How many neurons are in the human brain?

How to get close to this number

This is an order-of-magnitude question, so aim at the exponent. A brain is about a litre and a half of tissue, neurons are a few tens of micrometres across, and only a fraction of the volume is cell bodies rather than connections and support cells. Volume divided by cell size, discounted for what else is in there, lands you in the tens of billions.

The history in the explanation is the more interesting story. The round hundred billion was repeated in textbooks for decades with no traceable source, until someone actually counted using a method that dissolves the tissue and counts nuclei. The correction was modest in size and enormous in principle — a foundational number in neuroscience turned out to be folklore.

The distribution is the genuinely surprising part: roughly four in five neurons are in the cerebellum, which occupies a small fraction of the brain's volume. The structures associated with thought hold a minority of the cells, because cerebellar neurons are tiny and densely packed. As with red blood cells in the body-cell question here, the count is dominated by the smallest type.