Interest on the federal debt is 1.4% of GDP; $200 billion. But, it is projected to ramp up fast for 2 reasons. 1st we add $1.4 tillion to it every year and 2nd interest rates are at historic lows. If the annual deficit remains unchanged for 3 years and if interest rates double interest on the debt would climb to close to 4% of GDP or $500 billion; and would consume about 15% of all federal spending.