Fan Llia is a subsidiary summit of Fan Fawr in the Fforest Fawr section of the Brecon Beacons National Park, Wales.In common with other peaks in the Fforest Fawr uplands it lies within the county of Powys.The hill has the form of a broad ridge aligned south-south-west to north-north-east peaking at an altitude of 632m. It has a subsidiary top to the north which is known as Fan Dringarth and which attains a height of 617m. Its broad northern ridge is eventually truncated by the cliffs of Craig Cwm-du.
The hill is formed largely from sandstones of the Brownstones Formation of the Old Red Sandstone dating from the Devonian period. It southernmost slopes are formed in Plateau Beds Formation sandstones and an area in the north-east is underlain by sandstones and mudstones of the Senni Beds Formation. Rock exposures are infrequent - the hill being draped with glacial till and accumulations of peat.