Buskerud extends from Hurum at the Oslofjord to the Halling mountains and Hardanger. The county is conventionally divided into traditional districts. These are Eiker, Ringerike, Numedal and Hallingdal. Hønefoss is the district capital of Ringerike.

Its western part is a mountainous plateau with forested valleys and high, grassy pastures; its eastern part contains a lowland basin with many lakes and streams. Tyrifjorden and Krøderen are the biggest lakes. Numedalslågen, the third longest river in Norway, starting in Hordaland, runs through Buskerud unto Vestfold where it reaches the sea, while river Begna sweeps into lake Sperillen.