Draw an irregular shape on a piece of paper. Choose some point inside the shape and call that the "center."

Find the lengths from the center to different points on the edge of the shape. [If a segment goes outside the shape, add the lengths of the pieces that are inside the shape.] Find the average of all the lengths.

Let this average be the "radius" of the shape. Use the formula for the areas of a circle ( × radius × radius, or about 3.14 × r × r). This should be a good estimate of the area.