How Much Does the Human Adult Brain Weigh?

The human brain makes up 2% of the body's weight but uses about 20% of its
energy.

A human adult's brain weighs about 3 pounds (1.5 kg). Scientists believe
that the modern human adult's brain is about 10% smaller than it was 10,000
years ago, which is thought to be due to the introduction of agriculture
and less of a need to be hunter-gatherers. A human adult's brain is about
three times as large in relation to the rest of the body when compared with
the brains of other similarly sized mammals. It also uses up a large amount
of energy, requiring about 20% of the body’s energy supply even though
the brain is just 2% of the body’s mass.

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