Waun Fach is a Hewitt of 811 m in Powys. Its drop of 622 m is 77% of its height: the col linking it to higher ground is only 189.1 m above sea level.
It overtops all four of its nearest neighbours; the tallest of them, Waun Rydd, reaches 769.2 m. It stands 202 m above its nearest neighbour, Mynydd Troed, 5.0 km to the west. Of the register hills in Powys, only Pen y Fan and Cadair Berwyn stand higher.