Most retirees know their Social Security benefits might be taxed. Fewer know exactly which number determines how much. That number is called combined income, and where you land relative to the IRS thresholds will decide whether 0%, 50%, or 85% of your benefit is subject to federal tax in 2026. The Number That Determines Everything Combined income is your adjusted gross income, plus any tax-exempt interest (such as municipal bond interest), plus 50% of your annual Social Security benefit. That sum is the ...