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