Why a $900,000+ 401(k) Can Add About $5,200 to a Single Retiree’s Tax Bill When Social Security Begins

Starting Social Security can seem like getting a raise, but retirees with large traditional 401(k) balances may find that the first year of benefits activates two tax rules at once, turning that added income into a costly surprise.

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Consider a 67-year-old single retiree with more than $900,000 in a traditional 401(k) who takes out $68,000 a year. In the same year she begins receiving $30,000 in Social Security, her 401(k) withdrawal stays the same, yet her federal tax bill rises by nearly $5,200.

Nothing about her savings or spending changed. The increase comes from two tax rules kicking in together: the formula that determines how much of her Social Security is taxable, and the income phaseout for the temporary senior deduction. The example below uses 2026 federal rules and assumes she has no other income.

Her tax bill before Social Security begins

With only the 401(k) withdrawal, her deductions line up neatly. The 2026 standard deduction for a single filer is $16,100, and taxpayers age 65 or older get an additional $2,050.

She also qualifies for the full $6,000 senior deduction because her income is below the $75,000 phaseout start. That leaves taxable income of $43,850 and a federal tax bill of about $5,014.

How $30,000 in benefits turns into $25,500 of taxable income

The IRS uses combined income to decide how much Social Security is taxable. Combined income equals adjusted gross income, plus tax-exempt interest, plus half of Social Security benefits. In her case, that total comes to $83,000.

For single filers, the first threshold is $25,000, where some benefits become taxable. At $34,000, as much as 85% of benefits can be taxed.

She is well above the upper threshold, so $25,500 of her benefits becomes taxable income. That $34,000 cutoff is fixed and does not rise with inflation. For people with large 401(k) balances, required withdrawals can easily push combined income beyond it.

The senior deduction starts to phase out

Her adjusted gross income rises to $93,500. The senior deduction is reduced by 6% of modified AGI above $75,000, which cuts $1,110 from the deduction and leaves her with $4,890.

Her taxable income climbs to $70,460, and part of it now falls into the 22% bracket, which starts at $50,400 for single filers. Her federal tax reaches about $10,213, an increase of $5,199.

Why $30,000 of benefits acts more like $24,801

Once the extra tax is counted, her $30,000 Social Security benefit effectively nets about $24,801. That is roughly a 17% reduction in income many retirees expect to receive nearly untouched.

The 401(k) withdrawal is what creates the problem. Without it, her combined income would stay below the key thresholds, and most of her benefits would be minimally taxed. Pre-tax withdrawals raise combined income, which pulls Social Security into taxable income.

There is one benefit to being this far along: she has already hit the 85% ceiling, so additional 401(k) withdrawals cannot make any more of her Social Security taxable. Her marginal federal rate is about 23%, reflecting the 22% bracket plus the continuing phaseout effect, which lasts until modified AGI reaches $175,000.

Her Medicare cushion is only $15,500

At this income level, she would pay the standard 2026 Part B premium of $202.90 a month. Above $109,000, single filers pay an additional $81.20 for Part B and $14.50 for Part D. Medicare determines these surcharges, known as income-related monthly adjustment amounts, using income from two years earlier.

That leaves her with $15,500 of room below the 2026 threshold. A Roth conversion or a large one-time withdrawal that crosses that line could add about $1,148 a year in premiums at 2026 rates, on top of income tax.

Three ways to reduce the overlap

1. Withdraw more before claiming benefits. If you are in your early to mid-60s and still delaying Social Security, 401(k) withdrawals and Roth conversions during those years do not create tax on benefits. Every dollar moved earlier is a dollar that will not sit on top of your benefits later, and delaying Social Security until 70 extends that window.
2. Calculate your own combined income. Add your adjusted gross income to half of your expected benefit. If the total is well above $34,000, as much as 85% of your benefits may be taxable. From there, try to manage withdrawals around the top of the 22% bracket, $105,700 for single filers, and the $109,000 IRMAA threshold.
3. Consider qualified charitable distributions from an IRA starting at age 70½. QCDs count toward required minimum distributions but are excluded from AGI. That can lower combined income and leave more room under the IRMAA brackets.

These examples use 2026 federal rules and assume no other income. The senior deduction applies only to tax years beginning before January 1, 2029. After it expires, the same withdrawals and benefits would lose the remaining $4,890 deduction, and the tax bill could rise again even if spending does not change.