In these lessons, we will learn

- the diameter of the circle
- the radius of the circle

- the diameter of the circle
- the radius of the circle
- the area of the circle

A circle is a closed curve formed by a set of points on a plane that are the same distance from its center. The circumference of a circle is the distance around the circle. It is sometimes called the perimeter of a circle.

Calculating the circumference of a circle involves a constant called *pi* with the symbol *π*. Pi (*π*) is the ratio of the circumference of a circle to its diameter. Pi is always the same number for any circle.

The value of *π* (pi) is approximately
3.14159265358979323846...
but usually rounding to 3.142 should be sufficient. (see a mnemonic for *π*)

The formula for the circumference of a circle is

C = πd(see a mnemonic for this formula)or

C =2πr

where *C* is the circumference, *d* is the diameter and *r* is the radius.

The diameter of a circle is a line segment that passes through the center of the circle and has its endpoints on the circle. The radius of the circle is a line segment from the center of the circle to a point on the circle. The diameter of a circle is twice the length of its radius.

If you are given the diameter then use the formula *C = πd*

If you are given the radius then use the formula *C = *2*πr*

The following video shows a fun way to remember the formulas for the circumference of a circle and the area of a circle.

The formula for the circumference of a circle is the product of pi (*π*) and the diameter or twice the product of pi (*π*) and the radius.

Example:

Find the circumference of the circle with a diameter of 8 inches.

Solution:

Step 1: Write down the formula: | C = πd |

Step 2: Plug in the value: | C = 8π |

Answer: The circumference of the circle is 8*π* ≈ 25.163 inches.

Example:

Find the circumference of the circle with a radius of 5 inches.

Solution:

Step 1: Write down the formula: | C = 2πr |

Step 2: Plug in the value: | C = 10π |

Answer: The circumference of the circle is 10 *π* ≈ 31.42 inches.

Worksheet to calculate circumference of circle when given diameter or radius. | Worksheet to calculate circumference and area of circle when given diameter or radius. |

From the formula* C = *2*πr*, we see that we can find the radius of a circle by dividing its circumference by 2*π*.

Step 1: Find the circumference and substitute.

Step 2: Divide by π

Step 3: Divide by 2

Step 4: Write the units

From the formula* C = **πd*, we see that we can find the diameter of a circle by dividing its circumference by *π*.

To find the area of a circle when given the circumference, we first use the circumference to find the radius. Then, we use the radius to find the area of the circle.

Worksheet Calculate problems that involve the radius, diameter, circumference and area of circle. | Worksheet 1Word problems that involve circles. |

Worksheet 2 Word problems that involve circles. |

Rotate to landscape screen format on a mobile phone or small tablet to use the **Mathway** widget, a free math problem solver that **answers your questions with step-by-step explanations**.

You can use the free Mathway widget below to practice Algebra or other math topics. Try the given examples, or type in your own problem and check your answer with the step-by-step explanations.

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