Consider a couple in their mid-60s, both retired, with a $1.4 million traditional 401(k) and about to start Social Security. The plan looked airtight: pull 4% a year, collect benefits, and let the rest compound. Then the tax return arrives and roughly $51,000 of Social Security they thought was mostly theirs shows up as taxable income. This scenario surfaces constantly in Bogleheads and r/retirement threads, and the source of the surprise is almost always the same formula.
The Thresholds That Never Move
Social Security benefits become taxable when “combined income” (AGI, plus tax-exempt interest, plus half your benefits) crosses two thresholds set for joint filers in the 1980s and 1990s: $32,000 pulls up to 50% of benefits into taxable income, and $44,000 pulls up to 85% in. Those numbers were never indexed to inflation. They are the same today as when the rule was written. Meanwhile, the 2026 Social Security COLA came in at 2.8%, the standard deduction for a joint filer climbed to $32,200, and the 22% bracket starts at $100,800 of taxable income. The rest of the code inflated around a Social Security taxation rule that stood still.
A 401(k) balance above $1 million quietly ambushes couples because it generates enough required or discretionary withdrawals that combined income lands well above $44,000 before the first IRS check.
Running the Numbers on $1.4 Million
Take a couple, both 67, with $1.4 million in a traditional 401(k) and $60,000 in combined annual Social Security. Assume a 4% withdrawal of $56,000 and a small bond sleeve throwing off about $5,000 at a 10-year Treasury yield near 5%.
Combined income equals $56,000 from the 401(k), $5,000 in interest, and $30,000 (half of Social Security), or $91,000. That is more than double the $44,000 threshold. Under the formula, up to 85% of the couple’s benefits, roughly $51,000 of the $60,000, gets pulled into taxable income. AGI now sits near $112,000. Subtract the $32,200 standard deduction and taxable income lands around $79,800, still inside the 12% bracket that runs to $100,800 for joint filers.
Hold the same $1.4 million in a Roth 401(k) instead. The $56,000 withdrawal never touches the combined-income formula. Combined income drops to about $35,000, below the $44,000 threshold, and no more than 50% of Social Security is exposed. Same lifestyle, same portfolio, a very different tax bill.
The gap widens once RMDs start at age 73 under SECURE Act 2.0. A $1.4 million balance grown to roughly $1.7 million by RMD age produces a first-year required distribution near $64,150 using the current Uniform Lifetime Table divisor of 26.5. That distribution is not optional, and it locks the couple into 85% taxation of Social Security for the rest of their lives unless they act earlier.
Where the Tax Bomb Gets Bigger
Pile IRMAA on top. The 2026 first-tier Medicare surcharge kicks in at $218,000 of modified AGI for joint filers, and a two-year lookback means a conversion done in 2026 shows up on the 2028 premium bill. Standard Part B is $202.90 a month, and top-tier IRMAA can push premiums toward $689.90 per person. A Roth conversion sized to fix the Social Security problem can trigger the Medicare one, which is why households in this balance range must model both at the same time.
What This Couple Should Actually Do
- Fill the 12% bracket with Roth conversions between retirement and age 73. Convert enough each year to approach the top of the 12% bracket at $100,800 of taxable income without crossing the $218,000 IRMAA line. This shrinks future RMDs and permanently reduces the share of Social Security exposed to the 85% haircut.
- Delay Social Security to 70 if the 401(k) balance exceeds $1 million. Each year of delay past full retirement age adds 8% to the benefit and compresses the taxable-benefit window into fewer years.
- Use qualified charitable distributions once RMDs begin. The 2026 QCD limit is $111,000 per person. Dollars sent directly from the IRA to charity satisfy the RMD without touching AGI or the combined-income formula.
Pull last year’s tax return, add half of expected Social Security to projected AGI, and see how far above $44,000 the number lands. That distance is the size of the problem the 1984 thresholds are quietly creating.
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