The combined income thresholds that decide how much of your Social Security check the IRS gets to tax were written into law in 1983 and last adjusted in 1993. They have never been indexed for inflation. The single-filer threshold for any taxation still sits at $25,000, and the threshold at which up to 85% of benefits become taxable sits at $34,000. For joint filers, the figures are $32,000 and $44,000. That math is the reason a 401(k) balance that sounds like a retirement success story now quietly drags most of a Social Security check into taxable territory.
The $700,000 math
Start with a $700,000 401(k) and apply the standard 4% safe withdrawal guideline. That produces $28,000 in annual distributions, all of which count as ordinary income. Add the average Social Security retirement benefit, which the Social Security Administration reported at about $2,081 per month in April 2026, or roughly $24,972 a year. The IRS combined income formula adds your adjusted gross income, tax-exempt interest, and half of your Social Security benefits.
For a single retiree, that comes to roughly $40,486, comfortably above the $34,000 line. Up to 85% of the Social Security benefit is then subject to federal income tax. The trigger is the interaction between an unindexed 1983 threshold and a balance that today funds a middle-class retirement. The Consumer Price Index reached 335.1 in May 2026, using the 1982 to 1984 base. Prices have more than tripled since the taxation rules were written, while the thresholds have not moved a dollar.
Why $700,000 is the “average American” trap
A $700,000 balance is well above the typical 401(k) balance. Fidelity’s Q1 2026 retirement analysis put the average 401(k) balance at $141,000. The $700,000 figure is closer to what long-tenured savers actually accumulate. Fidelity reported that workers continuously enrolled in the same plan for 15 years averaged balances exceeding $600,000.
That means the taxation rule increasingly affects disciplined middle-class savers rather than the wealthy retirees Congress had in mind in 1983. Per capita disposable personal income rose to approximately $68,391 in the first quarter of 2026, and median weekly earnings for full-time workers reached $1,235. A working household near those medians, retiring with a fully funded 401(k), now sits well inside the 85% racket on day one of retirement.
Yields and COLAs keep pushing more retirees over the line
The 2026 Social Security cost-of-living adjustment was 2.8%, which raises benefits but also raises combined income. The federal funds target rate sits at 3.75%, down from a peak of 4.5% last September. Even at the lower rate, fixed-income holdings in a 401(k) generate taxable interest that factors into the combined income formula on the way out. Each COLA and each yield bump narrows the cushion between a retiree’s combined income and the $34,000 line.
Levers that change the outcome
Three actions change the calculation directly. First, Roth conversions executed before claiming Social Security shift future distributions out of AGI, since qualified Roth withdrawals are excluded from combined income. Second, delaying Social Security to age 70 raises the benefit by about 8% per year after full retirement age, which can allow a retiree to draw less from taxable 401(k) accounts in the early years. Third, qualified charitable distributions from an IRA after age 70 1/2 satisfy required minimum distributions without adding to AGI, keeping combined income lower.
The thresholds are unlikely to move. The Social Security Administration’s own solvency provisions list a proposal to phase out the lower-income thresholds from 2026 to 2045, which would expand taxation rather than shrink it. Planning around the $25,000 and $34,000 lines, not waiting for Congress to raise them, is what determines how much of a Social Security check a retiree actually keeps.
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