year's conversion so modified adjusted gross income stops short of the threshold, and the second test produces nothing for the surtax to apply to.
Move dividend-paying holdings into tax-sheltered accounts. Investment income earned inside an IRA or Roth is not net investment income for surtax purposes. Holding the highest-yielding positions there shrinks the first bucket.
Harvest losses in the conversion year. Realized losses offset realized gains within net investment income. A smaller first bucket means a smaller surtax base even if the threshold is cleared.
Time other large income events away from the conversion. Stacking a home sale or a concentrated-position sale into the same tax year compounds the problem rather than spreading it.
Consider charitable strategies. Deductions that reduce adjusted gross income can pull modified adjusted gross income back toward the threshold, though not every deduction works this way.

Why this is showing up more in 2026

Conversion interest tends to track two things: the size of traditional IRA balances and expectations about future tax rates. Both have been working in the same direction. Equity markets closed the most recent session higher across the board, with the S&P 500 tracker (NYSEARCA: SPY) finishing at $764.29, up 0.85% on the day from a prior close of $757.83, according to market data as of Fri, 11 Sep 2026 20:00 GMT. The Nasdaq 100 fund (NASDAQ: QQQ) ended at $714.88, up 0.87%, and the Dow tracker (NYSEARCA: DIA) at $525.79, up 0.97%.

Bigger balances mean bigger conversions, and bigger conversions mean a larger one-year income spike. They also mean more accumulated unrealized gain, which matters if a retiree sells anything in the same year. The two effects compound: the portfolio that makes a conversion attractive is the same portfolio most likely to generate the dividends and gains that the surtax will reach.

What to check before signing the paperwork

The practical step is to build a projected tax return for the conversion year before executing, not after. That means totaling expected dividends, interest, realized gains and any rental income — the first bucket — and then adding the proposed conversion on top of all other ordinary income to see where modified adjusted gross income lands against the threshold.

If the second number clears the line, the surtax applies to the smaller of the two amounts, and the arithmetic will show immediately whether trimming the conversion, deferring part of it, or offsetting gains changes the outcome. A conversion executed in December leaves no room to adjust. One modeled in the spring leaves the whole year.

The broader point is that the 3.8% surtax is a threshold tax, not an activity tax. It does not care that a Roth conversion is not investment income. It only cares what the conversion did to the number at the bottom of the income page.

Key facts

Surtax rate: 3.8% net investment income tax
Conversion treatment: Ordinary income — never itself subject to the NIIT
S&P 500 (SPY): $764.29, +0.85%, last close Fri, 11 Sep 2026 20:00 GMT
Who is exposed: Retirees converting a large traditional IRA balance in a single year

Frequently asked questions

Is a Roth conversion subject to the 3.8% surtax?

No. A Roth conversion is ordinary income, not net investment income, so the converted amount itself is never charged the 3.8% net investment income tax. The problem is indirect: the conversion raises modified adjusted gross income, and that higher figure can push dividends, interest and capital gains collected in the same year into the surtax.

How is the 3.8% surtax actually calculated?

It applies to the lesser of two amounts: your total net investment income for the year, or the amount by which your modified adjusted gross income exceeds a fixed statutory threshold. Because the tax takes the smaller of the two, a household can clear the threshold and still owe little if its investment income is small.

What counts as net investment income?

Interest, dividends, capital gains, rental and royalty income, non-qualified annuity income and income from passive business activities. Wages, Social Security benefits, IRA distributions and Roth conversions do not count. Investment income earned inside a tax-sheltered account such as an IRA or Roth is also excluded from the calculation.

Can splitting a conversion over several years avoid the surtax?

Often, yes. Because the surtax depends on how far modified adjusted gross income exceeds a fixed threshold, sizing each year's conversion to stay beneath that line can leave nothing for the surtax to apply to. A single large conversion clears the threshold in one step and exposes that year's dividends and gains.

Does a conversion affect anything besides the surtax?

It can. A higher modified adjusted gross income in the conversion year may increase the share of Social Security benefits included in taxable income and can trigger the income-related surcharges applied to Medicare premiums, which are assessed with a lag based on a prior year's return. Each operates under its own separate rules.

When should a retiree model the tax impact of a conversion?

Well before executing it, ideally early in the calendar year. Building a projected return that totals expected dividends, interest and realized gains alongside the proposed conversion shows where income lands against the threshold. A conversion done in December leaves no room to trim it, defer part of it, or harvest offsetting losses.

Sources

A Roth Conversion Isn’t Investment Income. It Can Still Trigger the 3.8% Surtax on the Dividends You Collect That Year — 24/7 Wall St

Photo: Kampus Production · Pexels Licence — source

A Roth Conversion Can Pull Your Dividends Into the 3.8% Surtax