Brain

Processes sensory input, controls movement, regulates the organs and generates everything experienced as thought. It runs on roughly 20 watts — about the same as a dim light bulb — while consuming a fifth of the body’s total energy budget.

Where it is

Inside the skull, floating in cerebrospinal fluid.

The numbers

Mass

1.3–1.4 kg

Neurons

about 86 billion

Energy use

~20% of total, ~20 W

Water content

about 73%

Worth knowing

The brain has no pain receptors of its own, which is why brain surgery can be performed on a conscious patient. The 86-billion neuron figure comes from a 2009 counting method that replaced the older, much-repeated 100 billion, which was never based on a direct count.

A common claim that is wrong

You do not use only ten per cent of your brain. Imaging shows essentially all of it active over a day, and there is no dormant reserve — damage to almost any region produces deficits.

Structural reference only — this is not medical advice, and individual variation in every figure here is substantial. Anything concerning your own health belongs with a clinician rather than a reference page.