How Big Was a Tyrannosaurus Rex?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
The best preserved specimens suggest an animal around twelve metres long, roughly four metres tall at the hip, and weighing something like eight tonnes. Those numbers come from real skeletons rather than imagination, and the methods used to reach them are worth understanding.
The numbers
The largest well studied specimens are around twelve to thirteen metres from snout to tail tip. Hip height, which is a better guide than total height for a two-legged animal, is close to four metres, so the head of a walking T. rex would have been roughly four to five metres up.
Weight estimates have ranged widely over the years and have generally been revised upwards. Current estimates for large adults cluster around eight tonnes, with some analyses suggesting the very biggest individuals could have exceeded that. The skull alone was about one and a half metres long.
How weight is estimated
You cannot weigh a fossil, so researchers use two main approaches. One builds a three dimensional model of the animal from the mounted skeleton, estimates the volume of flesh, and multiplies by a plausible density.
The other uses a relationship measured in living animals between the circumference of weight bearing limb bones and body mass. Both methods carry uncertainty, which is why published figures differ, but they now broadly agree with each other.
The bite
Tyrannosaurus rex had one of the strongest bites of any land animal ever measured or modelled. Estimates for the force at the back teeth run into the tens of thousands of newtons, far beyond any living predator.
The evidence is not just modelling. Fossil bones of other dinosaurs have been found with T. rex tooth marks that go straight through, and fossilised droppings contain crushed bone fragments. This was an animal that could bite through the skeleton of its prey.
What the arms were for
The famous short arms are around a metre long, which is small relative to the body but not small in absolute terms. Suggested functions include:
- •Holding prey close during feeding
- •Helping the animal rise from a lying position
- •Grip during mating
- •A shrinking leftover as the head took over the killing role
How we know about growth
Slicing thin sections of fossil bone reveals growth lines, a little like tree rings. Counting them across specimens of different sizes has produced a growth curve for the species.
It suggests T. rex grew slowly as a juvenile, then went through a rapid growth spurt in its teens, adding hundreds of kilograms a year, before levelling off. Most known individuals died in their twenties, and the oldest well studied specimen was around twenty eight.
How fast could it run
Estimates of Tyrannosaurus speed have come down considerably. Early suggestions of over forty kilometres per hour were based on limb proportions, but biomechanical modelling of the forces involved gives lower figures.
Current work generally puts its top speed somewhere in the range of a fast human jog, perhaps around twenty kilometres per hour, and several studies conclude it could not truly run with both feet off the ground at once. At eight tonnes, a fall at speed would likely have been fatal, which is a strong argument against the chase scenes in film.
The takeaway
A Tyrannosaurus rex was about twelve metres long, four metres tall at the hip and eight tonnes in weight, with a bite strong enough to crush bone, and those figures come from skeletons measured and modelled rather than guessed.