How heavy is a T. rex?

The Natural History Museum in London puts a normal adult Tyrannosaurus rex at 5,000 to 7,000 kg, with the largest specimens going over 8,000 kg. A 2011 study led by John Hutchinson reached a similar answer: adults of around 6,000 to 8,000 kg, and the biggest known skeleton, Sue, perhaps about 9,500 kg.

So 8 tonnes is a fair figure for a big, fully grown T. rex, a little above an average adult and a little below the record holders. Adults were roughly 12 m (40 ft) long, and the animal lived in North America 68 to 66 million years ago.

Sue, at the Field Museum in Chicago, is about 90% complete, 40.5 ft (12.3 m) long and 13 ft (4 m) tall at the hip. The museum says Sue probably weighed about nine tons in life. Digital models of the skeleton in the 2011 Hutchinson study gave anywhere from 9,500 to 18,500 kg, depending on how much flesh was added, and the authors considered the leaner models more plausible.

Scotty, at the Royal Saskatchewan Museum, was described in 2019 by a team led by Scott Persons of the University of Alberta. Its femur measures 590 mm around against Sue’s 580 mm, which works out to 8,870 kg for Scotty and 8,462 kg for Sue. That made Scotty the heaviest known T. rex on paper, but the gap is only about 5%, and scientists including Hutchinson and the Field Museum’s Pete Makovicky called the two too close to separate. Guinness World Records also calls it too close to call.

There are two main methods. The first measures the circumference of the big weight-bearing leg bones, mainly the femur, and plugs it into equations built from living mammals and reptiles of known weight. Those equations were made for four-legged animals, so for two-legged dinosaurs like T. rex a correction is applied. This is how Scotty’s 8,870 kg was calculated.

The second builds a 3D model of the whole animal around a scanned skeleton, adds muscle, organs and air spaces, and works out the weight from the volume. The results swing a lot with how bulky the model is made, which is why Sue’s estimates range from 9.5 to 18.5 tonnes. A 2020 review by Nicolás Campione and colleagues compared more than 400 reconstructions from 115 years of research and found the two methods tend to agree.

Older estimates were often lower. A 2004 study in Nature by Gregory Erickson’s team put Sue at about 5,650 kg.

T. rex hatched at well under 10 kg and became a multi-tonne giant in less than 20 years. By counting growth rings in fossil bones, Erickson’s team found a growth spurt that began at about age 14 and lasted around four years, with a top rate of 2.1 kg a day. The 2011 Hutchinson study revised that peak to about 1,790 kg a year, nearly 5 kg a day. The Field Museum estimates Sue gained 35 to 45 lb a week as a teenager, reached adult size by about 20 and lived to about 33.

Sue and Scotty are probably not the biggest T. rex that ever lived. In 2024, Jordan Mallon and David Hone modelled a population of 140 million T. rex and estimated the very largest individuals could have reached about 15,000 kg and 15 m, around 70% heavier than Scotty. They stress this is a starting hypothesis, not a measured fossil, and that a find that big may be thousands of years away at current collecting rates.