The federal bracket and the IRMAA tier get all the attention. But a Roth conversion raises your income, and two quieter thresholds react to that: the Net Investment Income Tax, and the share of your Social Security benefits that becomes taxable. Neither is on most calculators, and both can add real cost.
NIIT: a 3.8% surtax the conversion can trigger — indirectly
The Net Investment Income Tax adds 3.8% on top of your investment income once your MAGI exceeds a statutory threshold:
- Single / Head of Household: $200,000
- Married filing jointly: $250,000
- Married filing separately: $125,000
The key subtlety: a Roth conversion is ordinary income, not investment income, so the converted dollars themselves are never subject to NIIT. But NIIT is applied to the lesser of your net investment income and the amount by which your MAGI exceeds the threshold. By raising your MAGI, a conversion can pull your other investment income — interest, dividends, capital gains — over the line, creating a 3.8% charge on that income that wouldn't have applied otherwise. It's a surtax you incur on money you didn't convert, which is exactly why it surprises people.
Notably, unlike the federal brackets and IRMAA tiers, the NIIT thresholds are fixed in statute and are not indexed for inflation — they've stood at $200k/$250k/$125k since the tax took effect. Over time, more households drift over them simply because incomes rise while the thresholds don't.
Social Security: the 0% → 50% → 85% ramp
How much of your Social Security is taxable depends on your "combined income" — your adjusted gross income, plus any tax-exempt interest, plus half of your Social Security benefits. As combined income rises past two set of thresholds, the taxable share of your benefits steps up from 0% to as much as 50% and then up to 85%.
Because a Roth conversion increases your AGI, it increases combined income, which can move you up this ramp. And the ramp is narrow: in the transition bands, each extra dollar of conversion can make an additional 50 or 85 cents of Social Security benefits taxable on top of the conversion dollar itself. The result is a stretch of income where your effective marginal rate is noticeably higher than your bracket implies — sometimes called the Social Security "tax torpedo."
In the Social Security transition bands, one dollar of conversion can add far more than one dollar to your taxable income.
For very low income the benefits are fully untaxed and this isn't binding; once you're comfortably above the top threshold, 85% of benefits are already taxable and the ramp is behind you. It's the middle — where many single-income retirees sit — that a conversion can push you up the ramp at an unusually high effective cost.
Why these two belong on the cliff map
Individually, neither NIIT nor the Social Security ramp is usually the single biggest cost of a conversion. But they compound with the bracket and IRMAA effects, and because they're triggered indirectly — by income you didn't convert — they're easy to miss and easy to underestimate. Seeing them plotted alongside the other cliffs is the point: the true marginal cost of the next $10,000 of conversion is the sum of every threshold it crosses, not just the tax bracket.
See all the cliffs at once
The Roth conversion cliff calculator plots the federal bracket, IRMAA tier, NIIT threshold, ACA cliff, and Social Security band together, so you can see the largest conversion that clears them all. Keep reading: the IRMAA two-year lookback and the two 5-year rules.
Educational only, not tax or investment advice. Thresholds and rules change — verify against current IRS guidance. See our disclaimer.