How heavy is a Stonehenge trilithon?

A trilithon is three stones: two uprights with a horizontal lintel resting on top. Stonehenge has five of them, set in a horseshoe about 13.7 m (45 ft) across inside the outer sarsen circle. Nobody has ever weighed one, so the figure is a sum: 25 + 25 + 7 = 57 tonnes.

The 25 tonnes is English Heritage’s average for a Stonehenge sarsen. The 7 tonnes for a lintel checks out against the stones’ size. An outer circle lintel is about 3.2 m long, 1 m wide and 0.8 m thick, roughly 2.6 cubic metres, and sarsen weighs about 2,380 kg per cubic metre, which gives just over 6 tonnes.

Treat 57 tonnes as the low end. The 25-tonne average covers every sarsen, including the outer circle uprights, which Wikipedia puts at about 4.1 m tall and 26 tons each, while the trilithon uprights rise from about 6 m (20 ft) for the smallest pair to 7.3 m (24 ft) for the largest. Sources disagree on how much more they weigh. English Heritage says the horseshoe sarsens typically weigh 20 tonnes, Wikipedia says the trilithon stones weigh up to 50 tons each, and a stone-by-stone survey puts the above-ground parts of the three trilithons still standing complete at about 63 to 69 tons each, before counting their buried bases.

The tallest trilithon stood at the head of the horseshoe, in the south-west, and reached about 7.3 m (24 ft). Only one of its uprights, stone 56, still stands. About 6.7 m (22 ft) of it shows above ground and more than 2 m is buried, for a total length of 8.71 m according to English Heritage. Its partner, stone 55, has fallen, and the lintel, stone 156, lies on the ground; two of these fallen stones partly cover the Altar Stone.

In 1994, archaeologist Julian Richards and engineer Mark Whitby built full-size concrete copies of the Great Trilithon for the BBC series Secrets of Lost Empires. They modelled both uprights on stone 56 and used concrete with a density of 2,380 kg per cubic metre, very close to sarsen. Each upright came out at about 40 tonnes and the lintel at 10 tonnes, so the whole trilithon weighs roughly 90 tonnes (198,000 lb).

How much of the buried part is counted makes a big difference. The same stone-by-stone survey puts just the above-ground part of stone 56 at about 23 tons. Guinness World Records says single sarsen blocks in the trilithons weigh over 45 tonnes, while English Heritage names the Heel Stone outside the circle as the largest stone, at about 30 tons on one page and more than 36 tonnes on another. The true figure for the largest stones is uncertain.

The sarsens went up around 2500 BC. A 2020 chemical study matched 50 of the 52 surviving sarsens to West Woods in Wiltshire, about 25 km (15 miles) north of Stonehenge. Nobody knows exactly how they made the trip, but the idea most often tested is a wooden sledge dragged along a greased timber track.

In the Richards and Whitby experiment, 130 volunteers hauled a 40-tonne upright on an oak sledge up a 1 in 20 slope along a greased timber track. With the animal fat the builders would have had, they estimated about 200 people were needed, and that such a team could move a 40-tonne stone 30 km in about 12 days.

To stand a stone up, the builders dug a hole with one sloping side and tipped the stone in, then hauled it upright with plant-fibre ropes. The lintels were raised on timber platforms or ramps and locked on with joints borrowed from woodworking: tenons on top of the uprights fit into hollows under the lintels.