For circular areas you only need one measurement, the distance across the circle, or diameter (as shown in the image at the right). The formula then is:
Area = (Diameter / 2)² x 3.14

Oval areas need measurements similar to a rectangle. Simply take the length from one end to the other, and the width at the widest point. The formula you need then is:
Area = 0.8 x Width x Length

Triangles are another fairly simple shape. As illustrated at the right it's simply a matter of taking the height at the tallest point and the width at the base of the triangle and then multiplying those by half. So in this case the formula you need is:
Area = 0.5 x Width x Height