Nyandarua County is a county in the former Central Province of Kenya, with its capital and largest town being Ol Kalou. Formerly the capital was Nyahururu, which is now part of Laikipia County. Nyandarua County had a population of 596,268 by 2009 and 638,289 by 2019 and an area of 3,304 km2. The county is located on the northwestern part of the old Central Province, and contains the Aberdare Ranges.

Kiambu County is a county in the former Central Province of Kenya. Its capital is Kiambu and its largest town is Thika town. Kiambu County is the second most populous county in Kenya after Nairobi County. Kiambu County borders Nairobi and Kajiado Counties to the South, Machakos to the East, Murang'a to the North and North East, Nyandarua to the North West, and Nakuru to the West. It has a population of 2,417,735.