**Why do we use the radius ruler?**

In school we are taught the mathematical equation as how to find the radius of an object.

We have compiled some examples from the internet. These examples are prepared from Wikihow.com.

You can learn all the equations by simply following this link

How to Calculate the Radius of a Circle

The radius of a circle is the distance from the center of the circle to its edge. The diameter of the circle is the distance all the way across it, and is equal to two times the radius. You will often be asked to calculate the radius of the circle based on other measurements. This article will teach you to calculate the radius of a circle when you know its diameter, circumference, and its area. It will then teach you a more advanced method of finding the center and radius of a when you know the coordinates of three points that lie on the circle.

**Method 1: Calculating the Radius when you know the Diameter**

1: Remember what a diameter is. The diameter of a circle is the length of the line drawn through the center of a circle that touches the circle at two points. The diameter is the longest line that can be drawn through a circle, and cuts the circle into two equal halves. It is also two times the length of the radius. The formula for the diameter is D = 2r, where “D” stands for diameter, and “r” stands for radius. The same formula in terms of r is r = D/2.

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2: Divide the diameter by 2 to find the radius. If you are given the diameter of the circle, simply divide it by 2 to find the radius. For example, if the diameter of a circle is 4, the radius is equal to 4/2, or 2.

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**Method 2: Calculating the Radius when you know the Circumference**

1: Recall the formula for the circumference of a circle. The circumference of the circle is the distance around it. Another way to think of it is that the circumference is the length of the line you would get if you cut the circle open and stretched it out flat. The formula for the circumference of a circle is C = 2πr, where “r” is the radius, and π is the constant pi, or 3.14159... The formula for the radius in terms of the circumference is then r = C/2π.

It is usually OK to round pi off the hundredths place (3.14), but check with your teacher to find out how many places she would like you to use.

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2: Calculate the radius from the circumference. To calculate the radius based on the circumference, simply divide the circumference by 2π, or 6.28.

For example, if the circumference of a circle is 15, the radius r = 15/2π, or 2.39.

http://www.wikihow.com/Calculate-the-Radius-of-a-Circle

**Method 3: Calculating the Radius When You Know the Area**

1: Recall the formula for the area of a circle. The area of a circle is defined as A = πr2. If we restate that formula in terms of r, it becomes: r = √(A/π) (“r equals the square root of the Area divided by pi”).

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2: Plug the area into the formula. For example, if the area of the circle is 21 in 2 ; when we plug that value into the formula, we get: r = √(21/π).

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3: Divide the area by π (3.14). 21 / 3.14 = 6.69.

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4: Use your calculator to find the square root of this number. The result will be the radius of your circle. In our example, √6.69 = 2.59, the radius of our circle.

http://www.wikihow.com/Calculate-the-Radius-of-a-Circle

We have presented to you 3 methods on how to find the radius of a round or cylindrical object for further details and explanations we encourage you to use the website http://www.wikihow.com

The Waltz Radius Ruler in no way wants or has an intention to not teach you the formula’s presented in school, from the internet or from this website http://www.wikihow.com

The Waltz Radius Ruler is a ruler built as a tool the same way a measuring tape is used.