Let's start by explaining how Uncle Sam taxes Social Security benefits. The federal government can tax up to 85% of your Social Security benefits. To figure the tax on your benefits, you first need to determine your "provisional income." This is equal to the total of (1) 50% of your Social Security benefits, (2) your tax-exempt interest, and (3) your adjusted gross income (not including the student loan interest deduction or the tuition and fees deduction).

If you're single, none of your Social Security benefits are taxed if your provisional income is less than $25,000. If your provisional income is between $25,000 and $34,000, then up to 50% of your Social Security benefits may be taxable. If your provisional income is more than $34,000, then up to 85% of your benefits may be taxed.

For married couples filing a join return, your Social Security benefits are safe if your provisional income is less than $32,000. The 50% rate applies if your provisional income is between $32,000 and $44,000. If your provisional income exceeds $44,000, up to 85% of your benefits may be taxable.

Stimulus Check's Impact

Any additional taxable income will increase your adjusted gross income, which then increases your provisional income for Social Security tax purposes. If your provisional income goes up enough to move you from the 0% to the 50% bracket, or from the 50% to the 85% bracket, then you're looking at a tax increase. So, the key question is whether your stimulus checks boost your taxable income.