Koga is a city located in Ibaraki Prefecture, Japan. As of 1 January 2024, the city had an estimated population of 137,512 in 58,276 households and a population density of 1113 persons per km². The percentage of the population aged over 65 was 28.98%. The total area of the city is 123.58 square kilometres (47.71 sq mi).

Ibaraki Prefecture is a prefecture of Japan located in the Kantō region of Honshu. Ibaraki Prefecture has a population of 2,828,086 and has a geographic area of 6,097.19 square kilometres. Ibaraki Prefecture borders Fukushima Prefecture to the north, Tochigi Prefecture to the northwest, Saitama Prefecture to the southwest, Chiba Prefecture to the south, and the Pacific Ocean to the east.