 It was dammed about northwest of Llangefni, just below the confluence of the tributary from Tregaian with other streams flowing from the northwest, and the reservoir has a catchment area of about .The reservoir is fed by two main streams, the Afon Frogwy entering from the west and the Afon Erddreiniog from the north east end.With a maximum length of and a total water surface area of , Llyn Cefni is the second largest body of water on the island after Llyn Alaw, also a reservoir.