Flexi Says: The brain is the most complex organ in the body. The brain contains about 100 billion neurons—each of which can be connected to tens of thousands of other neurons within the brain. An average adult human brain weighs between 1 and 1.5 kg (3 lb). An adult brain uses about 20-25% of the total energy used by the body, while the developing brain of an infant consumes around 60% of the total energy used by the body. The average adult human brain is approximately 140mm wide, 160mm long and 90 mm high.