In 2004 a team measured how fast T. rex grew by counting growth rings in bone. The study looked at more than 60 bones from 20 tyrannosaur specimens that died between ages 2 and 28. It measured seven T. rex and 13 specimens of three smaller relatives. Big bones lose their early growth rings as the marrow cavity widens, so the team read the rings in ribs, shin bones and hip bones, which stay solid and keep every ring. Sue's solid ribs gave the team the idea for the method. Researchers checked the ring method against modern alligators and lizards, whose bone rings form in a similar way. They estimated how heavy each dinosaur was by measuring the distance around its thigh bone, which carries the body's weight.

The result was a growth curve with a steep middle. T. rex grew from about 1 ton to about 6 tons in roughly four years, it grew fastest between about age 14 and age 18, and it stopped growing at about age 20, spending about two-thirds of its life growing. It gained about 70 percent of its adult weight during the teenage spurt. T. rex grew very fast as a teenager, and it lived fast and died young. A grown T. rex could weigh about six tons, like an elephant, and its teenage growth was like that of an African elephant, but elephants live far longer, and smaller tyrannosaur relatives had growth spurts too, but gained far less weight each day.

Weight is a hard number to pin down. Because nobody knows how plump a living T. rex was, researchers built weight models that range from severely undernourished to overly obese. A 2011 mass study built digital models of five T. rex skeletons, including Sue, from laser scans accurate to less than half an inch for skeletons up to 40 feet long. As of November 2026, those digital models put Sue at over nine tons, more than older guesses made with scale models, which can magnify even minor errors. A typical adult T. rex probably weighed between 5,000 and 7,000 kilograms, and it stood about 13 feet tall at the hips; estimates for the length of the biggest T. rex run from about 40 to 46 feet. The answer depends on the method and its assumptions.

Speed estimates for T. rex have changed as the models improved. As of November 2026, older models allowed 11 to 33 miles an hour, while a 2017 model put the likely top near 12 miles an hour. The 2017 speed model added bone stress as a new factor and assumed muscles were not the limit at top speed; it found T. rex was typically restricted to a brisk walk. Some scientists think an adult T. rex could not run well, and that younger and smaller ones may have been faster. Scientists think T. rex was not very fast, perhaps 10 to 20 miles an hour at its top speed. T. rex had very small arms for its big body, and scientists still debate what T. rex used them for. Young T. rex had arms closer in proportion to their bodies, so the arms might have helped grab food and may have mattered less as the animals grew.

The bite was the weapon, and it has been measured in several ways. As of May 2017, scientists put the bite of T. rex at about 8,000 pounds of force, more than twice that of the largest living crocodiles, which bite with about 3,700 pounds. Bite force alone does not tell how well an animal can crush bone: tooth pressure matters more, and T. rex tooth tips reached 431,000 pounds per square inch. T. rex is unique among reptiles in crushing bone like wolves and hyenas do, but without teeth that fit together tightly. Researchers built 3-D computer models of T. rex jaw muscles from what they measured on living crocodiles, and the bone-crushing bite let T. rex eat bones and marrow that smaller meat-eating dinosaurs could not use.

Scientists have argued over whether T. rex hunted its food or mostly ate animals that were already dead. One study suggests the vision of T. rex was too good for a pure scavenger, which supports the idea that it was a hunter. A healed bite wound in a plant-eater is read as a probable sign of predatory behavior, and failed attacks count as evidence too, because prey that got away with a wound that later healed show that tyrannosaurs hunted live animals. Holes in the lower jaw of Sue were once thought to be bite marks from another T. rex; a scientific paper argued that jaw holes like Sue's match a parasite infection that today harms birds, called trichomonosis. As of November 2026, a bone growth-ring study of the Jane specimen put her at 11 to 13 years old and about 21 feet long, which is why she is treated as a young animal.

Words to know

• specimen: one fossil animal
• scavenger: eats dead animals
• marrow: soft bone center