By Jon Zamboni • Updated Aug 30, 2022
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The radius is the distance from the center of a circle to any point on its circumference. All points on the edge share the same radius. If a problem gives you the diameter instead of the radius, simply divide the diameter by two. For example, a diameter of 8 units yields a radius of 4 units.
Pi is a mathematical constant that represents the ratio of a circle’s circumference to its diameter. Its value is approximately 3.14159, but for most educational calculations we use 3.14 as a practical approximation. When higher precision is required, you can refer to official mathematical tables or software for more digits.
The circumference (C) is the length around the circle. The formula is C = 2 π r. Using π = 3.14, the equation can also be written as C = 6.28 r. For a circle with a radius of 4 inches, the circumference is 25.12 inches (4 × 6.28).
The area (A) is the space enclosed by the circle. The formula is A = π r². This means you multiply the radius by itself and then by π. For a radius of 3 inches: 3 × 3 = 9, and 9 × 3.14 = 28.26, so the area is 28.26 square inches.
