What qualifies for being called a fundamental quantity and having its own fundamental unit? For example length is considered fundamental quantity and it has a unit of meter. But Area isn't considered fundamental. Is it because we know area can be CALCULATED by multiplying the length of the two sides of a rectangle? Suppose they didn't know the formula. Then, would they have called area fundamental and defined a unit, lets say Ar, as the area of a square whose length is 1m? And then measured areas of figures by comparing to the area of the square? My question is "do some quantities fail to be fundamental because we know how to calculate them from other fundamental quantity?" If yes, is there any chance that few of today's fundamental quantity be called derived in future? I feel like I am missing something very fundamental and I am feeling quite ashamed for asking these questions.