The Town of New Castle is a Home Rule Municipality in Garfield County, Colorado, United States. The population was 4,518 at the 2010 census, up from 1,984 at the 2000 census.
New Castle is located at 39°34′20″N 107°32′6″W (39.572304, -107.534941), on the north side of the Colorado River, just east of where the river cuts through the Grand Hogback. Interstate 70 passes through the town following the river, with access from Exit 105. I-70 leads east 12 miles (19 km) to Glenwood Springs, the county seat, east 169 miles (272 km) to Denver, and west 74 miles (119 km) to Grand Junction. U.S. Route 6 is Main Street in New Castle and forms a parallel route to I-70 for local traffic. US-6 joins I-70 5 miles (8 km) to the east at Chacra and 17 miles (27 km) to the west at a point west of Rifle.
According to the United States Census Bureau, the town has a total area of 2.7 square miles (7.0 km2), of which 2.7 square miles (6.9 km2) is land and 0.04 square miles (0.1 km2), or 1.21%, is water.
As of the census of 2000, there were 1,984 people, 705 households, and 537 families residing in the town. The population density was 850.6 people per square mile (328.8/km2). There were 731 housing units at an average density of 313.4 per square mile (121.1/km2).