The 401(k) Bracket Smoothing Math: Why a 65 Year Old With $1.6 Million Should Convert Exactly $43,000 a Year Until 73

A 65 year old single retiree posted a familiar question on a Bogleheads thread last month: “I have $1.6 million in a traditional 401(k) and $30,000 in Social Security. Every article tells me to do Roth conversions to fill the 12% bracket. Nobody tells me the actual dollar amount.” The honest answer for that exact profile is $43,000 a year, every year, from age 65 through 72. Here is the line by line math that gets there.

Under the 2026 rules, a single filer pays 12% on taxable income up to $50,400. Anything above that jumps to 22%. The deductions stack as follows: the regular standard deduction of $16,100, the age 65 add on of $2,050, and the new senior bonus deduction of $6,000 created by the One Big Beautiful Bill, available in full because this retiree’s MAGI sits well under the $75,000 phase out for singles. Total deductions: $24,150.

That means the ceiling on gross income before tipping into the 22% bracket is $50,400 plus $24,150, or $74,550. Social Security eats some of that ceiling. With $30,000 in benefits and provisional income well past the upper threshold, 85% becomes taxable. That is $25,500 of ordinary income before a single dollar is converted.

Why $43,000 Is the Right Conversion Size

Subtract the taxable Social Security from the bracket ceiling: $74,550 minus $25,500 leaves roughly $49,000 of headroom for a Roth conversion. The temptation is to convert the full amount. Resist it. A taxable brokerage account throws off dividends and the occasional capital gain distribution in December that the retiree cannot forecast in March. Bond interest, a Treasury maturing, a mutual fund kicking out a surprise gain: any of these can push the last dollar of the conversion into the 22% bracket retroactively.

Converting $43,000 leaves a buffer of roughly $6,000 for that noise. The federal tax on the conversion itself is $43,000 times 12%, or $5,160 a year. Run that for eight years from age 65 through 72 (the last year before RMDs begin at 73 for this birth cohort) and the retiree has shifted $344,000 out of the traditional 401(k) at a known, locked in rate, for a cumulative tax cost near $41,000.

The Bracket Arbitrage Nobody Writes Down

Left alone, that same $344,000 compounds inside the 401(k). At a 6% return over eight years, the untouched balance would grow to roughly $548,000 by age 73, then start coming out as required minimum distributions on top of Social Security and any other income. At that point, the marginal rate on the top slice of RMDs is almost certainly 22% or 24%, well above today’s 12%. The arbitrage is paying 12% now to avoid 22% to 24% later on the same dollars plus all their growth.

Pushing the conversion higher destroys the trade. A $70,000 conversion would put roughly $20,000 into the 22% bracket. That marginal slice would be taxed at a rate identical to the future RMD rate this strategy is designed to dodge. There is no arbitrage left.

The IRMAA Window Stays Wide Open

The 2026 IRMAA first tier kicks in at $109,000 of MAGI for single filers, adding surcharges on top of the standard $202.90 Part B premium. A $43,000 conversion plus $25,500 of taxable Social Security puts AGI near $68,500, leaving more than $40,000 of cushion before Medicare premiums get hit. That cushion matters because IRMAA uses a two year lookback: the conversion done at 65 affects the Part B premium at 67.

One mechanical rule: pay the conversion tax from a taxable brokerage account rather than by withholding from the IRA itself. Withholding shrinks the amount that lands in the Roth and, before age 59 and a half, can trigger penalties. After 59 and a half there is no penalty, but every dollar held back for taxes is a dollar that stops compounding tax free for the rest of the retiree’s life.

Three Things to Do This Quarter

1. Project this year’s gross income line by line: Social Security, interest, dividends, capital gains, any pension, then back into the conversion size that lands AGI between $65,000 and $70,000. Do not eyeball it.
2. Set the conversion to execute in November or December, once the year’s dividends and capital gain distributions are known instead of guessed at.
3. Earmark a taxable account holding (a money market fund works) to cover the roughly $5,000 federal tax bill so the full $43,000 reaches the Roth.