How do you find the the area of a circle when you know the diameter?

Answer 1

The area of a circle is found with the equation: Area = Pi x radius2.

To find the radius, divide the diameter by two. The next step of the equation is to square the radius. Finally, multiply that by 3.14.

Example: Diameter 20

Find the radius: 20 divided by 2 = 10

Square: 10 times 10 = 100

Multiply by 3.14 for the final answer: 314

(Don't forget to include units! If your original diameter was measured in cm, the area will be in cm2.)
if you have the diameter, just multiply by 3.14 (pi) because that's how long around the outside the circle is no matter what. But if you want to find the area...that's different.

C=pi x D
So let's call the diameter d and you have that then then circumference is the constant Pi multiplied by the diameter or Pixd.

There are many approximations to Pi. It is a constant and an irrational number which means it goes on forever without repeating and cannot be written as a fraction. It can, however, be approximated using fractions. Many people use 22/7 but an even better approximation is 355/113. That is is easy to remember by just writing the number 113355 which are the first 3 odd numbers each written twice and split it down the middle. Put the bigger part on top and you have a very good approximation to Pi.

So for example, say the diameter is 113 inches, then the circumference is approximately 355 inches.
The Circumference of a Circle when the diameter is a known, fixed number is Pi or about 3.14 multiplied by the Diameter 3.14 x D