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. What follows is the line-by-line math that gets there.

Under 2026 rules, a single filer owes 12% on taxable income up to $50,400. One dollar above that ceiling tips into the 22% bracket. Three deductions determine how much gross income this retiree can absorb before hitting that line. First, the standard deduction of $16,100. Second, the age-65 add-on of $2,050. Third, the temporary senior bonus deduction of $6,000 created by the One Big Beautiful Bill Act, signed into law on July 4, 2025 and available in full for tax years 2025 through 2028 to single filers whose MAGI stays below $75,000. The deduction phases out at 6 cents per dollar of MAGI above that floor and disappears entirely at $175,000 for single filers. All three layers sum to $24,150 of total relief. Because the OBBBA also made the TCJA’s seven-bracket rate structure permanent, the 12% rate is no longer a planning window that might evaporate with the next Congress. It is the law going forward.

That puts the ceiling on gross income before entering 22% territory at $50,400 plus $24,150, which works out to $74,550. Social Security then claims a portion of that headroom. With $30,000 in benefits and provisional income well past the upper threshold, 85% of those benefits count as taxable income, producing $25,500 of ordinary income before a single dollar of conversion is processed.

Why $43,000 Is the Right Conversion Size

Subtract taxable Social Security from the bracket ceiling: $74,550 minus $25,500 leaves roughly $49,000 of room for a Roth conversion. The temptation is to convert the full amount, and it is the wrong move. A taxable brokerage account throws off dividends and capital gain distributions in December that nobody can forecast accurately in March. Bond interest, a maturing Treasury, a mutual fund distributing a surprise year-end gain: any of those can push the last dollar of the conversion into the 22% bracket and wipe out the rate advantage entirely.

Converting $43,000 instead preserves a buffer of roughly $6,000 for that unpredictable income. The federal tax on the conversion comes to $43,000 multiplied by 12%, or $5,160 a year. Run that for eight years, from age 65 through 72 (the last full year before required minimum distributions begin at 73 for those born between 1951 and 1959, per SECURE 2.0), and the retiree has moved $344,000 out of the traditional 401(k) at a known, locked-in rate. The cumulative federal tax cost for all eight years comes to roughly $41,000.

The Bracket Arbitrage Nobody Writes Down

Left alone, that same $344,000 would compound inside the 401(k). At a 6% annual return over eight years, the untouched balance would grow to roughly $548,000 by age 73 and then begin flowing out as required minimum distributions stacked on top of Social Security. At that point, the marginal rate on the top slice of RMDs would almost certainly land at 22% or 24%, well above the 12% being locked in today. The trade is straightforward: pay 12% now to avoid paying 22% to 24% later on the same dollars, while those dollars compound tax-free inside the Roth in the meantime.

Pushing conversions higher eliminates the trade. A $70,000 conversion would drop roughly $20,000 into the 22% bracket, taxing that slice at precisely the rate this strategy was designed to avoid. Once the rate spread disappears, so does the reason to convert.

The IRMAA Window Stays Wide Open

The 2026 IRMAA surcharge kicks in at $109,000 of MAGI for single filers, adding costs on top of the standard $202.90 Part B monthly premium. A $43,000 conversion combined with $25,500 of taxable Social Security puts AGI near $68,500, leaving more than $40,000 of cushion before Medicare premiums face any surcharge. That buffer matters considerably more than most retirees appreciate.

IRMAA functions as a cliff, not a slope. A single dollar of MAGI above $109,000 triggers the full Tier 1 surcharge: $81.20 per month added to Part B, plus $14.50 per month added to Part D, totaling roughly $1,148 in extra Medicare costs over a full calendar year for a single filer. There is no partial surcharge for being slightly over the line.

The two-year lookback makes disciplined planning essential. The Social Security Administration uses MAGI from two years prior when setting Part B premiums, so income reported on the 2026 return is exactly what SSA will examine when it sets 2028 premiums. Keeping AGI comfortably below $109,000 today is direct protection against a premium spike two years out.

One mechanical rule applies regardless of conversion size: pay the conversion tax from a taxable brokerage account rather than by withholding from the IRA itself. Withholding shrinks the amount that actually lands in the Roth. After age 59 and a half there is no early-withdrawal penalty, but every dollar diverted for taxes is a dollar that stops compounding tax-free for the rest of the retiree’s life. That opportunity cost accumulates quietly and permanently.

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 work backward to find 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 rather than estimated.
3. Earmark a taxable account holding (a money market fund works well) to cover the roughly $5,000 federal tax bill so the full $43,000 reaches the Roth intact.

Editor’s note: This update confirmed that the 2026 IRMAA Tier 1 threshold is $109,000 MAGI for single filers (up from $106,000 in 2025), with a Tier 1 Part B surcharge of $81.20 per month and a Part D surcharge of $14.50 per month totaling $1,148 annually, and added context that those born in 1960 or later face an RMD start age of 75, not 73, under SECURE 2.0.

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