Thus, to find the circumference of a circle, multiply the diameter by Π. If you know only the radius (a more likely scenario), multiply the radius by 2 to find the diameter: c = 2×Π×r. To find the diameter of a circle, divide the circumference by Π. Use 3.14 for Π.

Try it! Find a pan, trash can, or other large circular object. Measure around the edge, and then measure the diameter. The circumference divided by the diameter should be roughly equal to Π.

Formula for the Area of a Circle

Interestingly enough, Π is also the ratio between the area of a circle and the square of its radius. Thus, Π = A/r2; A = Π×r2; and r = . The most important equation to remember is the middle equation, A = Π×r2. Thus, to find the area of a circle, square the radius and multiply by Π. If the radius is unknown but the diameter is known, divide the diameter by 2 to find the radius.

Examples:

What is the circumference of a circle with diameter 5?
c = d×Π = 5×3.14 = 15.7

What is the circumference of a circle with radius 3?
d = 3×2 = 6;c = d×Π = 6×3.14 = 18.8

What is the area of a circle with radius 3?
A = Π×r2 = 3.14×32 = 28.3

What is the area of a circle with diameter 5?
r = 5/2 = 2.5;A = Π×r2 = 3.14×2.52 = 19.6

What is the diameter of a circle with circumference 11?
d = c/Π = 11/3.14 = 3.50

What is the radius of a circle with circumference 11?
r = d /2 = 3.50/2 = 1.75

What is the area of a circle with circumference 11?
A = Π×r2 = 3.14×1.752 = 9.62