 In the Nebraska license plate system, Clay County is represented by the prefix 30 (it had the 30th-largest number of vehicles registered in the county when the license plate system was established in 1922).According to the US Census Bureau, the county has an area of , of which is land and (0.2%) is water.As of the 2000 United States Census, there were 7,039 people, 2,756 households, and 1,981 families in the county.