Retiring high earners can convert traditional 401(k) assets to Roth IRAs during market downturns at lower account values, generating the same tax bill but acquiring 25% more shares that grow tax-free—a strategy that produced $75,000 in permanent tax-free gains during the April 2025 market selloff when the S&P 500 fell 20%. SPDR S&P 500 ETF (SPY) exemplifies the funds held in these accounts.

IRMAA Medicare premium surcharges triggered by high modified adjusted gross income create a hard ceiling on conversions: single filers must stay below $109,000 MAGI to avoid surcharges entirely, or deliberately convert up to just under $137,000 to accept only the first tier ($1,148 annually) rather than the third tier ($4,620), making split-year conversions across two calendar years essential for larger transfers.

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A cardiologist who retires at 60 with $2 million in a traditional 401(k) and delays Social Security until 67 has something most investors do not: a multi-year window of artificially low taxable income. When the market drops 20%, the instinct is to hold on and wait for recovery. The strategic move is to convert.

Between retirement and Social Security, a high earner who spent decades in the 37% bracket can suddenly find themselves in the 22% or 24% bracket. The 2026 federal income tax brackets place single filers in the 22% bracket from $50,401 to $105,700, and in the 24% bracket from $105,701 to $201,775. For someone with no W-2 income, a $150,000 Roth conversion lands squarely in that range. That is the window. The question is when to use it.

The answer is during a market downturn. The S&P 500 fell sharply enough in April 2025 that the VIX hit nearly 41, a level associated with genuine panic. The VIX remained elevated above 30 from April 10 through April 23, 2025, a sustained period of market stress. Most investors froze. Physicians who understood the mechanics converted.

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A physician holds a broad index fund inside her traditional 401(k). Shares are trading at $50. She converts $150,000, which buys 3,000 shares. The tax bill is the same regardless of market conditions: $150,000 of ordinary income at her marginal rate.

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During a 20% market downturn, shares drop to $40. The same $150,000 converts to 3,750 shares. The tax cost is identical. But when the market recovers, and shares reach $100 a decade later, the downturn converter holds 3,750 shares worth $375,000 inside the Roth, versus 3,000 shares worth $300,000 for the bull-market converter. The difference is $75,000, and every dollar of it is permanently tax-free.

The mechanism works in two directions simultaneously. The converted dollar amount is lower in real terms because assets are depressed, and the recovery gains accrue inside the Roth rather than in the traditional account, where they would eventually be subject to ordinary income tax upon withdrawal. Converting during a downturn produces more tax-free shares for the same dollar outlay than converting at higher prices.

The conversion window is real, but it has a ceiling. Medicare premium surcharges under IRMAA use a two-year lookback, so income decisions made today affect your premiums in 2028. For 2026, IRMAA surcharges begin at a MAGI of $109,000 for single filers. The first tier adds about $1,148 per year per person in combined Part B and Part D surcharges. The second tier, triggered at or above $137,000, costs roughly $2,885 per person annually.

A $150,000 Roth conversion on top of even modest other income can easily push you past multiple IRMAA tiers. For example, a single filer with $30,000 in dividend income who converts $150,000 ends up with $180,000 MAGI and lands in the third tier, triggering an annual surcharge of about $4,620 per person. That extra cost shows up two years later on the Medicare premium notice, and most people never connect it back to the conversion.

The smart planning target for a single retiree is to keep total MAGI below $109,000 to avoid IRMAA entirely, or to deliberately convert up to just under $137,000 and accept only the first-tier cost while still gaining the Roth benefit. For a married couple filing jointly, the threshold is $218,000 before any surcharge applies, which gives considerably more room for conversions.

Calculating MAGI for the current year before any conversion, including dividends, interest, capital gains distributions, and any part-time income, establishes the baseline. The gap between that number and $109,000 (single) or $218,000 (joint) is the IRMAA-safe conversion ceiling for 2026, with the surcharge landing in 2028.

When the market drops more than 15%, the share-count advantage becomes material. Dividing the target conversion amount by the current depressed share price of the fund being held, then comparing it to the same division at the pre-correction price, reveals the difference in shares, and multiplying that difference by a reasonable long-term recovery price shows the incremental tax-free gain from converting during the downturn rather than after recovery.

If MAGI after a full conversion exceeds $137,000 for a single filer, splitting the conversion across two calendar years can keep income within a single IRMAA tier. The jump from Tier 1 ($1,148 annually) to Tier 2 ($2,886 annually) is triggered by exceeding $137,000 in MAGI, and avoiding that threshold for two years saves more than $3,400 in Medicare premiums per person.

The VIX spiked again to around 31 in late March 2026, before retreating to around 19 by early April. Those windows open and close quickly, and so the investors who act during them are applying discipline, not abandoning it.

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