<P> The Congressional Budget Office (CBO) projects budget data such as revenues, expenses, deficits, and debt as part of its "Long - term Budget Outlook" which is released annually . The 2014 Outlook included projections for debt through 2039 and beyond . CBO outlined several scenarios that result in a range of outcomes . The "Extended Baseline" scenario and "Extended Alternative Fiscal" scenario both result in a much higher level of debt relative to the size of the economy (GDP) as the country ages and healthcare costs rise faster than the rate of economic growth . CBO also identified scenarios involving significant austerity measures, which maintain or reduce the debt relative to GDP over time . </P> <P> The sooner austerity steps are taken (e.g., raising revenues or reducing spending or a combination of both), the smaller those changes can be to maintain or reduce the debt level projected in 2039 . For example, under the baseline scenario, stabilizing the ratio of debt to GDP at 74% (its current level) would require non-interest spending cuts and / or tax hikes of 1.2% of GDP annually if implemented in 2015, or 1.5% of GDP annually if begun in 2020 . To bring the debt - to - GDP ratio back to its 40 - year average of 39%, the austerity measures required would be 2.6% GDP if begun in 2015 and 3.2% GDP if begun in 2020 . Over the 1974 to 2013 period, federal spending averaged 20.5% GDP and revenue 17.4% GDP . For scale, a 2% GDP reduction in spending would be approximately a 10% spending cut . </P> <P> During FY2016, the federal government collected approximately $3.27 trillion in tax revenue, up $18 B (billion) or 1% versus FY2015 . Primary receipt categories included individual income taxes ($1,546 B or 47% of total receipts), Social Security / Social Insurance taxes ($1,115 B or 34%), and corporate taxes ($300 B or 10%). Other revenue types included excise, estate and gift taxes . </P> <P> FY 2016 revenues were 17.8% of gross domestic product (GDP), versus 18.2% in FY 2015 . Tax revenues averaged approximately 17.4% GDP over the 1980 - 2015 period, generally ranging plus or minus 2% from that level . </P>

What are the biggest sources of revenue for the federal government