Koala Brain vs Human Brain: Size, Folds and What It Means
A koala brain is about 79 times lighter than a human brain, at 19.2 g, and its cortex is smooth. The popular claim that it fills only 60% of the skull does not hold for living koalas: MRI found 83% to 90%.
What you need to know
- In a koala brain vs human brain comparison, the koala brain weighs 19.2 g (27 adults, De Miguel and Henneberg, 1998) and the average adult man's brain 1,508 g, about 79 times more. That is a size gap, not an IQ gap.
- The koala cortex is smooth (lissencephalic). A 2026 MRI study of one koala brain, 15.72 cm3 in volume, found the cerebral hemispheres make up 59.39% of it.
- The claim that a koala brain fills only about 60% of its skull does not hold in living koalas. MRI gave 83% to 90% (Taylor and colleagues, 2006), and the authors contrast it with cadaver specimens.
- No koala neuron count or IQ exists. Its brain size relative to its body is 35% to 50% of the mammal average by general equations, but 108% to 156% against marsupial or insectivore baselines.
A koala brain is about 79 times lighter than a human brain: 19.2 g in 27 adult koalas, against 1,508 g for the average adult man. It is also smooth, with no folds in the cortex. Neither fact ranks intelligence. The popular claim that the brain fills only 60% of the skull does not hold for living koalas: MRI found 83% to 90%.
Below: the sizes side by side, what we could and could not find on neurons, why the cortex is smooth, and what a small smooth brain does and does not mean.
Koala brain vs human brain: how big is the size gap?
Koala brain size is about 19 g, against about 1.5 kg for a human brain. De Miguel and Henneberg weighed fresh brains of 27 adult koalas from South Australia in Australian Mammalogy (1998). The average was 19.2 g, in animals that averaged 8.0 kg. The human figure is the adult male average of 1,508 g, from Herculano-Houzel's 2009 review that we also use in our human brain vs dog brain guide. Divide one by the other and you get 78.5 (our arithmetic).
- Koala, 27 adults, fresh weight (1998): brain 19.2 g; body 8.0 kg
- Koala, one female, MRI of a fixed brain (Goh and colleagues, 2026): volume 15.72 cm3 (15,720.98 mm3)
- Common wombat, MRI (Anthoniel and colleagues, 2026): volume 41.75 cm3 (41,745.46 mm3)
- Human, adult: brain 1,508 g (adult male average); volume about 1,330 cm3, the value Goh and colleagues use
By volume the gap is about the same. 1,330 cm3 against 15.72 cm3 is about 85 times (our arithmetic). The two koala figures come from different animals and methods, so they will not match exactly, and a brain fixed in formalin can differ from a fresh one. The wombat, a close relative of the koala, has a brain about 2.7 times the koala's volume (two single animals). For the wider picture, see our piece on brain size and IQ.
How many neurons does a koala brain have?
We found no published count, so we do not give one. The human brain has about 86.1 billion neurons, with only 19% of them in the cortex (Azevedo and colleagues, Journal of Comparative Neurology, 2009). The koala MRI paper measures volumes, not cells. Our smartest animals data set lists neuron counts for the species that have them, and the koala is not one of them.
This matters because brain weight is a poor guide to neuron count. A golden retriever brain of 114.5 g holds about 2.6 billion neurons, as we show in the dog-brain guide. Without a count, a koala brain tells us its size and its shape, nothing about how many cells do the work.
Why is the koala brain smooth?
The koala smooth brain has no folds in its cortex, which anatomists call lissencephalic. A 2026 MRI study by Goh and colleagues in Frontiers in Neurology states this for the one adult female brain they scanned. Its cerebellum, by contrast, is folded into fine ridges called folia. The cerebral hemispheres made up 59.39% of the brain volume, the cerebellum 14.66% and the olfactory bulbs 2.66%.
The proportions differ from ours as well. In the same scan the koala brainstem was 11.92% of the brain volume, against about 2.6% of weight in humans, a gap the authors suggest may reflect the much larger expansion of the human cortex. The koala's olfactory bulbs, which handle smell, were 2.66% of the brain volume. That is a smaller share than in the brushtail possum (3.1% of weight) and the quokka (3.4%).
Smoothness varies across marsupials. A 2007 review by Karlen and Krubitzer in Progress in Neurobiology notes that the Virginia opossum has a small smooth brain, while the Tasmanian wombat and the Western gray kangaroo have folded ones. Koalas and wombats are close relatives, so a smooth koala next to a folded wombat shows that folding does not simply run in a family.
Why is the koala brain smooth? Nobody has measured it. A koala brain atlas from the University of New South Wales suggests that a small, smooth brain helps the koala save energy on a poor eucalyptus diet. That is a suggestion, not a tested result.
Does a smooth brain mean koalas are dumb?
No. A smooth brain tells you the surface is not folded, not how well the animal solves problems. The mouse brain is also smooth (Goh and colleagues, 2026). Are koalas dumb? The brain data cannot say, and nobody has given koalas an intelligence test, because no standard IQ test exists for animals. An IQ is a rank within a group of people, which you can check with the IQ percentile calculator.
What the data show is that "braininess" depends on the yardstick. De Miguel and Henneberg worked out the koala's encephalization, its brain size relative to its body size, with several published equations. The answer changed with the equation:
- General mammal equations: 35.3% (Martin), 38.9% (Jerison) and 49.7% (Eisenberg) of the mammal average
- Marsupial baselines: 107.9%, 108.4% and 116.5%, using the equations of Haight and Nelson, Nelson and Stephan, and Pirlot
- Insectivore baselines: 131.7% (Martin) and 155.9% (Stephan)
The authors conclude that the choice of sample and equation changes the conclusion. So "koalas have tiny brains" and "koalas have a normal marsupial brain" can both be defended, depending on who the koala is compared with.
A small, smooth brain is a fact about anatomy. It is not a score, and nobody has given a koala a test.
Does a koala brain really fill only part of its skull?
Not as little as the popular claim says. The ratio of brain volume to cranial volume was reported as low, about 60%, as Taylor and colleagues note; a 1987 book chapter by Haight and Nelson is titled "A brain that doesn't fit its skull". We could not read that chapter. Taylor and colleagues scanned living koalas with MRI in Australian Mammalogy (2006) and found 83% to 90%.
They conclude there is a substantial difference between the living brain and cadaver specimens, which shows why studies of living animals matter. So the "60%" figure should not be repeated as fact for living koalas.
How do other marsupials and primates compare?
Marsupial brains range widely in size and in smoothness, as the opossum, wombat and kangaroo show. Among primates, our data set gives a chimpanzee brain of 344 g and a capuchin monkey brain of 52.2 g, against 1,508 g for a human. The koala's 19.2 g is about 37% of the capuchin's 52.2 g (our arithmetic). No animal on that list has an IQ. Read our guide to the smartest primate for what primate tests did measure.
For the human side, our human brain facts page lists the numbers, and the sections above show why a bigger or more folded brain does not give an animal a score.
Where would your own score land?
Brain size does not give anyone an IQ. A test does. The IQ Metrics test reports a score with its scale and a percentile against the general population.
Find your IQ score now! →Take the IQ Metrics test, or train your memory with the memory game.
Common questions
How big is a koala brain compared with a human brain?
A koala brain weighs about 19.2 g (27 adults, 1998), against 1,508 g for the average adult man, so the human brain is about 78.5 times heavier. By MRI volume the gap is about 85 times: 15.72 cm3 for one koala against about 1,330 cm3 for a human.
Why is the koala brain smooth?
The koala cortex has no folds (lissencephalic), and nobody has measured why. Marsupials differ: the wombat, a close relative, has a folded brain. A koala brain atlas suggests the small smooth brain saves energy on a poor eucalyptus diet, which is a suggestion rather than a tested result.
Are koalas dumb?
The brain data cannot say. A smooth cortex and a small brain describe anatomy, not ability, and no standard intelligence test exists for koalas. Their encephalization is 35% to 50% of the mammal average by general equations but 108% to 156% against marsupial or insectivore baselines.
Does a koala brain fill only 60% of its skull?
That figure was reported in earlier work. A 2006 MRI study of living koalas found the brain fills 83% to 90% of the cranial volume, and its authors say the living brain differs substantially from cadaver specimens.
Sources for this story
- De Miguel C, Henneberg M. Encephalization of the koala, Phascolarctos cinereus. Australian Mammalogy 20 (1998), doi 10.1071/AM98315 — CSIRO Publishing
- Taylor J and colleagues. MR imaging of brain morphology, vascularisation and encephalization in the koala. Australian Mammalogy 28 (2006), doi 10.1071/AM06034 — CSIRO Publishing
- Goh KRQ and colleagues. Description of the koala (Phascolarctos cinereus) brain based on magnetic resonance images. Frontiers in Neurology 17 (2026), doi 10.3389/fneur.2026.1733030 — Frontiers Media
- Karlen SJ, Krubitzer L. The functional and anatomical organization of marsupial neocortex. Progress in Neurobiology 82 (2007), doi 10.1016/j.pneurobio.2007.03.003 — Elsevier
- Anthoniel NB and colleagues. Creation of an MRI-based three-dimensional common wombat (Vombatus ursinus) brain atlas. Anatomia, Histologia, Embryologia 55 (2026), doi 10.1111/ahe.70150 — Wiley
- Azevedo FAC and colleagues. Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. Journal of Comparative Neurology 513 (2009), doi 10.1002/cne.21974 — Wiley
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