How Big Was a T rex Brain? T rex Brain Size, Intelligence and a Human Comparison

A T rex brain weighed about 97 to 174 grams on the 2024 estimate, 6% to 12% of a human brain, and no study has given it an IQ. T rex brain size is an estimate, because the fossil shows the brain cavity and not the brain. Published estimates of its forebrain neurons run from 0.25 to 3.3 billion.

New here: the cavity is measured, but the brain inside it is assumed. For one skull, AMNH 5029, the 2023 study gives 343 g and the 2024 study 118 to 160 g. Published forebrain neuron estimates run about 13 times apart, from 0.25 to 3.3 billion. Below: what a skull can and cannot show, and how a T rex brain compares with a human’s, a crocodile’s and a raven’s.

Key takeaways

• Weight: about 97 to 174 g on the 2024 estimate (31% to 42% of the skull cavity), against 343 g and 202 g for two skulls in the 2023 estimate and 1,508 g for a human brain.
• Method: scientists measure the cavity (the endocast) and assume what share was brain: 31% to 42% in the 2024 study, as in a crocodile. The study also tried other ratios; we show only the ones we could check.
• Neurons: published forebrain estimates run from 0.25 to 3.3 billion, a 13-fold gap.
• Intelligence: neuron counts are not intelligence. A giraffe (1.7 billion) out-counts tool-using capuchin monkeys (1.1 billion).
• IQ: none. No IQ score exists for any dinosaur.

How big was a T rex brain?

About 97 to 174 grams on the 2024 estimate by Caspar and 10 colleagues. That T rex brain weight comes from the cavity in the skull, which held 314 to 414 milliliters in the three skulls measured.

The 2024 study used three adult skulls: AMNH 5117 (314 mL), AMNH 5029 (382 mL) and the skeleton known as Sue, whose 414 mL cavity is the largest. More cavity does not settle the question, because what counts is how much of it was brain.

Each skull gives a range. At 31% to 42% of the cavity, AMNH 5117 gives 97 to 132 g, AMNH 5029 gives 118 to 160 g and Sue gives 128 to 174 g (Table 1 of the 2024 preprint).

The 2023 estimate by Herculano-Houzel is higher: 343 g for the AMNH 5029 skull and 202 g for Sue, as listed in the 2024 paper. The gap between the 2023 and 2024 estimates is the heart of the debate, and the section on measuring a dinosaur brain explains it.

How does a T rex brain compare with a human brain?

A T rex brain weighed 6% to 12% of a human brain on the 2024 estimate, and even the 2023 study’s largest guess of 343 g is under a quarter. That is the T rex brain vs human brain answer in numbers. This dinosaur brain size comparison puts it beside other animals.

Weight alone misleads, because a T rex weighed tonnes: the three skeletons in the 2024 study were estimated at 5.5 to 8.1 tonnes. Our dinosaur brains study puts 31 dinosaurs on one scale of brain size for body size.

How do scientists measure a dinosaur brain?

They measure the empty brain cavity in the skull, called an endocast, because brain tissue is only rarely fossilized. With the bony braincase preserved, CT scanning gives the cavity’s volume.

The cavity is not the brain. In crocodiles the brain fills only part of it, so the 2024 study assumed a brain of 31% to 42% of the cavity for T rex. The lowest ratio it cites for the three longest American alligators measured earlier is 31%. Many earlier studies assumed 50%, and some assumed 100%. The study also tried a 57% ratio on one skull, which we leave out because we could not confirm its value.

For small bird-like dinosaurs the 2024 study assumed the brain filled the whole cavity, as in modern birds. For a large theropod like T rex it did not, and that choice moves the answer: for the AMNH 5029 skull the 2023 study gives 343 g and the 2024 study 118 to 160 g. The neuron estimates also depend on how tightly the cells were packed, crocodile-like or bird-like, and nobody can see that.

Few dinosaurs have even this much data. Only 45 of 1,851 named dinosaur genera have a measured brain, by our count from the Paleobiology Database on 2 November 2026.

Was T rex smart?

Nobody can measure it. The 2024 study found much lower neuron counts for T rex than the 2023 one, and called neuron counts flawed proxies for cognition.

In 2023 Herculano-Houzel estimated 2.2 to 3.3 billion forebrain neurons for T rex if its cells were packed like a bird’s. She called these baboon-like numbers and said they would make the animal long-lived and endowed with flexible cognition.

In 2024 a team of 11 researchers re-measured the skulls. At 31% to 42% of the cavity they found 0.25 to 0.36 billion neurons if the brain was packed like a crocodile’s, or 1.1 to 1.7 billion if like a bird’s. They also checked the 2023 life-history predictions, maturity at 4 to 5 years and a life of 42 to 49 years, and found the fossil evidence contradicts them.

Did a bigger brain mean a smarter dinosaur?

Not by itself. Caspar and colleagues reviewed relative brain size and neuron counts and called them flawed proxies for cognitive complexity.

Their example is a giraffe, with about 1.7 billion forebrain neurons, more than tool-proficient capuchin monkeys (1.1 billion) and corvids (0.4 to 1.2 billion). The authors say it would be appropriate to be skeptical of any claim of macaque-like cognition for giraffes from that count alone. Many other traits, most strikingly body size, separate these animals.

On the scale in our dinosaur brains study, the most brain for the body went to small feathered hunters such as Troodon, not to giants like T rex, which sits in the middle. For what brain size does and does not say about IQ in people, see our news report on brain size and IQ and our human brain facts.

What do birds tell us about T rex brain size?

Birds show how many neurons a small brain can hold. A blue-and-yellow macaw has about 1.9 billion forebrain neurons, more than the bird-like T rex estimates at 31% to 42% of the cavity, which reach 1.7 billion.

A common raven’s brain weighs 14 g and holds about 1.2 billion forebrain neurons. The lowest T rex brain estimate is about 7 times heavier but has a similar bird-like neuron count of 1.1 billion. Birds descend from small feathered theropods, so living birds are the nearest check we have on T rex brain size. See crow IQ and parrot IQ.

Other brains in our series: squid and cuttlefish and orcas, and the famous brains weighed study for human brains.

Is there a T rex IQ?

No. IQ ranks people against people of the same age, and no test or norm group exists for a dinosaur. Any “T rex IQ” number is invented. Try the IQ test to see how a person’s score is built.

Quick answers

• How big was a T rex brain? About 97 to 174 grams on the 2024 estimate, 6% to 12% of a human brain.
• What was the T rex brain weight in the 2023 study? 343 g for the AMNH 5029 skull and 202 g for Sue, as listed in the 2024 paper.
• Was T rex smart? Nobody can measure it. Neuron estimates run from 0.25 to 3.3 billion, and neuron counts are not intelligence.
• What is a T rex IQ? There is none. No IQ test exists for any dinosaur.
• How does a T rex brain compare with a raven’s? It was 7 to 12 times heavier, yet the lowest bird-like estimate has about the same forebrain neurons.

Sources

• Caspar KR and 10 others (2024). How smart was T. rex? Testing claims of exceptional cognition in dinosaurs and the application of neuron count estimates in palaeontological research. The Anatomical Record 307(12), 3685-3716 (abstract read; methods and Tables 1 and 2 read in the bioRxiv preprint, doi 10.1101/2024.01.10.575006, which may differ from the published version).
• Herculano-Houzel S (2023). Theropod dinosaurs had primate-like numbers of telencephalic neurons. Journal of Comparative Neurology 531(9), 962-974 (abstract read).
• Olkowicz S and others (2016). Birds have primate-like numbers of neurons in the forebrain. PNAS 113(26), 7255-7260.
• Azevedo FAC and others (2009). Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. Journal of Comparative Neurology 513(5), 532-541.
• Kverkova K and others (2022), the evolution of brain neuron numbers in amniotes, PNAS 119(11), and the dinosaur and living-animal figures on our dinosaur brains study and smartest animals study.