Princeton Asset Management modeled a $65,000 Roth conversion for a retiree buying insurance on an ACA marketplace plan. The income tax on the conversion was $10,315.87. That is the number most conversion calculators would show you. But the conversion also raised the retiree's modified adjusted gross income, which cut the premium tax credit, adding $7,179.99 in lost health subsidy. The true cost of that "tax-efficient" conversion was $17,495.86, an effective rate of 26.92% on money moved during what was supposed to be a low-tax window. If you are running a Roth conversion ladder and buying ACA coverage, this is the interaction that decides whether the ladder is brilliant or expensive.
A Roth conversion ladder and ACA subsidies are natural enemies. The ladder works by manufacturing income in low-income years, and the subsidy works by rewarding low income. Every rung of the ladder raises the same number the marketplace uses to price your insurance.
How a Roth conversion hits your ACA subsidy: the MAGI math
A conversion is ordinary income in the year you make it. It lands on your return, flows into adjusted gross income, and with the ACA's additions becomes the modified adjusted gross income that the marketplace uses to size your premium tax credit. Below 400% of the federal poverty level, the credit phases out gradually as MAGI rises. Above 400%, it stops. The pandemic-era expansion that softened that cliff has expired, and the cliff is back in full force for 2026 and 2027.
The cruelty is in the mechanics. If you accepted advance premium tax credits during the year, your insurer got paid monthly based on your estimated income. File your return above 400% FPL and you repay the full amount, not a prorated share. One dollar over the line can mean zero credit plus a repayment bill. A late-year conversion you sized for the 12% bracket can quietly detonate a subsidy worth several thousand dollars. This is exactly what the modeled case above demonstrates: the tax bracket math said "convert," and the total cost math said otherwise.
For scale, 400% of the federal poverty level runs roughly in the upper $50,000s for a single-person household and the mid-$70,000s for a two-person household, using the 2025 HHS guidelines that set 2026 plan thresholds. Verify your exact figure at healthcare.gov before sizing anything; the line moves every year and with household size.
The conversion cap rule
So what do you actually do? Three situations, three rules.
If you are on an ACA plan now: cap the year's conversion so MAGI stays under 400% FPL. Your conversion room is not the top of your target tax bracket anymore. It is the smaller of that number and the poverty-level line. In many early-retirement households the subsidy line binds first, which means the right conversion is smaller than the tax math alone suggests.
If you are between jobs with no marketplace plan: convert normally, bracket-filling and all. The trap only exists when a subsidy is at stake. Years with employer coverage, COBRA you are paying full price for anyway, or a spouse's plan are conversion-friendly years.
If you are approaching 65: remember IRMAA. Medicare's income surcharge looks back two years, so a huge conversion at 63 inflates premiums at 65. The ages 60 to 64 are the danger zone twice over: subsidies matter most and the IRMAA clock is already ticking.
The sequencing I would use: convert aggressively in the years right after retiring and before marketplace coverage begins, run small subsidy-safe conversions during the ACA years to keep the five-year pipeline alive, then resume larger conversions after Medicare. You lose some bracket-filling efficiency in the middle years. You keep thousands in premium credits. That is a trade worth making.
One more thing worth saying plainly: the 5-year clocks do not care about any of this. Every conversion still starts its own five-year clock from January 1 of the conversion year, and a December conversion gets nearly a full year's head start. Size the conversion for the subsidy, time it for the clock. They are independent levers.
Model your Roth conversion ladder free
The calculator lets you plug in your balance, annual conversion, and other income, then shows the tax per year, the total converted, and when each rung unlocks. When you are on an ACA plan, run your conversion amount through the cap rule above first, then model the capped number.
Frequently asked questions
How does a Roth conversion affect ACA premium tax credits?
A Roth conversion counts as ordinary income, which raises your modified adjusted gross income. ACA premium tax credits are calculated from MAGI, so the conversion shrinks your subsidy, sometimes dollar for dollar, and can push you over the 400% federal poverty level cliff where the credit disappears entirely.
What is the ACA subsidy cliff in 2026?
Above 400% of the federal poverty level, premium tax credits end completely. The pandemic-era expansion that smoothed this cliff has expired, so crossing the line by even one dollar can mean losing the full credit and repaying advance payments received during the year.
How can I keep my ACA subsidy while running a Roth conversion ladder?
Cap each year's conversion so your MAGI stays under 400% of the federal poverty level for your household size. Convert aggressively in the years before you enter the marketplace or after you reach Medicare at 65, and run smaller maintenance conversions during the ACA years.
Do Roth conversions affect Medicare premiums too?
Yes, through IRMAA, the income-related surcharge on Medicare Parts B and D. IRMAA is based on your MAGI from two years earlier, so a large conversion at 63 raises your premiums at 65. Coordinate conversion size with both the ACA years and the two-year IRMAA lookback.
Should I pause my ladder while on an ACA plan?
Not necessarily. Small conversions sized to stay under the subsidy cliff still build your five-year pipeline. What you should not do is run a bracket-filling conversion sized for tax optimization alone, because the lost health subsidy can cost more than the tax savings.
Sources: Princeton Asset Management, "How Roth Conversions Affect ACA and IRMAA Subsidies" (modeled $65,000 conversion case); Fidser, "ACA Subsidy Cliff 400% FPL"; Considerable, "Retiring Before 65? ACA Marketplace Premiums Are Jumping for 2027." FPL figures are 2025 HHS guidelines for 2026 plans; verify at healthcare.gov. Figures verified October 2026.