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Calculating the radius of a circle is a fundamental skill in geometry and mathematics. Understanding the concepts and formulas involved in finding the radius allows us to measure, analyze, and solve problems related to circles. The radius of a circle can be determined through various methods, depending on the information you have available. Whether you are working with the circumference, diameter, or area of a circle, knowing how to calculate the radius accurately is essential. In this guide, we will explore different approaches to calculate the radius of a circle, equipping you with the necessary skills to tackle circle-related problems with confidence.

This article was co-written by Grace Imson, MA. Grace Imson is a math teacher with over 40 years of teaching experience. Grace is currently a math teacher at City University of San Francisco and previously worked in the math department of Saint Louis University. She has taught math at the elementary, middle, high, and college levels. She holds a master’s degree in education from Saint Louis University, majoring in management and supervision in education.

This article has been viewed 315,867 times.

The radius of a circle is the distance from the center of the circle to any point on the circumference of the circle. ^{[1] X Research Source} The easiest way to calculate the radius of a circle is to divide the diameter of the circle in half. If you don’t know the diameter of the circle but know other measurements, such as circumference ( OLD=2π(r){displaystyle C=2pi (r)} ) or area ( A=π(r2){displaystyle A=pi (r^{2})} ) of the circle, you can still find the radius of the circle using formulas and split variables r{displaystyle r} .

## Steps

### Find the radius when the circumference of the circle is known

**Write the formula for the circumference of a circle.**This formula is

, in there OLD{displaystyle C} is the circumference, and r{displaystyle r} is the radius. ^{[2] X Research Source}

- Symbol pi{displaystyle pi} (“pi”) is a special number, approximately 3.14. You can use this value (3,14) in calculations or use the symbol pi{displaystyle pi} on the computer.

**Calculate r (radius).**Use algebra to convert the formula for the circumference of a circle until only

*r*(radius) remains on one side of the equation:

For example *OLD**=**2**π**r**{displaystyle C=2pi r}**OLD**2**π**=**2**π**r**2**π**{displaystyle {frac {C}{2pi }}={frac {2pi r}{2pi }}}**OLD**2**π**=**r**{displaystyle {frac {C}{2pi }}=r}**r**=**OLD**2**π**{displaystyle r={frac {C}{2pi }}}** *

**Substitute the perimeter value into the formula.**When the problem tells you the value of

*C*for the circumference of a circle, you can use this equation to find the radius

*r*. We will substitute the value

*C*of the circumference of the circle in the math problem into the equation:

For example

If the circumference of the circle is 15 cm, we will have the formula: r=152π{displaystyle r={frac {15}{2pi }}} cm

**Round to decimal.**Enter the results into the calculator with the button π{displaystyle pi } and round the number. If you don’t have a calculator, you can do the math by hand, using 3.14 as an approximate value of π{displaystyle pi } .

For example

r=152π={displaystyle r={frac {15}{2pi }}=} about 7.52∗3,14={displaystyle {frac {7.5}{2*3.14}}=} approximately 2.39 cm

### Find the radius when the area of the circle is known

**Write down the formula for calculating the area of a circle.**This formula is

, in there A{displaystyle A} is the area of the circle, and r{displaystyle r} is the radius. ^{[3] X Research Sources}

**Solve the equation to find the radius.**Use algebra to return

*r*to one side of the equation:

For example

Divide both sides by π{displaystyle pi } :

A=πr2{displaystyle A=pi r^{2}}

Aπ=r2{displaystyle {frac {A}{pi }}=r^{2}}

Take the square root of both sides:

Aπ=r{displaystyle {sqrt {frac {A}{pi }}}=r}

r=Aπ{displaystyle r={sqrt {frac {A}{pi }}}}

**Substitute the area value into the formula.**Use this formula to find the radius if the problem gives the area value of the circle. We will substitute the value of the area of the circle for the variable A{displaystyle A} .

For example

If the area of the circle is 21 square centimeters, this formula would be:r=21π{displaystyle r={sqrt {frac {21}{pi }}}}

**Divide the area by the number**

**π**

**{displaystyle pi }****.**Start by reducing the part below the square root sign ( Aπ){displaystyle {frac {A}{pi }})} . Use the calculator with the . button π{displaystyle pi } if possible. If we don’t have a calculator, we’ll use 3.14 as the value of . π{displaystyle pi } .

For example

If 3.14 is used instead of π{displaystyle pi } , we have the calculation:

r=213,14{displaystyle r={sqrt {frac {21}{3,14}}}}

r=6,69{displaystyle r={sqrt {6,69}}}

If the calculator allows entering the entire formula in a row, we will have a more accurate answer.

**Calculate square root.**

, since this is a decimal. The result will be the radius of the circle.

For example

r=6,69=2,59{displaystyle r={sqrt {6.69}}=2.59} . Thus, the radius of a circle with an area of 21 square centimeters is about 2.59 centimeters.

Area always uses square units (like square centimeters), but radius always uses length units (like centimeters). If you look at the units in this problem, you’ll notice csquare meter2=csquare meter{displaystyle {sqrt {cm^{2}}}=cm} .

### Find the radius when the diameter of the circle is known

**Find the diameter of the circle in the problem.**The radius of a circle will be very easy to calculate if the problem is given the diameter data. If you are calculating on a specific circle,

, touch both opposite points on the circle. ^{[4] X Research Sources}

- If you are not sure where the center of the circle is, place the ruler across the circle according to your estimate. Keep the zero line on the ruler always close to the circle and slowly move the other end of the ruler around the circle. The largest measurement you will find will be the diameter measurement.
- For example, your circle might have a diameter of 4 cm.

**Divide the diameter in half.**Radius of the circle

^{[5] X Research Sources}

- For example, if the circle’s diameter is 4 cm then its radius will be 4 cm ÷ 2 =
**2 cm**. - In the mathematical formula, the radius is denoted by
*r*and the diameter is*d*. This formula in a textbook can be written as follows: r=d2{displaystyle r={frac {d}{2}}} .

### Calculate the radius when knowing the area and the angle at the center of the fan shape

**Write the formula for calculating the area of a fan.**This formula is

, in there ASector{displaystyle A_{sector}} is the fan-shaped area, θ{displaystyle theta } is the angle at the center of the fan in degrees, and r{displaystyle r} is the radius of the circle. ^{[6] X Research Source}

**Substitute the area and angle at the center of the fan into the formula.**This information is in the title.

We will substitute the value of the fan area for the variable ASector{displaystyle A_{sector}} and the central angle for the variable θ{displaystyle theta } .

For example

If the area of the fan is 50 square centimeters, and the angle at the center is 120 degrees, we have the following formula:

50=120360(π)(r2){displaystyle 50={frac {120}{360}}(pi )(r^{2})} .

**Divide the central angle by 360.**That way we will know how much of the circle the fan occupies.

For example

120360=first3{displaystyle {frac {120}{360}}={frac {1}{3}}} , that is, the fan shape is equal to first3{displaystyle {frac {1}{3}}} circle.

We will have the following equation: 50=first3(π)(r2){displaystyle 50={frac {1}{3}}(pi )(r^{2})}

**Split number**

**(**

**π**

**)**

**(**

**r**

**2**

**)**

**{displaystyle (pi )(r^{2})}****.**To do this, we divide both sides of the equation by the fraction or decimal we just calculated above.

For example

50=first3(π)(r2){displaystyle 50={frac {1}{3}}(pi )(r^{2})}

50first3=first3(π)(r2)first3{displaystyle {frac {50}{frac {1}{3}}}={frac {{frac {1}{3}}(pi )(r^{2})}{frac {1}{3}} }}

150=(π)(r2){displaystyle 150=(pi )(r^{2})}

**Divide both sides of the equation by the number**

**π**

**{displaystyle pi }****.**This step will separate the variable r{displaystyle r} . For more accurate results, you can use a calculator. You can also round numbers π{displaystyle pi } to 3.14.

For example

150=(π)(r2){displaystyle 150=(pi )(r^{2})}

150π=(π)(r2)π{displaystyle {frac {150}{pi }}={frac {(pi )(r^{2})}{pi }}}

47.7=r2{displaystyle 47.7=r^{2}}

**Calculate the square root of both sides.**The result of the calculation will be the radius of the circle.

For example

47,7=r2{displaystyle 47.7=r^{2}}

47,7=r2{displaystyle {sqrt {47,7}}={sqrt {r^{2}}}}

6,91=r{displaystyle 6.91=r}

Thus, the radius of the circle will be about 6.91 cm.

## Advice

- Number pi{displaystyle pi} actually in a circle. If we measure the circumference
*C*and diameter*d*of the circle very precisely, then the calculation OLD÷d{displaystyle Cdiv d} will return the number pi{displaystyle pi} .

This article was co-written by Grace Imson, MA. Grace Imson is a math teacher with over 40 years of teaching experience. Grace is currently a math teacher at City University of San Francisco and previously worked in the math department of Saint Louis University. She has taught math at the elementary, middle, high, and college levels. She holds a master’s degree in education from Saint Louis University, majoring in management and supervision in education.

This article has been viewed 315,867 times.

The radius of a circle is the distance from the center of the circle to any point on the circumference of the circle. ^{[1] X Research Source} The easiest way to calculate the radius of a circle is to divide the diameter of the circle in half. If you don’t know the diameter of the circle but know other measurements, such as circumference ( OLD=2π(r){displaystyle C=2pi (r)} ) or area ( A=π(r2){displaystyle A=pi (r^{2})} ) of the circle, you can still find the radius of the circle using formulas and split variables r{displaystyle r} .

In conclusion, calculating the radius of a circle is a fundamental concept in geometry and can be done using a few different methods. The most common method is to use the formula r = d/2, where r represents the radius and d represents the diameter of the circle. This method is quick and straightforward, allowing for easy calculations. Alternatively, the radius can also be found using the formula r = sqrt(A/π), where r represents the radius and A represents the area of the circle. This method is useful when the area is given and can provide a more precise measurement. Lastly, the radius of a circle can also be determined by measuring the distance from the center point to any point on the circumference using a ruler or measuring tape. Overall, understanding how to calculate the radius of a circle is essential in various fields, including mathematics, engineering, and architecture, and can be applied to numerous real-life situations.

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