Pre-Tax Dollars Become Taxable Income on Conversion
Convert $50,000 from a Traditional IRA to a Roth and the federal tax math works out fine. Convert $55,000 instead and that extra $5,000 crosses an IRMAA threshold, adding thousands in Medicare premiums two years later that never show up on the 1040 at all.
A Roth conversion moves pre-tax retirement dollars, whether from a Traditional IRA, SEP IRA, SIMPLE IRA, or an employer plan like a 401(k), into a Roth IRA. The IRS treats the converted amount as taxable income for that year. Once the funds land in the Roth, they grow tax-free and come out tax-free in retirement, assuming the account meets the five-year holding rule and the owner is past age 59 and a half.
Calling this a loophole gets it wrong. It's a timing trade: the IRS collects its tax revenue now instead of later, and in exchange, the account owner locks in today's tax rate on that balance rather than gambling on what rates look like in 10 or 20 years. For someone in a low-income year, that trade can be worth tens of thousands of dollars over a lifetime. For someone in a high-income year, the same move pushes them into a bracket they never intended to enter.
Move money, pay tax, grow tax-free forever, that's the pitch. So why do so many conversions end up costing people more than they expected? The answer starts with how someone picks the conversion amount in the first place.
Five Steps That Determine Where the Tax Bill Lands
Stonewood Financial's 2026 guidance lays out a four-step calculation before anyone touches the account: estimate the client's taxable income for the year from salary, pensions, Social Security, capital gains, and required minimum distributions; identify which federal bracket that income falls into; calculate the gap between current taxable income and the top of the next bracket the client is comfortable filling; then treat that gap as the maximum conversion amount at that rate. Someone with taxable income in the mid-$70,000s, with the 22% bracket generally topping out somewhere in the low $100,000s (figures adjusted annually for inflation), typically has a few tens of thousands of dollars of room to convert before spilling into the 24% bracket.
Once the amount is set, the mechanical steps through a provider like Vanguard are straightforward:
- Open a Roth IRA if one doesn't already exist, and do it before the transfer, not during it.
- Log into the account with the plan provider and select the Traditional IRA to convert from.
- Choose the conversion amount, informed by the bracket math above, not an arbitrary round number.
- Decide on tax withholding. Withholding directly from the conversion reduces the amount that actually reaches the Roth, which shrinks future tax-free growth.
- Pay the tax bill owed on the converted amount, ideally from cash outside the IRA, not from the conversion itself.
- And keep the confirmation the provider sends once the transaction completes. That document matters at tax time and in case of an IRS inquiry.
Two mistakes show up repeatedly in the Stonewood guidance. First, clients ignore state income taxes: federal bracket math gets all the attention, but state rates add another 3% to 13% depending on where the client lives, and skipping that calculation means underestimating the real cost. Second, clients convert during an unexpectedly high-income year, say one that includes a large bonus, a property sale, or a spike in capital gains, without re-running the numbers. A conversion that made sense in January turns expensive by November if income came in higher than projected.
The bracket math and the mechanical steps are both manageable on paper. The harder question is what the conversion does to income-linked costs that sit outside the tax return entirely, specifically Medicare premiums.
Medicare Surcharges That Follow a Big Conversion Year
A 68-year-old converting $150,000 from a Traditional IRA to a Roth can get the federal and state tax bill exactly right and still get hit with a bill they never budgeted for: an IRMAA surcharge on Medicare Part B and Part D premiums. Roth conversions raise modified adjusted gross income (MAGI), and Medicare looks back a couple of years when using MAGI to set premium tiers. A conversion that looks clean on the 1040 can substantially increase Medicare premiums two years later.
A basic tax-bracket calculator won't catch this. You need the full picture: current income, projected income two years out, and whether a given conversion amount pushes MAGI across an IRMAA threshold. As in the opening example, converting $50,000 might stay under a threshold while converting $55,000 crosses it and adds thousands in annual Medicare costs that persist for a full year.
Then there's the question of where the tax payment itself comes from. Paying the conversion tax bill out of the IRA funds being converted shrinks the amount that actually lands in the Roth, cutting into the tax-free growth that was the whole point of converting. Paying from a separate taxable account preserves the full conversion amount and generally produces a better long-term outcome, but it requires having that cash available.
A few other scenarios change the math entirely. Someone with a shorter life expectancy, due to a life-limiting illness or starting retirement savings late, may not live long enough to recoup the upfront tax cost through years of tax-free growth. Someone facing an unusually high-income year from a bonus, property sale, or capital gains spike should run the numbers before converting rather than after, since that income may already be filling the bracket space a conversion was meant to use.
Roth conversions still make sense for plenty of people. But the $5,000 difference between a $50,000 and a $55,000 conversion is not a rounding error: it's the line between a clean tax trade and a Medicare bill that shows up two years later with no obvious connection to the decision that caused it. The fix isn't avoiding conversions, it's running the IRMAA threshold check and the state tax calculation before filing the paperwork, not after.