Glossary
Provisional income
AGI without benefits + tax-exempt interest + half your Social Security — decides how much of your benefit is taxed.
Provisional income (the IRS also says "combined income") is adjusted gross income excluding Social Security, plus tax-exempt interest, plus 50% of your Social Security benefits for the year.
If it is under $25,000 (single) or $32,000 (joint), benefits are tax-free. Between that and $34,000/$44,000, up to 50% is taxable; above, up to 85%.