---
title: "Brain — what it does and the numbers"
description: "About 1.4 kg, using roughly 20% of the body’s energy at 2% of its mass. What it does, where it is, the figures people look up, and the widely repeated claims that tu"
url: https://atoneplace.net/body/brain/
site: "At One Place"
---

# Brain

**Brain: About 1.4 kg, using roughly 20% of the body’s energy at 2% of its mass.**

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.

## Common questions

### What does the brain do?

Processes sensory input, controls movement, regulates the organs and generates everything experienced as thought.

### Brain — key numbers

Mass: 1.3–1.4 kg. Neurons: about 86 billion. Energy use: ~20% of total, ~20 W. Water content: about 73%.

### Is it true that…

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.

## Related pages

- [Heart](https://atoneplace.net/body/heart/)
- [Liver](https://atoneplace.net/body/liver/)
- [Lungs](https://atoneplace.net/body/lungs/)
- [Skin](https://atoneplace.net/body/skin/)
- [Kidneys](https://atoneplace.net/body/kidneys/)
- [Bones](https://atoneplace.net/body/bones/)
- [Blood types](https://atoneplace.net/body/blood-types/)
- [Teeth](https://atoneplace.net/body/teeth/)
- [Blood](https://atoneplace.net/body/blood/)
- [The senses](https://atoneplace.net/body/senses/)
- [Taste](https://atoneplace.net/body/taste/)

## Sources

- Typical range compiled from manufacturer specifications — At One Place (/about/)