Human Brain Weight: Average, Proportion & Energy Demand

The average adult human brain weighs 1,300–1,400 grams (2.9–3.1 lbs).

Key Metrics

• Weight range: 1,100–1,400 g (sex, body size, age variation).
• % Body weight: ~2% (70 kg person → 1.4 kg brain).
• % Energy consumption: ~20% of basal metabolic rate (BMR).
• Neurons: ~86 billion; glia ~85 billion.
• Volume: ~1,200 cm³.

Why So Energy-Hungry?

• Ion gradient maintenance (Na⁺/K⁺-ATPase) for resting potentials & signaling.
• Synaptic transmission (vesicle cycling, receptor turnover).
• Protein synthesis, axonal transport, cytoskeletal dynamics.

Clinical Notes

• Brain weight ↓ with age (~5% per decade after 40).
• Microcephaly, macrocephaly, atrophy (Alzheimer's, TBI) alter weight.
• High metabolic rate explains vulnerability to hypoxia/ischemia (minutes).

Source: NIH / U.S. National Library of Medicine.