Find the diameter of the circle. This is the largest distance across the circle or the distance across the circle at the widest part. Use an object with a 90-degree angle (e.g., a T-square or a sheet of paper). Put the object over the circle so that the 90 degree angle just touches the circle. The other two places that the object touches the circle are the ends of a diameter. Mark these two points and connect the marks with a straight edge. Measure the distance between the marks to get the diameter.
Approximate pi. Because pi goes on forever without repeating, you will need to use an approximation (e.g., 3.14159265). The number of digits to which you approximate pi will depend on how precise you need your calculation of the circumference to be. (See Tips.) Most calculations are done with rough approximations of pi (e.g., 3.1416).
Calculate the circumference. Because the distance around the circle is curved, it is hard to measure directly. To calculate the circumference, multiply the diameter from Step 1 by your approximation of pi from Step 2.
Calculate radius. Divide the diameter from Step 1 of the "Calculate Circumference" section. This value is the radius.
Approximate pi. As in Step 2 of the "Calculate Circumference" section, your approximation of pi will depend on how precise you need your calculation of area to be.
Calculate area. The formula to calculate a circle's area is pi times the radius squared (i.e., pi x radius x radius). Multiply the radius from Step 1 by itself, then multiply by the approximation of pi from Step 2. This value is the area of the circle.