For both area and perimeter -- in a circle this is called the circumference -- you need to know the radius of the circle, which is HALF the distance across the circle through the midpoint. Here are the formulas:
CIRCUMFERENCE: pi (3.14) x 2 x radius
AREA: pi x (radius squared)
So, as an example, let's say we have a circle with a radius of 4. Here are the calculations:
Circumference: 3.14 x 2 x 4 = 25.12
Area: 3.14 x 4^2 = 50.24
Of course, both the circumference and area of a semi-circle are one-half that of the whole circle.