About how many neurons are estimated to make up the adult human brain?

The adult human brain is estimated to contain about 86 billion neurons.

This estimate comes from work led by Brazilian neuroscientist Suzana Herculano-Houzel, who developed a method for counting nuclei in dissolved brain tissue. Her 2009 research challenged the widely repeated figure of 100 billion neurons and produced an average estimate of roughly 86 billion for the human brain.

Neurons are electrically excitable cells that receive, process, and transmit information. They are supported by many types of glial cells, including astrocytes, oligodendrocytes, and microglia. The brain’s neurons are not distributed evenly: the cerebellum contains a very large share, while the cerebral cortex contains many billions involved in perception, planning, memory, and language.

The number is an estimate rather than a literal census of every living brain. Individual brains vary, and counting methods and definitions can affect the result. The figure also does not mean that every neuron is unique in its role or connected to every other neuron.