The largest brains are those of sperm whales, weighing about 8 kg (18 lb). An elephant 's brain weighs just over 5 kg (11 lb), a bottlenose dolphin 's 1.5 to 1.7 kg (3.3 to 3.7 lb), whereas a human brain is around 1.3 to 1.5 kg (2.9 to 3.3 lb). Brain size tends to vary according to body size. The relationship is not proportional, though: the brain-to-body mass ratio varies.