Short answer- all of it. It's a common myth that only 10 or 20% of the brain is used. From an evolutionary standpoint, having an organ which only used 20% of it's capacity would be incredibly wasteful and would have favored a much smaller brain which used less energy.
Short answer We only use 90% at a time.
A better question might be "how much of the brain is used at once?" Just looking at a computer screen, reading the text, and typing a response is going to be using significant portions of the occipital lobe (visual regions), motor areas (typing), temporal regions (language), and pre-frontal areas (planning). If you put someone in an fMRI scanner (used to image brain activity) while they typed away at that task, you might get 30% lighting up as being more active than when they were just lying there doing nothing. But those areas are simply *more* active during a task than when you're doing "nothing". But even when you're doing "nothing" you're still looking at things, thinking about what you'll have for lunch and so on- the brain is always active. True, not every neuron in your brain is firing all of the time, but it's fair to say that the whole brain is being used.
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Previous answer: between six and 10 percent... 4 percent if you are a politician ;).
Popular misconceptions don't come any more popular and misconceptiony than this one. Indeed, its origins lie so far back that nobody knows who said it first (various 19th Century quacks believed it, based on skull measurements and whatnot). The concept has since been plugged by everyone from advertisers to psychics, but - boring as it sounds - actual science has shown that we do indeed use just about all of our brains throughout the course of every single day. The mechanisms of the brain may not be fully understood, but the idea of a vast bulk of grey matter lying dormant and mysterious is a myth.
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15% of total resting cardiac output is supplied to the brain. This represents 50-55ml blood flow per 100 grams brain tissue per minute, or 750ml total blood flow per minute. Data from Guyton Textbook of Medical Physiology.
All of it, though not all at once. (If every neuron in your brain was firing uncontrollably all at once, you'd have an epileptic seizure.)
The "10%" figure you'll occasionally see is an old wives' tale/urban legend/gross oversimplification of a misinterpretation of someone who actually had some idea of what they were talking about said.
You have heard it said that we only use 10% of our brain. That is untrue, we use 100%.