How heavy is the iceberg that sank the Titanic?

No one weighed or surveyed the iceberg. It was seen only briefly, at close range, on a moonless night. The figure of 2 million tonnes comes from Grant Bigg and David Wilton of the University of Sheffield, writing in the journal Weather in April 2014. They started from what survivors reported: a berg about 50 to 100 ft (15 to 31 m) high and about 400 ft (122 m) long.

Next they worked out how much of it was hidden. The density of ice alone says only about 13% of an iceberg sits above water, but icebergs are worn into irregular shapes, so the hidden depth is more like 5 times the visible height. That makes the visible part about one-sixth of the total height, and puts the Titanic’s berg at least 90 to 185 m (300 to 600 ft) deep.

Then they applied a stability limit. Icebergs melt mostly from the sides, and once one gets too narrow for its height its centre of gravity sits too high and it rolls over. For a berg about 125 m (410 ft) long, that caps the total thickness at about 100 m (330 ft), so the part above water was probably 15 to 17 m (50 to 60 ft) tall, in line with lookout Frederick Fleet’s 50 or 60 ft. That size gave them a mass of about 2 million tonnes. Separately, their drift model’s best match for the berg had melted down to 2.1 million tonnes by mid-April 1912.

A quick estimate from the visible part alone lands far lower. Take the part above water as a box 120 m long, 40 m wide and 20 m tall. A jagged, peaked berg fills well under half of that box; at 45% it holds about 43,000 m³ of ice. Glacier ice weighs about 900 kg per m³, so that is about 39,000 tonnes above the surface.

Ice is about 88% as dense as seawater, so only about one-eighth of a floating berg’s volume shows. Multiply by eight: 39,000 × 8 is about 310,000 tonnes, or roughly 300,000 tonnes. Change the height and width within what witnesses said and you get anything from about 150,000 to 550,000 tonnes.

The gap comes from the hidden shape. The box method treats the underwater part as a bigger copy of the ragged peak. Bigg and Wilton’s approach gives the berg a solid body up to 100 m thick under a peak only 15 to 17 m tall, which holds far more ice. Size classes back the higher figure: Statistics Canada lists the class of “medium” icebergs, 16 to 45 m above the water and 61 to 120 m long, at about 2 megatonnes.

Lookout Frederick Fleet, who spotted it from the crow’s nest, agreed at the US Senate inquiry that it was about 50 or 60 feet (15 to 18 m) high. His fellow lookout Reginald Lee told the British inquiry it was higher than the forecastle and came out of the haze as a dark mass, with only a white fringe along the top.

Quartermaster Alfred Olliver, who saw it pass alongside, said it was about the height of the boat deck, “if anything, just a little higher”, and dark blue rather than white. He only saw the top and could not judge its length. Quartermaster George Rowe put it at about 100 ft (30 m), and passenger Eleanor Cassebeer at 75 to 100 ft (23 to 30 m) out of the sea.

Bigg and Wilton’s ocean and iceberg model traced the most likely candidate to southwest Greenland. It broke off in early autumn 1911 as a berg about 500 m long, 300 m deep and 75 million tonnes, then melted to about 2.1 million tonnes on its way south. Another study pointed to the Jakobshavn Glacier on Greenland’s west coast.

Several bergs near the wreck site were photographed in the following days, some with what looked like a streak of red paint near the waterline, including one taken from the cable ship Minia that Bigg and Wilton say fits their size estimate. Once the berg reached the warmer water of the Gulf Stream, it probably lasted only a week or two more.