Short Answer: 100%
Explanation: It has been misattributed to many people (including Albert Einstein) that we only use 10% of our brain. This myth was originated in the 1890’s by the father of American psychology, William James. He said “most of us do not meet our mental potential” and this misunderstanding stuck for a long time.
The truth is, we use virtually every part of the brain and most of the part is active all the time. In fact, during sleep, all parts of your brain remain active. Although, it is true that every part of the brain is not concurrently active at any given moment. For maximum efficiency and in order to be conscious at all, 1 to 16% of the cell should be active. Furthermore, most of the cells are used to control unconscious activities such as heart rate, dreaming etc.
The human brain requires 20% (more than any other organ) of the body’s energy, in children that figure is 50% and in infants, it is 60%. It contains more neurons (around 86 billion) than any other species and this dense neuron packing is what makes us so smart.