Newcastle is an unincorporated town and census-designated place (CDP) in Placer County, California. Newcastle is located 8 miles (12.9 km) northeast of Rocklin. It is nestled in the Sierra Nevada foothills, 31 miles northeast of Sacramento. Newcastle generally has moderate winters and warm summers. The latitude of Newcastle is 38.874N, the longitude is -121.132W, and an elevation of 945 ft. The town's zip code is 95658 and area code 916. The population of the CDP as of the 2010 U.S.