URDAIBAI covers an area of 220 km² with around 45,000 inhabitants. It has 22 municipalities, 11 of which lie wholly within its borders. To the north its border is the coast, where capes Matxitxako and Ogoño protect the entrance to the estuary and the island of Izaro stands guard. The rest of its borders follow the watershed of the River Oka. It is twelve kilometres in width and twenty in length.