It cannot be done. First of all you cannot estimate what you don't know. And you don't know what you haven't done yet. Unknown unknowns is what Fred Brooks ex

It cannot be done. First of all you cannot estimate what you don't know. And you don't know what you haven't done yet. Unknown unknowns is what Fred Brooks explained in detail in his Mythical Man Month. Then there is the psychological aspect which is what is called optimistic bias. Which is that humans are optimistic beings and will always believe they are better than they really are at some things. The planning fallacy is based upon this. All of this is why we try to iterate and validate on feedback, instead of doing fixed timed development. That is simply because we have no idea about the future and guessing on it is more often than not a waste of time.