Which account a retiree taps for spending money can quietly decide how much of a Social Security check the IRS gets to tax. The reason is a calculation called combined, or provisional, income — a formula that adds up certain income sources to determine what share of benefits is taxable. Qualified withdrawals from a Roth account do not appear in that formula at all. A retiree who pulls a year’s living expenses from a Roth rather than from a traditional IRA can keep provisional income lower, and a lower provisional income can mean a smaller taxable slice of Social Security. The account choice, repeated year after year, adds up to real money.
How the combined-income formula decides what gets taxed
Social Security benefits are not automatically tax-free, and they are not automatically taxed either. The tax turns on a single figure: combined income, which the IRS defines as a person’s adjusted gross income plus any tax-exempt interest plus one-half of the Social Security benefits received during the year. That total is measured against two thresholds. Below the lower threshold, none of the benefit is taxable. Between the two thresholds, up to half of the benefit becomes taxable. Above the higher threshold, up to 85 percent of the benefit can be taxed — never more than 85 percent, but that is the ceiling.
The thresholds are set in statute and depend on filing status, with separate figures for single filers and married couples filing jointly. Because they are fixed rather than indexed, more retirees drift into the taxable ranges over time as ordinary incomes rise. What a retiree can still control is the income side of the formula, and that is where the account a withdrawal comes from becomes the lever.
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Why a Roth withdrawal stays out of the calculation
A traditional IRA or 401(k) was funded with pre-tax dollars, so every withdrawal is ordinary taxable income that flows straight into adjusted gross income — and therefore into the combined-income formula. A qualified Roth withdrawal is different. Roth accounts are funded with money already taxed, and once the account is at least five years old and the owner is 59½ or older, the distributions come out entirely tax-free. Because they are not taxable income, they never enter adjusted gross income, and they never raise provisional income. The IRS explanation of how Social Security benefits are taxed defines combined income around adjusted gross income, which a qualified Roth distribution leaves untouched.
The practical effect is that two retirees with identical spending can face very different tax bills on the same Social Security benefit. The one who covers expenses from a traditional IRA pushes combined income up with every withdrawal, potentially dragging a larger share of the benefit into the 50 percent or 85 percent taxable range. The one who covers the same expenses from a Roth adds nothing to the formula, and can hold the benefit below a threshold that the traditional-IRA retiree crosses. The Social Security Administration’s page on benefit taxation describes the same combined-income test from the benefits side.
Where the strategy helps and where it runs out
The tactic is most powerful for retirees sitting near a threshold rather than far above or below one. Someone whose income already sits well under the lower threshold owes no tax on benefits regardless of the account used, so the Roth choice changes nothing. Someone whose income is already high enough that 85 percent of the benefit is taxed has little room to fall back under a threshold from a single year’s withdrawal decision. It is the large middle group — retirees hovering where an extra few thousand dollars of ordinary income flips more of the benefit into the taxable column — that gains the most from routing spending through a Roth.
Required minimum distributions complicate the picture. Traditional IRAs and 401(k)s force taxable withdrawals starting at the age set in law whether the money is needed or not, and those mandatory distributions land in the combined-income formula. Roth IRAs carry no lifetime required distributions for the original owner, which is part of why they are useful for managing provisional income late in retirement. A retiree who converted some traditional savings to a Roth in earlier, lower-income years has more of this flexibility available; one who left everything in pre-tax accounts has less.
The larger point is that the tax on Social Security is not fixed by fate — it is shaped by the mix of accounts a household draws from and the order in which it draws from them. The Internal Revenue Service applies the same thresholds to everyone, but the amount of taxable income each retiree feeds into the formula is a choice made one withdrawal at a time. For households with both traditional and Roth balances, deciding which account funds a given year’s spending is not a minor bookkeeping detail; it is one of the few remaining levers over how much of a hard-earned benefit stays in the household.
This article was researched and drafted with the assistance of artificial intelligence.
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