How a 401(k) Balance Above $900,000 Can Quietly Add More Than $5,000 to a Single Retiree’s Tax Bill When Social Security Begins

Quick Read

• A $68,000 yearly 401(k) withdrawal lifts combined income to $83,000 once Social Security begins, causing 85% of benefits to be taxable, or $25,500.

• When two rules kick in at the same time, her federal tax bill rises by $5,199: Social Security taxation and the senior deduction phaseout together leave her $30,000 benefit worth only $24,801.

• Larger 401(k) withdrawals or Roth conversions before claiming Social Security can avoid the overlap and materially reduce long-run taxes.

• The biggest 401(k) choices often get made once and never revisited: when to roll it over, what to invest in, and when to start taking withdrawals. That is the problem the report 401k & IRA Tips and Answers was designed to help with, and it is free for now. (Sponsor)

A 67-year-old single retiree has more than $900,000 in a traditional 401(k) and takes out $68,000 a year. This year, she also starts receiving $30,000 in Social Security. Her 401(k) withdrawal is unchanged from the prior year, yet her federal tax bill increases by nearly $5,200.

Her savings and spending did not change. What changed is that two tax rules began working together: the formula used to determine how much of Social Security is taxable, and the income phaseout for the temporary senior deduction. The calculation below uses 2026 federal rules and assumes she has no other income.

Her Tax Bill Before Social Security Starts

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 threshold. That leaves her with taxable income of $43,850 and federal tax 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 of a benefit is taxable. Combined income equals adjusted gross income (AGI) plus tax-exempt interest and half of Social Security. In her case, that total is $83,000.

Now Available: The 401k Answers Nobody Gives You.

For many Americans, contributing to a 401(k) only creates more retirement questions. Should you roll your 401(k) into an IRA, and if so, when? What happens if the rollover check is made out incorrectly? When do required withdrawals start, and how much must be taken?

Single filers face two key thresholds. Above $25,000, part of Social Security becomes taxable. Above $34,000, up to 85% can be taxed.

She is well past the higher threshold, so $25,500 of her benefits counts as taxable income. That $34,000 line is fixed and does not rise with inflation. For anyone with a large 401(k), required withdrawals can push combined income beyond it.

A Second Hit From the Senior Deduction Phaseout

Her AGI rises to $93,500. The senior deduction is reduced by 6% of modified AGI above $75,000. That phaseout cuts $1,110 from the deduction, leaving 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 comes to about $10,213, an increase of $5,199.

Why $30,000 of Benefits Acts Like $24,801

After accounting for the extra tax tied to her Social Security check, her $30,000 in benefits effectively nets about $24,801, a reduction of roughly 17% in income many retirees expect to keep almost in full.

The 401(k) is what creates the effect. Without those withdrawals, her combined income would stay below the thresholds and her benefits would be mostly untaxed. Pre-tax withdrawals raise combined income, which in turn pulls more of her benefits into taxable income.

There is one partial cushion. She has already hit the 85% ceiling, so the next 401(k) dollar cannot make any more of her benefits taxable. Her marginal federal rate is about 23%: the 22% bracket plus the extra drag from the senior deduction phaseout, which continues 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 owe an additional $81.20 for Part B plus $14.50 for Part D. Medicare sets these income-related monthly adjustment amounts, or IRMAA, using income from two years earlier.

That leaves her with $15,500 of room before 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 Moves That Reduce the Overlap

1. Take larger withdrawals before claiming Social Security. If you are in your early to mid-60s and still waiting to file, 401(k) withdrawals and Roth conversions in those years do not trigger tax on benefits. Every dollar moved now is one less dollar stacked on top of benefits later, and waiting until age 70 makes that window longer.

2. Work out your own combined income. Add AGI to half of your expected benefit. If the total is far above $34,000, as much as 85% of Social Security may be taxable. From there, plan withdrawals around the top of the 22% bracket, $105,700 for single filers, and the $109,000 IRMAA threshold.

3. Consider qualified charitable distributions, or QCDs, from an IRA beginning at age 70½. QCDs count toward required minimum distributions but do not increase AGI. That can lower combined income and create more room under the IRMAA brackets.

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

Released: 401k & IRA Tips and Answers

(sponsor) 401(k)s are powerful retirement tools. But even experienced savers can make mistakes that quietly cost money for years, from choosing the wrong fund to overlooking small fees, or handling rollover steps in the wrong order.

That is exactly what 401k & IRA Tips and Answers was created to help prevent. It is a free Fisher Investments guide covering rollovers, contributions, required withdrawals, and the errors that do the most damage to a retirement account. Learn more here.