The Brain by the Numbers

About 86 billion neurons, roughly as many glial cells, around 1.4 kilograms, and a fifth of the body's energy: the headline statistics of the human brain — and where the numbers come from.

Numbers about the brain get repeated so often that it's worth knowing where they come from — and how confident we can be.

Cells

The best-known modern estimate is about 86 billion neurons in the adult human brain. It comes from a 2009 study led by Frederico Azevedo and Suzana Herculano-Houzel, who used a technique called the isotropic fractionator: dissolving brain tissue into a uniform soup of cell nuclei and counting a stained sample. The same study found about 85 billion non-neuronal cells (mostly glia), overturning the old claim that glia outnumber neurons ten to one. The long-quoted "100 billion neurons" had never actually been measured. Like all estimates, the 86 billion figure has a margin of error and is still debated.

Remarkably, about 69 billion of those neurons — roughly 80% — sit in the cerebellum, mostly as tiny granule cells. The cerebral cortex, home of much of what we think of as "thinking", has about 16 billion.

Size, energy and blood

An adult brain weighs about 1.2 to 1.4 kilograms and is about three-quarters water. It is roughly 2% of body mass but consumes around 20% of the body's resting energy — about 20 watts. It receives about 15% of the heart's output, roughly three-quarters of a litre of blood every minute, and if that flow stops, consciousness is lost in around ten seconds.

Connections and wiring

Estimates of the number of synapses run to the order of a hundred trillion or more. A typical cortical neuron receives thousands of synaptic inputs. The white matter contains on the order of 150,000 to 180,000 kilometres of myelinated fibres — enough to wrap around the Earth several times. The fastest nerve fibres conduct signals at about 120 metres per second; the slowest unmyelinated ones manage under one metre per second.

Folding

The cerebral cortex is a sheet only about 2 to 4 millimetres thick. Folded into ridges (gyri) and grooves (sulci), it fits a surface area of roughly 2,000 square centimetres — about a 45-centimetre square — inside the skull.