In mathematical terms, area is a way of measuring two-dimensional surfaces. For example, a square inch -- the most basic unit of measurement for area in the United States -- is literally a square that is one inch long by one inch wide. When you're presented with a polygon, such as a square or rectangle, you can quickly calculate its area in square inches by multiplying the length in inches times the width in inches.

Parallelograms use the same basic formula to determine area, but instead of multiplying the lengths of the sides, you multiply the length of just one side -- called the base -- by the overall height of the figure, or the distance between the base and the opposite side of the parallelogram. So for a parallelogram that has a base of 5 inches and a height of 2 inches, its area is 5 x 2 = 10 inches squared. The area for a triangle is 1/2(base x height), so for a triangle with a base of 5 inches and a height of 2 inches, you'd calculate the area with the formula of 1/2(5 x 2) or 5 inches squared.

The formula for the area of a circle is pi x r^2, where r is the radius of the circle, or the distance from the center of the circle to any point on the circle. For most purposes, you can abbreviate pi to 3.14. So, for a circle with a radius 2 inches, the area in square inches is pi x 2^2 or 3.14 x 4, or 12.56 inches squared.