here's an answer I once posted to another forum:

As far as we can tell we always use our brain's full capacity.

The issue starts with that there does not exist a single measure of brain "usage" or "activity". Neural functioning can be measured on the scale of genes, proteins, single neurons, groups of cells, or entire brains. Neuroscientists look at blood flow, glucose use, oxygen use, neurotransmitter levels, number of action potentials, intensity of various brain rhythms, connectivity patterns between areas, and hundreds, if not thousands, of things more. Clearly, there is no single all-encompassing way of quantifying brain usage.

That said, distinct brain regions and cell types certainly vary in their levels of activity on many of these measures. Activity levels vary across time, and they vary with the particular task the organism is performing. Indeed, this variation reflects how the brain processes information: in a way, it's how the brain works.

For other brain regions we have less of an idea "what they're for". But it has long been recognized that some brain functions aren't localized to any specific region. Rather, these functions are distributed over many areas. So you may be able to cut away a certain piece of brain tissue without grave consequences, but you will have reduced the brain's overall capacity for information processing.

Also, more is not always better. Bigger brains aren't smarter. Moreover, much higher blood flow through the brain would result in blood vessel ruptures, greatly increased neurotransmitter levels are toxic to the brain (cells die), and when neurons all over the brain fire action potentials as quickly as they possibly can we call that an epileptic seizure.

All of this doesn't mean we can't try out new psychological/cognitive strategies that optimize or enhance our functioning somehow (presumably by rearranging the information flow in our brains). And yes, "zapping" the brain with electric or magnetic pulses as well as some drugs can impressively alter behavior. But "impressive" for neuroscientists usually means a 50 percent change (of whatever it is we're looking at) that lasts for as long as the stimulation is on, or however long the drug remains active. And naturally, scientists then only look at what's improved, not at all the brain functions that may simultaneously be taking a beating...

Finally, there's an evolutionary argument. The human brain uses a massive amount of energy compared to our other organs. And foraging for food is no joke. So it's very unlikely nature would have put in 90 percent of idle brain tissue unless we really desperately needed it in order to survive.

Great TED clip attached as link

http://ed.ted.com/lessons/what-percentage-of-your-brain-do-you-use-richard-e-cytowic

here's an answer I once posted to another forum:

As far as we can tell we always use our brain's full capacity.

The issue starts with that there does not exist a single measure of brain "usage" or "activity". Neural functioning can be measured on the scale of genes, proteins, single neurons, groups of cells, or entire brains. Neuroscientists look at blood flow, glucose use, oxygen use, neurotransmitter levels, number of action potentials, intensity of various brain rhythms, connectivity patterns between areas, and hundreds, if not thousands, of things more. Clearly, there is no single all-encompassing way of quantifying brain usage.

That said, distinct brain regions and cell types certainly vary in their levels of activity on many of these measures. Activity levels vary across time, and they vary with the particular task the organism is performing. Indeed, this variation reflects how the brain processes information: in a way, it's how the brain works.

For other brain regions we have less of an idea "what they're for". But it has long been recognized that some brain functions aren't localized to any specific region. Rather, these functions are distributed over many areas. So you may be able to cut away a certain piece of brain tissue without grave consequences, but you will have reduced the brain's overall capacity for information processing.

Also, more is not always better. Bigger brains aren't smarter. Moreover, much higher blood flow through the brain would result in blood vessel ruptures, greatly increased neurotransmitter levels are toxic to the brain (cells die), and when neurons all over the brain fire action potentials as quickly as they possibly can we call that an epileptic seizure.

All of this doesn't mean we can't try out new psychological/cognitive strategies that optimize or enhance our functioning somehow (presumably by rearranging the information flow in our brains). And yes, "zapping" the brain with electric or magnetic pulses as well as some drugs can impressively alter behavior. But "impressive" for neuroscientists usually means a 50 percent change (of whatever it is we're looking at) that lasts for as long as the stimulation is on, or however long the drug remains active. And naturally, scientists then only look at what's improved, not at all the brain functions that may simultaneously be taking a beating...

Finally, there's an evolutionary argument. The human brain uses a massive amount of energy compared to our other organs. And foraging for food is no joke. So it's very unlikely nature would have put in 90 percent of idle brain tissue unless we really desperately needed it in order to survive.

Great TED clip attached as link

http://ed.ted.com/lessons/what-percentage-of-your-brain-do-you-use-richard-e-cytowic

Dear ROY COX Sir, That was an excellent information which gave me a lot of insights. Thank you very much.

Very exciting question! 

"we use a very small percentage of our brain" or "we use 10% of our brain abilities" were the statements used by trainers to motivate us to be more creative. However, after I read about the topic, it is a myth! 

The below paragraph was quoted from the attached article, which explain this issue.

"It turns out though, that we use virtually every part of the brain, and that [most of] the brain is active almost all the time," Gordon adds. "Let's put it this way: the brain represents three percent of the body's weight and uses 20 percent of the body's energy."

http://www.scientificamerican.com/article/do-people-only-use-10-percent-of-their-brains/

Dear Sir, very useful information and so happy to know that the full capacity of our brain nd the wrong belief of less percentage of brain use is totally proven wrong. Thank you very much sir.

Dear Madam Aparna --- you are right --- we surely need the whole 100% usage of the brain for all in life we need o manage every situation and circumstances. Thanks for your reply.

Brain PET with fludeoxyglucose shows quite well the acivity of the different brain regions.

http://www.coma.ulg.ac.be/papers/vs/Stender_jcbfm2014.pdf

This is a quantitative measurement.

There is no formula & their will not be any formula for measuring brain .Although it is microscopic but it has an energy force of within .For understanding our energy force of brain we have to view the storage of fruits of our action on our head joining with our MIND helping us to co-relates with our BRAIN within a divinity within us.

In order to appreciate the 3 said function of our body we cannot afford to ignore our forehead .All the said areas are related to our memories which can also be part of our intelligence.

When we have touched the forehead it has been observed that when we desire to recall our memory we with our semi closed eyes ,concentrate on the central part of our forehead & we find the result of our forgotten memory .

Our mind gets tired & require the such a change but our brain never gets tire but it has been observed that individual are using the brain power not fully but in limited percentage of his intelligence,capacity ,intuition & last but not least the power of visualization which has to be developed ,cultivated ,by creating our living in a holy worthy mode.

As we know that with the visualization we have also developed  the AURA  & for the measurement of AURAS long year back Russian scientist SEMYON DAVIDOVICH KIRLIAN have research a camera namely Kirlian photography .

We have observed that highly intellectual individual so to say we may call GENIUS  have with them the revised memory of their previous lives & their with their contribution they create wonders of BRAIN for us & for the society .

This is my personal opinion 

Completely agree with Roy Cox's answer. Where this limited percentage of brain usage myth seems to stem from is the change in activation of certain brain areas in response to an experimental condition (for example a cognitive task). This, however, is a change in allocation in activity of different brain areas within a background of activity. The total brain activity as measured by metabolic demand is quite stable, even during sleep. So the brain is always fully at use, it only changes its pattern of activity based on need. This is also what happens during learning and memory, when this resource allocation is more long lasting. How efficient and flexible this resource allocation occurs is different from person to person, by age, and seems to be the basis of mental capabilities. There are also indications that in mental disorders this is what doesn't work properly (i.e. decreased ability for the task at hand to overcome the background activity).