Measure the diameter of the bottom head, in feet. The diameter is the widest part of the circle, from one edge to the other, going straight through the center of the circle.
Divide the diameter by two to obtain the radius. For example, if your measurement is 70 feet, then the radius is
70/2 = 35 feet.
Square the radius you obtained in Step 2. For example: 35 feet x 35 feet = 1,225 square feet
Multiply the number in Step 3 by pi (pi = 3.14159): 1,225 square feet x pi = 3,848 square feet. (Your calculator may have a pi key.) This is the number of square feet in the bottom head.