Federal interest expense, reported monthly by the U.S. Treasury, represents payments on the national debt across various security types. This includes marketable securities (Treasury bills, notes, bonds, and TIPS, approximately 75% of payments), non-marketable securities (savings bonds, special securities for state and local governments), and intragovernmental holdings. Interest costs are influenced by debt volume, maturity structure (average 5.7 years), and prevailing market rates. The weighted average interest rate on federal debt was 3.4% in 2023, with significant variation across security types. These expenses constitute a growing share of federal outlays (approximately 8% in 2023), projected by the CBO to reach 13% by 2033. The metric's sensitivity to interest rate changes makes it a crucial factor in fiscal planning, with each 1% rate increase adding approximately $240 billion to annual costs at current debt levels. Payment patterns show seasonal variations tied to security issuance schedules and interest rate cycles.