Social Security is the largest single line in the federal budget — about $1.46 trillion in FY2024 — and is funded by a dedicated tax: the 6.2% FICA Social Security contribution (matched 6.2% by your employer, for 12.4% total), capped at the annual wage base.
Two programs make up almost all the spending: Old-Age and Survivors Insurance (OASI, ~$1.31T) pays monthly retirement benefits to about 52 million retirees plus survivor benefits to their families; Social Security Disability Insurance (SSDI, ~$151B) pays benefits to about 8 million working-age adults unable to work due to disability. Administrative costs of running the Social Security Administration are a small fraction of the total.