What is the formula for area of a half circle but you don't have the radius?

If you have the diameter, then the formula for the area of a semi-circle is:

(pi x (diameter/2)2)/2

That is,

1. Divide the diameter in two to find the radius (diameter/2)
2. Square the radius (radius x radius)
3. Multiply it by pi (3.1416)
4. Divide the answer by two to find half the circle.
5. Remember your units. The answer will be in square inches, or square centimeters or square somethings.