Measure the radius of the circle with a ruler or measuring tape, depending on the size of your circle. Because all points on the perimeter are equidistant from the center of the circle, it does not matter which point on the circumference you measure.
Multiply the radius by 2 to find the diameter. For example, if your radius equals 13 feet, multiply 13 by 2 to find the diameter equals 26 feet.
Multiply the diameter by 3.14 to find the perimeter. In this example, multiply 26 by 3.14 to find the perimeter of the circle equals 81.64 feet.
Divide the angle measure of the part of the circle by 360 to find the portion of circle the part comprises. If you are not given the angle measure, use a protractor to find the angle measure. For example, if your part of the circle measures 144 degrees, divide 144 by 360 to get 0.4.
Multiply the circumference by the result to find the curved edge. In this example, multiply 81.64 feet by 0.4 to find the curved edge equals 32.656 feet.
Add the diameter to the result to find the perimeter of a part, or sector, of a circle. No matter how large the slice, the two straight edges will be radii and therefore the total will equal the diameter. Completing this example, add 32.656 feet to 26 feet to get 58.656 feet as the perimeter.