What is the area of a circle?

The area of a circle is the size of the 2-dimensional space inside the circle's
closed curved boundary.

The area can be calculated in terms of known linear measurements of the circle:

-- Area  =  (π) x (radius)²

-- Area  =  (π/4) x (diameter)²

-- Area  =  (1/2) x (circumference) x (radius)

-- Area  =  (1/4) x (circumference) x (diameter)

Any of these formulas will give you the area.  The one you decide to use
just depends on what you already know about the circle.