Retired Couples With $700,000 in a 401(k) Face a Tax Chain Reaction That Adds $3,100 a Year to Their Social Security Tax Bill
A routine 401(k) withdrawal can quietly trigger a tax bill on income a retired couple never actually touched, and most people don't see it coming until the damage is already done.
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A $28,455 withdrawal from a traditional 401(k) sounds like $28,455 in taxable income. But for one retired couple, it sets off a tax chain reaction, making another $24,187 of their Social Security taxable and adding roughly $3,100 to their federal tax bill.
Consider a hypothetical married couple, both 75, with $700,000 in a traditional 401(k). Each collects $3,750 a month from Social Security, while a pension and bank interest bring in another $48,900 a year. They already owe tax on some of their benefits, but their required withdrawal makes the bill considerably larger.
A $28,455 RMD Is What Flips the Switch
The tax shows up when money comes out, and required withdrawals have already begun for this couple. At 75, the IRS Uniform Lifetime Table divisor is 24.6, so a $700,000 balance from the previous December 31 produces a required minimum distribution (RMD) of $28,455.
If they skip it, they face a 25% penalty on the shortfall, falling to 10% if corrected in time.
How the RMD Pulls More Benefits Into the Tax Base
Combined income is other income plus half your Social Security benefits. For joint filers, taxation starts above $32,000. Once combined income passes $44,000, as much as 85% of benefits can become taxable. Here’s how the couple’s tax picture compares with and without the $28,455 withdrawal:
| Item | Before RMD | After RMD |
|---|---|---|
| Combined income | $93,900 | $122,355 |
| Taxable Social Security | $48,415 | $72,602 |
The couple was already in the 85% zone, so every RMD dollar pulls 85 cents of benefits onto the return with it. That adds $24,187 of newly taxable Social Security, all of it income they had already been receiving.
Where the Extra $3,100 Comes From
Under 2026 rules, the couple subtracts $32,200, plus $1,650 each for being 65 or older. Add the new $6,000 senior deduction each, for $47,500 in total.
The 12% bracket ends at $100,800 of taxable income, and the 22% bracket starts there.
- Without the RMD: taxable income of $49,815 produces federal tax of $5,482.
- Once the RMD hits, benefits held at the old level: taxable income rises to $78,270 and tax rises to $8,896. The $3,415 increase is the tax on the RMD itself.
- With the newly taxable benefits added: taxable income reaches $102,457, crossing into the 22% bracket, and tax hits $11,964. The $3,068 gap, roughly $3,100, is the tax on Social Security they didn’t expect to owe.
Altogether, a $28,455 withdrawal costs $6,483 at the federal level, about 23% of the RMD.
Their last RMD dollars are taxed even harder. Each dollar in the 22% bracket adds $1.85 of taxable income, which works out to an effective rate of 41%. The same math in the 12% bracket gives 22%.
Their adjusted gross income (AGI) lands at $149,957, just below the $150,000 threshold where the senior deduction starts phasing out. A slightly larger RMD could start shrinking that deduction, too. Medicare surcharges aren’t a concern at this income level. Under 2026 rules, the first surcharge starts above $218,000 for joint filers, with a standard Part B premium of about $203 a month each.
That’s the surprise: a mandatory 401(k) withdrawal doesn’t just create its own tax bill. It can make more of your Social Security taxable, too. A free guide covers how to prepare for that first withdrawal years ahead of time: The First-Year Tax Bomb.
Three Moves That Shrink the Surprise
1. Take withdrawals before Social Security starts. Between retirement and claiming benefits, 401(k) withdrawals are taxed at their own bracket rate, with no Social Security in the formula to increase them. Each dollar taken out in your 60s is a dollar that can’t later pull 85 cents of benefits into taxable income. Filling the 12% bracket in those years can help offset future RMDs.
2. Convert to Roth in the same window, with an eye on Medicare. Roth conversions reduce the pretax balance used to calculate future RMDs. Medicare generally uses income from two years earlier, so conversions from 63 forward can affect premiums. Under 2026 rules, joint income above $218,000 raises Part B to about $284 a month per person. Keep the applicable Medicare threshold in mind when sizing conversions.
3. Already taking RMDs? Use qualified charitable distributions (QCDs). If you’re giving to charity, rolling eligible 401(k) money into an IRA can help with future RMDs. You must first satisfy this year’s 401(k) RMD, which cannot be rolled over. A QCD sent directly from the IRA to charity can count toward future IRA RMDs while staying out of taxable income, up to $111,000 per person in 2026. That can also slash the tax on Social Security benefits.
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Gerelyn Terzo is the author of dividend investing handbook "Dividend Investing Strategies: How to Have Your Cake & Eat It Too." A veteran financial journalist, she covers agri-finance for outlets like Global AgInvesting and the broader stock market and personal finance for 24/7 Wall Street. She began at CNBC and later helped launch Fox Business in New York. Gerelyn currently resides in Woodland Park, Colorado and dabbles in nature photography as a hobby.
