How Big Was the Tyrannosaurus rex? They May Have Been Larger Than Fossil Evidence Suggests

Key Takeaways on How Big was the Tyrannosaurus rex?

• A Tyrannosaurus rex would have been about 40 feet long and stood 13 feet high.
• We know how big a T. rex is due to the fossils researchers have already discovered. From these fossils, researchers can estimate the average size of T. rex, but it’s possible the dinosaur could have been even bigger.
• The T. rex wasn’t the biggest theropod predator. Spinosaurus, Carcharodontosaurus, and Giganotosaurus likely had it beat.

Tyrannosaurus rex is undoubtedly one of the most famous dinosaurs. Whether it was the largest amongst the meat-eating theropods, however, is questionable and often part of intense debate.

Based on the fossil record, T. rex reached lengths of up to about 40 feet (12 or 13 meters), a height of around 13 feet (four meters), and weighed as much as 7 tons. Though research suggests this may just be scratching the surface of the truly largest T. rex that tyrannized the late Cretaceous period.

Researchers have estimated that during the roughly 2.4 million years that T. rex walked the Earth during the Late Cretaceous, over 2 billion of these dinosaurs existed. At any given time, they believe that around 20,000 adult T. rex were alive.

Those are staggering numbers. It also provides a clear indication that the fossil record of this dinosaur is minuscule, with only 30 to 40 known specimens.

“When you consider the fact that there were millions, perhaps even billions, of T. rex individuals that ever existed, that's just a drop in the bucket,” says Jordan Mallon, a paleontologist at the Canadian Museum of Nature.

Read More: What a Tyrannosaurus Rex Skull Tells Us About Its Intelligence

How Big Was the T. rex and How Do We Know

The exact number and what qualifies as a “good” specimen depends on who you ask, says Mallon.

In 2024, Mallon and David Hone, at Queen Mary University of London, published a paper suggesting that T. rex could well have reached even greater sizes, up to 70 percent heavier than what the fossil evidence indicates.Based on computer modeling, the study suggested T. rex at the ultimate size may have weighed as much as 15 tons and reached lengths of around 50 feet (around 15 meters).

“We're only talking about one individual,” he says. "You know, it's like the most extreme T. rex.”

In his view, this hypothesis opens up a host of questions and possibilities, such as whether a dinosaur of this size could even support its own weight and the ecological implications it raises.

“T. rex was probably feeding primarily on the duck-billed dinosaurs and the horned dinosaurs that lived in its environment,” Mallon says. “There's a potential that the biggest T. rex were able to take down sauropods.”

This study, which Mallon describes as a thought experiment, however, is difficult to test based on the current fossil record. Whether or not truly gigantic T. rex fossils of this kind are ever found remains to be seen.

“The likelihood that the one biggest T. rex, or even the 100 biggest T. rex that ever lived, are preserved in the fossil record and that we will find them is very, very low,” says Mallon.

Yet it leaves open the intriguing prospect that the already gigantic T. rex that stalked the big screen in Jurassic Park may have reached even more epic sizes during its time on Earth.

Was The T. Rex The Biggest Dinosaur?

Even considering the potential for an individual T. rex growing to about 50 feet (15 meters) in length, it still would not have claimed the title of the largest dinosaur. The towering sauropods, of course, reached far greater heights and lengths than any of the meat-eating theropods.

Dreadnoughtus, for example, likely reached lengths of up to about 85 feet (26 meters) and weighed as much as 65 tons.

Several other theropods grew to large sizes. Competing with T. rex for the title of largest theropod are other large dinosaurs such as Spinosaurus, Carcharodontosaurus, and Giganotosaurus.

Spinosaurus is typically considered the largest theropod, with estimates suggesting it weighed approximately 7.4 tons and reached lengths of up to 45 feet (14 meters). Carcharodontosaurus, meanwhile, was likely around the same size as T. rex but had a lighter build.

The Dinosaur Fossil Scarcity

A problem, though, says Mallon, is the scarcity of fossil samples and complete specimens for many species, including these other large theropods. Those several dozen T. rex specimens represent a tiny fraction of the actual number that existed, but when compared to other dinosaur fossil finds, it’s actually a “pretty good” collection, he adds.

“We really don't know what's out on the tail ends of those [size] distributions,” he says. “And the poorer the fossil record, the harder it is to say [their largest size].”

That’s part of the reason he steers away from the frequently recurring debates on which of the theropods tipped the scale as the largest. “It's really hard to know,” he says.

Importantly, the findings from his 2024 paper that T. rex could have reached even greater sizes than the fossil record suggests aren’t limited to this meat-eater.

“It applies to Spinosaurus, and the other big meat eaters out there, which are known from even fewer fossils,” he says. “The same argument applies to every fossil you can think of.”

What Animal Has The Closest DNA to The T. Rex

In 2008, researchers published a paper examining 68-million-year-old proteins found in collagen extracted from a fossilized T. rex femur. These findings, still often referenced today, appear to indicate that T. rex is most closely linked to the far less terrifying modern chicken and ostrich than other animals like alligators.

At that time, headlines ran suggesting the lowly chicken is the mighty T. rex’s closest living relative. That, however, is not entirely accurate as all birds are essentially modern-day dinosaurs. The authors of the paper sought to establish T. rex’s place in a larger web of life.

“The point is that among all living animals, T. rex is most closely related to birds because birds are dinosaur descendants,” says Mallon, who wasn’t involved in that paper. “There's no one particular bird that is more closely related to T. rex than other birds.”

At this stage, scientists have amassed a wide range of evidence from the fossil record, biochemical analyses, and more showing that modern-day birds are dinosaur descendants, he says. “The evidence just continues to grow, and at this stage, it's undeniable.”

Read More: The Tyrannosaurus Rex Origin Story May Not Have Started in North America

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• Natural History Museum. Dinosaurs in the Late Cretaceous
• Canadian Museum of Nature. Size of T. rex
• Britannica. Titanosaurs: 8 of the World’s Biggest Dinosaurs
• Natural History Museum. Carcharodontosaurus
• Natural History Museum. What was the biggest dinosaur?