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measure the distance around a circle and divide it by the distance across the circle through the center, you will always come close to a particular value, depending upon the accuracy of your measurement. This value is approximately 3.14159265358979323846... We use the Greek letter (pronounced Pi) to represent this value. The number goes on forever. However, using computers, mathematicians have been able to calculate the value of to thousands of places.

The distance around a circle is called the circumference. The distance across a circle through the center is called the diameter. is the ratio of the circumference of a circle to the diameter. Thus, for any circle, if you divide the circumference by the diameter, you get a value close to . This relationship is expressed in the following formula: where is circumference and is diameter. You can test this formula at home with a round dinner plate. If you measure the circumference and the diameter of the plate and then divide by , your quotient should come close to . Another way to write this formula is: where · means multiply. This second formula is commonly used in problems where the diameter is given and the circumference is not known (see the examples below).

The radius of a circle is the distance from the center of a circle to any point on the circle. If you place two radii end-to-end in a circle, you would have the same length as one diameter. Thus, the diameter of a circle is twice as long as the radius. This relationship is expressed in the following formula: , where is the diameter and is the radius.

Circumference, diameter and radii are measured in linear units, such as inches and centimeters. A circle has many different radii and many different diameters, each passing through the center. A real-life example of a radius is the spoke of a bicycle wheel. A 9-inch Pizza is an example of a diameter: when one makes the first cut to slice a round pizza pie in half, this cut is the diameter of the pizza. So a 9-inch pizza has a 9-inch diameter. Let's look at some examples of finding the circumference of a circle. In these examples, we will use = 3.14 to simplify our calculations.

Example 1: The radius of a circle is 2 inches. What is the diameter? Solution: = 2 · (2 in) = 4 in

Example 2: The diameter of a circle is 3 centimeters. What is the circumference? Solution: = 3.14 · (3 cm) = 9.42 cm

Example 3: The radius of a circle is 2 inches. What is the circumference? Solution: = 2 · (2 in) = 4 in = 3.14 ·(4 in) = 12.56 in

Example 4: The circumference of a circle is 15.7 centimeters. What is the diameter? Solution: 15.7 cm = 3.14 · 15.7 cm ÷ 3.14 = = 15.7 cm ÷ 3.14 = 5 cm

Courtesy of Ms. Goodies Math Page

A circle does not have a volume. It is a 2-dimensional thing, which has an area. and a perimeter which has a length (circumference). A cylinder, like a tube with closed ends has a volume equal to the area of the base (usually a circle, dicussed above) multiplied by the length of the cylinder.

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13y ago
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14y ago

The area of a circle is 'pi' multiplied by the square of the circle's radius.

The volume of a circle is zero. The volume of a sphere is (4/3 pi) multiplied by the cube of the sphere's radius.

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10y ago

A circle is Pi*Radius^2

So if you measure the diameter of a circle, for example it is 20. Then it is 3.14*10^2 or 314 square units.

Pi*r^2

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12y ago

pi(r)^2 (pi times the radius of the circle squared)

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14y ago

haha u cant find the volume of a circle, you can only find the volume of a 3-D shape

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14y ago

The rule for the area of a circle is πr squared which means pi x the radius squared

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14y ago

me eats cheese alot :)

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12y ago

Type your answer here... pi*^2

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12y ago

Pi x (radius of circle)2

Pi=3.146

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13y ago

Area = (circumference squared)/(4 pi)

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Q: What is the Formular for the area of a circle?
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Area of a circle: pi times radius squared Circumference of a circle: 2 times pi times radius or diameter times pi


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The perimeter of a circle is called the circumference and the formular of finding it is Pi D or 2Pi R


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