Composed mainly of chalk and clay excavated from the surrounding area, the mound stands 40 metres (130 ft) high and covers about 5 acres (2 ha). It is a display of immense technical skill and prolonged control over labour and resources. Archaeologists calculate that Silbury Hill was built about 4750 years ago and that it took 18 million man-hours, or 500 men working 15 years (Atkinson 1974:128) to deposit and shape 248,000 cubic metres (324,000 cu yd) of earth and fill on top of a natural hill. Euan W. Mackie asserts that no simple late Neolithic tribal structure as usually imagined could have sustained this and similar projects, and envisages an authoritarian theocratic power elite with broad-ranging control across southern Britain.
The base of the hill is circular and 167 metres (548 ft) in diameter. The summit is flat-topped and 30 metres (98 ft) in diameter. A smaller mound was constructed first, and in a later phase much enlarged. The initial structures at the base of the hill were perfectly circular: surveying reveals that the centre of the flat top and the centre of the cone that describes the hill lie within a metre of one another (Atkinson 1974:128). There are indications that the top originally had a rounded profile, but this was flattened in the medieval period to provide a base for a building, perhaps with a defensive purpose.
The first phase, carbon-dated to 2400 ±50 BC, consisted of a gravel core with a revetting kerb of stakes and sarsen boulders. Alternate layers of chalk rubble and earth were placed on top of this: the second phase involved heaping further chalk on top of the core, using material excavated from an encircling ditch. At some stage during this process, the ditch was backfilled and work was concentrated on increasing the size of the mound to its final height, using material from elsewhere. The step surrounding the summit dates from this phase of construction, either as a precaution against slippage, or as the remnants of a spiral path ascending from the base, used during construction to raise materials and later as a processional route.
Sillbury hill is similar in size to some of the smaller Egyptian pyramids of the Giza Necropolis. Its purpose however, is still highly debated.