She Moves Her Savings Into Treasuries to Earn More Interest. The Investment Pulls Another $10,074 of Her Social Security Into Taxable Income

A 10-year Treasury yield of 5.0% looks like a gift to a retiree with $250,000 sitting in a bank account. The national average 12-month CD pays just 1.8%, so moving the money feels like an easy call.

For a Social Security recipient, though, interest does double duty. It gets taxed on its own, and it can also pull more of her monthly check into taxable income. For a retiree with substantial savings, that means an attractive Treasury yield may come with a bigger tax bill than expected.

That $250,000 Actually Earns at Today’s Yields

Yields near 5% show up mostly at longer maturities. The 7-year Treasury trades at 4.9% and the 20-year at 5.35%, while the 1-year pays 4.4%.

At a 5% annual yield, $250,000 would generate roughly $12,500 in annualized returns if held to maturity. The amount and timing of her taxable interest depend on the securities she buys and what she pays for them. Treasury interest is generally exempt from state income tax but remains subject to federal income tax.

Combined Income Decides How Much of Your Check Gets Taxed

The IRS uses a figure often called combined income. It is your adjusted gross income, plus any tax-exempt interest, plus half of your Social Security. If that total tops $25,000 ($32,000 if married filing jointly), part of your benefits becomes taxable.

Up to 85% of benefits can be counted once combined income passes $34,000 ($44,000 if you are married filing jointly). Tax-exempt municipal bond interest still goes into the formula, so switching to munis won’t keep that income out of the calculation.

How $12,500 of Interest Drags $10,074 of Benefits Into the Tax Return

Picture a single filer getting the typical retirement benefit of $2,071 a month after the 2026 cost-of-living raise. That’s $24,852 a year. She also takes $20,000 a year from her IRA.

Before the Treasury purchase, her combined income is $32,426. Running the IRS worksheet, $3,713 of her benefits are taxable.

Add the $12,500 of interest and combined income rises to $44,926.

Now $13,787 of her benefits are taxable, an increase of $10,074. Each dollar of interest pulled about 80 cents in benefits into taxable income along with it.

The 50% and 85% figures control how much of your benefit counts as income, and your bracket decides the actual bill.

For 2026, a single filer’s 12% federal income-tax bracket covers taxable income above $12,400 through $50,400.

At 72, she qualifies for the larger standard deduction available to single filers 65 and older, plus the new $6,000 senior deduction if she meets the income requirements. Using 2026 rules, her taxable income would be about $22,137 after adding the Treasury interest and the newly taxable Social Security benefits.

That produces roughly $2,408 in federal income tax, compared with no tax in the original example. About 19% of her $12,500 in interest goes toward the additional federal bill, leaving an effective yield of roughly 4% before any other costs or adjustments.

Why IRA Withdrawals and Interest Compete for the Same Space

Every dollar from her IRA goes into the same formula as the interest. Once required minimum distributions (RMDs) kick in, those mandatory withdrawals could push her deeper into the zone where benefits become taxable, even without new investments. The extra standard deduction for filers 65 and older can soften the bill, but it doesn’t change how much of her benefit counts as income. This Social Security clawback is one of nine IRS rules that quietly drain retirement accounts, all charted in our free tax trap map.

Her income is well below the $109,000 threshold for Medicare surcharges in 2026. Medicare generally uses income from two years earlier to set those premiums, so a large increase today may affect a future bill. In her example, though, the immediate concern is how much of her Social Security becomes taxable.

Run the After-Tax Numbers Before Moving a Large Cash Balance

Before moving the money, run the IRS worksheet twice: once with your current income and once with the new interest added. Then compare after-tax yields across the options you’re considering.

Timing gives you some control. Treasury bills pay their interest when they mature. A bill bought this fall that matures next year moves that income into next year’s return. Longer bonds lock in the income stream, and the tax effect, for years, while spreading purchases across maturities makes it easier to adjust if your IRA withdrawals or benefits change.

Every return is a little different. Filing status, deductions and income mix can shift these results, so treat the example above as a starting framework for your own numbers.

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