Clear Money Guide
What this guide covers
A quick view of the questions and evidence developed below.
Updated July 28, 2026. Quick answer: The conversion itself is not net investment income, so it is not directly subject to the 3.8% tax. But it does raise modified adjusted gross income — which is what determines whether your other investment income becomes subject to it.
The indirect mechanism
The net investment income tax applies to the lesser of your net investment income or the amount by which your modified AGI exceeds a threshold. A conversion is retirement-plan distribution income, not investment income, so it is not in the first bucket.
But it lands squarely in modified AGI. So a conversion can push you over the threshold and make dividends, interest and capital gains you already had newly taxable at 3.8%.
This is why the true marginal cost of a conversion is often higher than your bracket suggests. Someone with meaningful taxable-account income can face their ordinary rate plus 3.8% on the collateral damage — and the 3.8% is not visible anywhere in a bracket table.
Who is exposed
People with substantial taxable investment accounts alongside their retirement accounts. Someone whose wealth is almost entirely inside retirement accounts has little net investment income for the tax to reach, and can largely ignore this.
Price the conversion before you make it
A conversion cannot be undone once it is done, so it is worth having someone model the bracket it fills, the knock-on effects on your other income and how long the money has to compound before you settle on an amount.
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Sources
IRC §408A (Roth IRAs); IRC §408A(d)(3) (conversions); IRC §1411 (net investment income tax); IRC §86 (taxation of Social Security benefits); IRC §6654 (estimated tax); Tax Cuts and Jobs Act (2017) §13611 (repeal of conversion recharacterisation). Cross-checked July 2026 against professional analyses. Indexed thresholds are described rather than asserted, because they change annually.
This states what the cited authority says. It is not tax advice, and a conversion interacts with the rest of your return in ways one page cannot see.
Related
Because the exposure turns on a threshold your modified income has to stay under, the practical step is to convert up to it and stop. The Roth conversion guardrail estimator takes the ceiling you want to respect and your income before the conversion, and returns the room beneath it and the estimated tax on the conversion that fits. Its optional cap field is labelled for Medicare’s IRMAA, but it is simply a ceiling you set.