Koga is a city located in Ibaraki Prefecture, Japan. As of January 1, 2008, the new city has an estimated population of 144,392 and a population density of 1,200 persons per km². The total area is 123.58 km². On September 12, 2005, Koga absorbed the towns of Sanwa and Sōwa (both from Sashima District) to create the new and expanded city of Koga. The new city hall is now located at the former town of Sōwa.