Divide each polygon into a set of triangles. Each polygon is different, so each triangle will be unique in size and shape.
Measure the sides of each triangle. Work one triangle at a time when calculating the area. The formula to find the area of a triangle is "(B * H) / 2" where "B" is base and "H" is height.
Multiply the base (the longest side) by the height (the distance between the base and the point formed by the two other sides). If the triangle is a right triangle, which is a triangle containing a 90-degree angle, the base and height are the two sides adjacent to the right angle.
Divide this answer by two. This is the area. For instance, if the base of the triangle is 4 inches and the height is 7 inches, the area is 14 square inches: (4 * 7) / 2 = 14.
Calculate the area of each triangle.
Add the areas of each triangle found within one of the polygons. For instance, if a polygon contains two triangles, one with an area of 16 square inches and another with an area of 3 square inches, the polygon has an area of 19 square inches: 16 + 3 = 19.
Calculate the area of each polygon.
Add together the areas of all the polygons to determine the total area of the Thiessen polygons.