San Marcos is located at 29°52'58" North, 97°56'26" West (29.882778, -97.940833).
According to the United States Census Bureau, the city has a total area of 18.32 mi2 (47.45 km2). 18.21 mi2 (47.16 km2) of it is land and 0.11 mi2 (0.29 km2) of it is covered by water. The total area is 0.62% water.
San Marcos is located in the Central Standard Time (CST) timezone, GMT-06:00. It observes Daylight Saving Time (DST).