San Marcos, TX -- U.S. city in Texas Population (2000): 34733 Housing Units (2000): 13340 Land area (2000): 18.208921 sq. miles (47.160886 sq. km) Water area (2000): 0.113435 sq. miles (0.293794 sq. km) Total area (2000): 18.322356 sq. miles (47.454680 sq. km) FIPS code: 65600 Located within: Texas (TX), FIPS 48 Location: 29.879387 N, 97.938829 W ZIP Codes (1990): 78666 Note: some ZIP codes may be omitted esp. for suburbs. Headwords: San Marcos, TX San Marcos