Combined income is defined as:
Your adjusted gross income
+ Nontaxable interest
+ ½ of your Social Security benefits
Below $25,000 combined income the SS income will not be taxed federally. Above $25,000 the following applies.
https://www.ssa.gov/benefits/retirement/planner/taxes.html#:~:text=between%20%2425%2C000%20and%20% 2434%2C000%2C%20you,your%20benefits%20may%20be%20t axable.
Some of you have to pay federal income taxes on your Social Security benefits. This usually happens only if you have other substantial income in addition to your benefits (such as wages, self-employment, interest, dividends and other taxable income that must be reported on your tax return).
You will pay tax on only 85 percent of your Social Security benefits, based on Internal Revenue Service (IRS) rules. If you:
file a federal tax return as an "individual" and your combined income* is between $25,000 and $34,000, you may have to pay income tax on up to 50 percent of your benefits.
more than $34,000, up to 85 percent of your benefits may be taxable.
file a joint return, and you and your spouse have a combined income* that is between $32,000 and $44,000, you may have to pay income tax on up to 50 percent of your benefits.
more than $44,000, up to 85 percent of your benefits may be taxable.