Overview: Not all Social Security recipients owe federal income tax on their benefits — but many do, and the formula for determining taxability catches some recipients by surprise.

Whether Social Security benefits are taxable depends on a figure the IRS calls combined income (also called provisional income):

Combined Income = Adjusted Gross Income (AGI) + Nontaxable Interest + ½ of Social Security Benefits

Note that tax-exempt municipal bond interest does count in this formula, even though it doesn't appear in AGI. This design means even income that is otherwise tax-free can trigger taxation of Social Security benefits.

The thresholds that determine what fraction of benefits may be taxable:

Filing Status	No Tax	Up to 50% Taxable	Up to 85% Taxable
Single / HOH / Qualifying Widow(er)	Under $25,000	$25,000–$34,000	Over $34,000
Married Filing Jointly	Under $32,000	$32,000–$44,000	Over $44,000
Married Filing Separately (lived together)	—	—	Virtually all taxable

Critical clarification: "Up to 85% taxable" does not mean an 85% tax rate. It means up to 85% of the benefit amount is included in taxable income, then taxed at the recipient's regular marginal bracket (10%, 12%, 22%, etc.). At least 15% of Social Security benefits is always federal-income-tax-free.

These thresholds were set in 1983 and 1993 and have never been indexed for inflation. As nominal incomes have risen over decades, an increasing share of beneficiaries have crossed into the taxable zone — a design feature that generates growing revenue for the Social Security trust funds without requiring Congress to vote for an explicit tax increase.

SSI benefits, in contrast, are never taxable. State taxation of Social Security benefits varies; most states do not tax them, but a handful do with varying exemptions.