She Shifted Savings Into Treasuries for Better Yield. The Move Also Pushed Another $10,074 of Social Security Into Taxable Income

Moving a big cash reserve into Treasuries for a higher return may look simple at first, but the interest can set off a ripple effect in Social Security taxation and quietly raise a retiree’s federal tax bill.

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A 10-year Treasury yield of 5.0% can seem irresistible for someone holding $250,000 in a bank account. With the national average 12-month CD paying only 1.8%, moving the money may appear to be an obvious choice.

For someone receiving Social Security, however, interest works in two ways. It is taxable on its own, and it can also cause more of the monthly benefit to become taxable. For retirees with substantial savings, that attractive Treasury rate can therefore come with a larger-than-expected tax bill.

What $250,000 Can Earn at Current Yields

Yields around 5% are mostly available at longer maturities. The 7-year Treasury yields 4.9% and the 20-year Treasury yields 5.35%, while the 1-year note pays 4.4%.

At a 5% annual yield, $250,000 would produce about $12,500 in annualized interest if held to maturity. The exact amount and timing of the taxable interest depend on which securities are purchased and the price paid. Treasury interest is generally exempt from state income tax, but it remains subject to federal income tax.

Combined Income Determines How Much of a Benefit Is Taxed

The IRS uses a measure commonly known as combined income. It equals adjusted gross income plus any tax-exempt interest plus half of Social Security benefits. If that total exceeds $25,000 for single filers or $32,000 for married couples filing jointly, part of the benefit becomes taxable.

Once combined income goes above $34,000 for singles or $44,000 for joint filers, as much as 85% of benefits can be included in taxable income. Tax-exempt municipal bond interest still counts in the formula, so moving money into munis would not keep that income out of the calculation.

How $12,500 of Interest Can Pull $10,074 of Benefits Into Taxable Income

Consider a single filer receiving the typical retirement benefit of $2,071 a month after the 2026 cost-of-living increase. That works out to $24,852 a year. She also withdraws $20,000 annually from her IRA.

Before buying the Treasury, her combined income comes to $32,426. Under the IRS worksheet, $3,713 of her Social Security benefits are taxable.

After adding the $12,500 in interest, combined income increases to $44,926.

At that point, $13,787 of her benefits become taxable, which is $10,074 more than before. In other words, each dollar of interest caused about 80 cents of additional Social Security benefits to be taxed.

The 50% and 85% thresholds determine how much of the benefit counts as income, while the tax bracket determines the final bill.

For 2026, the 12% federal income-tax bracket for a single filer applies to taxable income above $12,400 and up to $50,400.

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

That would create about $2,408 in federal income tax, compared with no tax in the original example. Roughly 19% of the $12,500 in interest would be absorbed by the higher federal tax bill, leaving an effective yield of about 4% before any other adjustments or costs.

Why IRA Withdrawals and Interest Compete for the Same Tax Space

Every dollar taken from the IRA feeds into the same formula as the interest. Once required minimum distributions, or RMDs, begin, those mandatory withdrawals can push a retiree deeper into the range where benefits become taxable, even without new investments. The extra standard deduction for filers age 65 and older can reduce the tax bill, but it does not change how much of the benefit counts as income. This Social Security clawback is one of nine IRS rules that can quietly drain retirement accounts, all covered in our free tax trap map.

Her income remains well below the $109,000 threshold for Medicare surcharges in 2026. Medicare generally uses income from two years earlier when setting premiums, so a large jump now could affect a future bill. In this example, though, the immediate issue is how much of her Social Security becomes taxable.

Check the After-Tax Result Before Moving a Large Cash Balance

Before shifting the money, run the IRS worksheet twice: once using your current income and once with the new interest included. Then compare the after-tax yield across the options you are considering.

Timing also matters. Treasury bills pay interest when they mature, so a bill bought this fall and maturing next year shifts that income into next year’s return. Longer bonds lock in both the income stream and the tax effect for years, while spreading purchases across maturities can make it easier to adapt if IRA withdrawals or benefit amounts change.

Every return is different. Filing status, deductions, and the mix of income can all change the outcome, so treat this example as a starting point for your own calculations.