Learn the basic terminology of circle measurement notation; it will greatly help you understand the formula required to measure a circle's area. The diameter of a circle is the measurement taken from one side of the circle to the other through the center of the circle. The radius is half the diameter. The formula to measure a circle's area is Pi times r squared. The "r" refers to the radius and Pi is a constant value of 3.141.
Calculate the area of the main circle by applying the formula: 3.141 times r squared. Establish the square root of the radius, then multiply that figure with 3.141. For example: if a circle has a radius of 7 inches, the square root is 49, which when multiplied by 3.141, equals a circle area of 153.91 square inches.
Apply the same formula to measure the areas of any circular holes within the main circle. For rectangular or square holes, it's a simple multiplication of the length times the width to establish the hole's area.
Add the areas of the combined holes. For example: if you have 5 holes measuring 3, 4, 6, 9 and 11 square inches, then the combined area of the holes is 33 square inches.
Subtract the area of the combined holes from the area of the large circle previously calculated. For example, subtracting 33 square inches from the large circle area of 153.91 square inches will result in 120.91 square inches.