Whether the IRS taxes your Social Security, and how much of it, is decided by a single figure called combined income. Below a fixed threshold, none of the benefit is taxed. Above it, up to 85 cents of every benefit dollar is pulled into your ordinary taxable income. The threshold does not move with your wealth or your spending. It moves with which accounts your retirement income comes out of.
We entered one single retiree into our calculator three times to price the effect. Each version held the same $1,000,000, spent the same $66,000 a year, and collected the same $30,000 Social Security check starting at 67. The only thing that changed was where the million sat. Held entirely in a traditional 401(k), the plan reached age 95 in 48% of simulated runs and owed federal tax every single year of retirement. Split half into a Roth, the identical plan reached 95 in 65% of runs and stopped owing tax altogether once the traditional account drained. Same money, same spending, same benefit. Combined income is the gap between those two outcomes.
Social Security taxation runs on a measure of income that appears nowhere else on your return. The IRS calls it combined income, and the term provisional income means the same thing.
The formula. Combined income = your adjusted gross income (AGI) + any tax-exempt interest you earned + one half of your annual Social Security benefits.
What counts and what does not is the whole story. Combined income includes withdrawals from a traditional 401(k) or IRA, pension payments, wages, interest, dividends, and capital gains. It even includes tax-exempt municipal bond interest, otherwise invisible on your return. It counts only half of the benefit itself. And it leaves out qualified Roth withdrawals entirely. Your total then lands in one of three bands, and the band sets the share of the benefit that becomes taxable.
| Combined income (single) | Combined income (married, joint) | Share of benefit taxable |
|---|---|---|
| Under $25,000 | Under $32,000 | 0% |
| $25,000 to $34,000 | $32,000 to $44,000 | Up to 50% |
| Over $34,000 | Over $44,000 | Up to 85% |
The top band is a share of the benefit, not a tax rate. Reaching it does not mean 85% of your check is taken. It means up to 85 cents of every benefit dollar joins your ordinary taxable income, which is then taxed at your regular rate, for most retirees 10%, 12%, or 22%. More outside income means more of the benefit counts.
In the version holding everything in a 401(k), every dollar of spending is a traditional withdrawal, and every one of those dollars lands in combined income. On the median path her combined income sits far above the $34,000 single ceiling for all thirty years, so the benefit is taxed at the 85% maximum throughout. At 72, roughly $28,850 of her $33,942 benefit is pulled into taxable income. The calculator's forced withdrawals at 75, the required minimum distributions on a large 401(k), only deepen the effect, driving taxable income up even in years she would rather have spent less. Over the full retirement, the median all-traditional path hands the IRS about $477,000, and reaches 95 in fewer than half of all runs.
A qualified Roth withdrawal is invisible twice. It is not taxed coming out, and it does not appear in adjusted gross income, so it never enters combined income. In the half-Roth version the retiree draws her traditional account first, paying the same tax as the all-traditional plan for about a decade, then crosses over to the Roth. Once she does, her only income counted by the formula is half her benefit, roughly $20,000, back under the $25,000 floor. The taxable share of her benefit falls to zero, her tax bill drops to zero with it, and the median lifetime tax lands near $122,000, about a quarter of the all-traditional figure. That shielding is why the same plan reaches 95 in 65% of runs rather than 48%.
A taxable brokerage account sits between the two. Only the gain portion of a brokerage withdrawal counts toward combined income, not the return of your original basis, and long-term gains are taxed at their own lower rates. A third run of the same retiree, holding a quarter of the money in a 401(k), half in a brokerage, and a quarter in a Roth, reached 95 in 70% of runs. Spreading the same million across account types with different tax treatment is the lever, and it is the case for holding both Roth and traditional accounts rather than betting everything on one.
The One Big Beautiful Bill Act, signed in July 2025, did not repeal the tax on benefits despite headlines to that effect. It added a deduction of up to $6,000 per person age 65 or older, up to $12,000 for a qualifying couple, available whether or not you itemize. It phases out above $75,000 of modified AGI for singles and $150,000 for joint filers, applies to tax years 2025 through 2028, and then expires. What it does not touch is the combined-income formula or the three bands. It works downstream, shrinking the taxable income that results, enough that the Treasury expects about 88% of beneficiaries to owe nothing on their benefits during the window. The relief is real, but it is temporary, it fades for higher earners, and the underlying tax returns in full in 2029.
Everything above is federal, and your state's treatment tells you nothing about it. The large majority of states do not tax Social Security at all. A small and shrinking number still do, each with its own carve-outs tied to age or income. If you are weighing where to retire, check your specific state's rule rather than assuming the federal answer carries over.
Whether your Social Security is taxed is neither random nor fixed. It tracks one number, your combined income, and the largest input to that number is which accounts your retirement income comes out of. A traditional withdrawal counts in full and can push the benefit to the 85% ceiling. A Roth withdrawal counts for nothing. Holding both, and drawing them in the right order, is what moved the same retiree from a plan taxed every year to one that stopped owing entirely. Enter your own benefit, balances, and spending in the RetirFi calculator and watch the drawdown order move your tax on Social Security across a full retirement. When to claim is a separate decision, covered in our guide to the break-even age for claiming at 62, 67, or 70.
Enter your numbers and run a Monte Carlo simulation to see how this plays out for your specific timeline and assets.
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