How Your 401(k) Is Quietly Making Up to 85% of Your Social Security Taxable

A retired couple collects $40,000 per year in Social Security and pulls $60,000 from a traditional 401(k) to cover living expenses. They expect to pay taxes on the 401(k) income. What they don’t expect is to also pay taxes on most of their Social Security.

The mechanism is provisional income, and it’s why traditional 401(k) balances quietly become more expensive than they appear.

How the IRS Counts Your Income Before You Do

Provisional income is the figure the IRS uses to determine how much of your Social Security is taxable. The formula adds all other income (wages, pensions, investment income, and every dollar of traditional 401(k) withdrawals) to 50% of your Social Security benefit. For the couple above, that’s $60,000 in 401(k) withdrawals plus $20,000 (half of $40,000 in Social Security), for a provisional income of $80,000.

The threshold that matters for married filers is $44,000. Above it, up to 85% of Social Security benefits become taxable. At $80,000 in provisional income, this couple clears that threshold by $36,000, so the full 85% applies. That’s $34,000 of Social Security added to their taxable income. At a 22% federal rate, the tax on that Social Security alone runs roughly $7,480 per year.

The thresholds that trigger thism 25,000 for single filers, $32,000 for couples at the first tier, $44,000 for couples at the 85% tier, have not changed since 1984. Inflation has continued to accumulate: headline PCE inflation was 2.8% year-over-year as of February 2026, and services inflation, which hits retirees hardest, was 3%. Retirees don’t need income growth to cross these thresholds. Inflation pushes them over automatically.

The One Withdrawal Type That Doesn’t Count

Roth 401(k) and Roth IRA withdrawals do not count toward provisional income. A dollar withdrawn from a Roth is not included in this calculation. That asymmetry is the core planning lever for anyone with a mix of traditional and Roth balances.

For retirees who have left the workforce, the window between retirement and age 73, when required minimum distributions (RMDs) begin, is often the best time to convert remaining traditional balances to Roth. Each year of conversion reduces the future RMD base, which reduces future provisional income. The tradeoff is paying ordinary income tax on the converted amount in the year of conversion, so sizing the conversion to stay within a bracket matters.

There’s also a Medicare cost dimension. IRMAA surcharges in 2026 begin at $109,000 MAGI for single filers and $218,000 for married filing jointly. A couple converting $80,000 in a single year could push their MAGI past that joint threshold, triggering $2,297 in additional Medicare Part B and Part D premiums per couple at Tier 1. Because IRMAA uses a two-year lookback, a large conversion in 2026 affects 2028 premiums. Sizing conversions to stay under the IRMAA floor each year avoids this entirely.

The QCD Strategy Most Retirees Skip

For those past 73 with RMDs already in effect, qualified charitable distributions (QCDs) offer a specific fix that provisional income planning alone cannot. A QCD transfers money directly from an IRA to a qualified charity. It satisfies the RMD requirement without the distribution appearing in adjusted gross income, which means it never enters the provisional income calculation.

The 2026 QCD limit is $111,000 per person, or up to $222,000 per couple. A retiree who rolls a prior 401(k) into a traditional IRA can then use QCDs from that IRA to satisfy RMDs, reducing provisional income and potentially keeping Social Security below the 85% threshold. The chain: 401(k) to rollover IRA, then QCDs from the IRA to charity. Each step is routine on its own, but the combination is rarely executed as a deliberate tax strategy.

Common Approaches to Reducing Provisional Income

1. Provisional income calculation: add all non-Social Security income to 50% of the annual Social Security benefit to determine the threshold. If the total exceeds $44,000 (married) or $34,000 (single), up to 85% of the benefit is taxable. Staying below the threshold may require limiting 401(k) withdrawals or carefully sizing Roth conversions.
2. Roth conversions in the gap between retirement and age 73: converting traditional balances at a controlled rate each year reduces future RMDs and future provisional income. For couples whose combined income already exceeds the first IRMAA threshold at $218,000 for joint filers, the Medicare premium math can factor into how conversions are sized.
3. For those 70½ or older with charitable giving plans, rolling over a 401(k) into an IRA and using QCDs of up to $111,000 per person to satisfy RMDs keeps distributions out of the calculation of provisional income. This can move Social Security taxation from 85% back toward 50%, or eliminate it entirely.