The city of Rockdale is located in east-central Texas approximately 206 miles south of the Texas-Oklahoma border and 144 miles north of the Gulf of Mexico The distance eastward to the Texas-Louisiana border is 204 miles, and Mexico lies 465 miles to the west. The exact location of the city is 30.39° north latitude and 97:0° west longitude, at an altitude of 467 feet above mean sea level.

The two largest cities nearest to Rockdale are Waco, which is 71 miles to the north, and Austin, which is 63 miles to the southwest. Houston, which is the largest city in Texas, is 134 miles southeast of Rockdale, and Dallas, the second largest city in Texas, is 170 miles to the north.

Rockdale is situated in the southwestern portion of Milam County. It is 14 miles east of the western boundary line and 9 miles north of the southern limit of the county. Milam County is bounded on the north by Bell and Falls counties, on the south by Burleson and Lee counties, on the west by Williamson County, and on the east by the Brazos River. The total area of the county iS 1,030 square miles, or 659,200 acres. The total land area is 1,027 miles, and the total inland water area is three square miles.