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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →Scan for outdated or missing drivers - takes under a minuteDriver Scan →About 86 billion neurons is a widely cited estimate—not an exact count for every person. It comes from a 2009 study of adult male brains that estimated an average of 86.1 billion neurons, with uncertainty of ±8.1 billion. Later work has reported different figures, in part because studies use different samples, methods and cell classifications.
1. The 86-billion figure is an estimate, not a universal count
Azevedo and colleagues’ 2009 study estimated an average of 86.1 ± 8.1 billion NeuN-positive cells, which the authors classified as neurons, in adult male human brains. The uncertainty is important: the result describes the study’s sample and method, not a census of every living person’s brain. Read the study abstract.
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2. The researchers used a method called the isotropic fractionator
The study used the isotropic fractionator to estimate total cell numbers. Its result depends on how the researchers sampled and classified cells, so it should not be treated as a direct count of each neuron in an intact brain. Comparisons between studies need to account for method as well as the population studied. A review of the method and its use discusses these considerations. See the 2009 review.
3. The brain contains a similar estimated number of non-neuronal cells
Azevedo and colleagues also reported 84.6 ± 9.8 billion NeuN-negative cells. These are non-neuronal cells, a broad category that includes glia as well as endothelial and other cell types. “Non-neuronal” and “glial” are therefore not interchangeable labels. The original study abstract gives both estimates; a later review discusses the range of cells included in non-neuronal counts. Read the review.
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4. The number is still debated across studies
A 2024 review by Alain Goriely discusses estimates ranging from 61 billion to 99 billion neurons. That is a range of estimates reviewed, not a new direct census. A separate 2023 paper estimated 88 billion neurons for its reference male brain. These results are not interchangeable: the underlying samples, methods and counting assumptions differ. Read Goriely’s 2024 review; Read the 2023 study.
5. Most neurons are not in the cerebral cortex
In Azevedo and colleagues’ analysis, the cerebral cortex accounted for 19% of all neurons while representing 82% of total brain mass. The cerebellum, though smaller, is an important neuron-rich region. These paired figures show why brain mass alone does not reveal where most neurons are located. See the study’s regional results.
6. The old “one trillion glia” figure reflects older assumptions
The often-repeated claim that the brain contains about one trillion glial cells grew from neuron-to-glia ratios that were not well supported by whole-brain counts. Later counting approaches produced different estimates and clarified that non-neuronal cells include more than glia. The history is best understood as a change in evidence and methods, not as a simple correction from one certain number to another. A review traces the changing estimates.
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7. The estimate does not mean humans break primate scaling rules
Herculano-Houzel’s 2009 review compared human counts with modeled primate and rodent brains and argued that human cellular composition is consistent with primate scaling. That comparison is about cell counts and brain organization; it is not a complete explanation for human intelligence, nor does neuron count alone measure cognitive ability. Read the review.
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8. Different studies count and define cells differently
For a useful comparison, look beyond the headline number. Ask what method was used, which people or reference brain were studied, what cell types were counted, and whether the number is a direct estimate for a sample or a range summarized by a review. Those distinctions help explain why a figure such as 88 billion does not automatically disprove the widely cited 86-billion estimate.
9. Neuron count is not a score for intelligence
A total neuron estimate cannot, by itself, tell you how intelligent someone is. The cited primate comparison describes patterns of cellular composition across species; it does not establish a simple causal link between an individual’s neuron count and cognitive abilities.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.10. The safest short answer is “roughly 86 billion”
If someone asks how many neurons the human brain has, “roughly 86 billion” is a reasonable shorthand when its status as an estimate is clear. The landmark 2009 result was an average for adult male brains, later studies have produced other figures, and sample, method and cell classification all affect comparisons.
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