Approximately how many neurons are found in the adult human brain?

The adult human brain contains approximately 86 billion neurons.

This estimate comes from quantitative work using the isotropic fractionator method, which counts neuronal nuclei in dissolved brain tissue rather than relying on a simple visual estimate. A widely repeated older figure was 100 billion, but modern measurements support a value close to 86 billion.

Neurons are electrically excitable cells that receive, process, and transmit information. They communicate at synapses using electrical signals and chemical neurotransmitters. Their shapes and functions differ substantially: motor neurons, sensory neurons, interneurons, and many specialized populations each contribute to neural circuits.

The neuron count is not the same as intelligence, and it does not describe the brain’s full cellular population. The brain also contains glial cells, blood vessels, and other cells. In addition, connectivity matters: the arrangement of circuits and the strength of synapses are central to how the brain operates.