Social Security income is only federally taxed if an individual or married couple's total combined income exceeds a certain threshold. Each state has its own unique rules about taxation of Social Security benefits.
The federal government uses combined income to determine if Social Security benefits can be taxed. For an individual, this amounts to adjusted gross income and non-taxable interest plus one half of the individual's benefits. As of 2015, if this amount is below $25,000, then benefits are not taxed. The threshold for married couples is $32,000. Between 50 to 85 percent of Social Security benefits can be taxed if the combined income exceeds these amounts.