Cadair Idris – Penygadair is a Hewitt of 892.7 m in Gwynedd. Only eight of the register's Wales hills are higher.
It overtops all four of its nearest neighbours; the tallest of them, Maesglase, reaches 678.5 m. It stands 225.8 m above its nearest neighbour, Tarren y Gesail, 7.1 km to the south. In the database's Dolgellau to Machynlleth region it is the highest of four register hills, above Maesglase, Tarren y Gesail and Tarrenhendre. The summit is marked by a trig point.