1. A line from the center of a circle to a point on the circle.

2. The distance from the center of a circle to a point on the circle.

2. The distance from the center of a circle to a point on the circle.

Try this Drag the orange dot. The blue line will always remain a radius of the circle.

The radius of a circle is the length of the line from the center to any point on its edge. The plural form is radii (pronounced "ray-dee-eye"). In the figure above, drag the orange dot around and see that the radius is always constant at any point on the circle.

Sometimes the word 'radius' is used to refer to the line itself. In that sense you may see "draw a radius of the circle". In the more recent sense, it is the length of the line, and so is referred to as "the radius of the circle is 1.7 centimeters"

where:

where:

ENTER ANY ONE VALUE | ||

Radius | clear | |

Diameter | clear | |

Area | clear | |

Circumference | clear | |

Use the calculator above to calculate the properties of a circle.

Enter any single value and the other three will be calculated. For example: enter the radius and press 'Calculate'. The area, diameter and circumference will be calculated.

Similarly, if you enter the area, the radius needed to get that area will be calculated, along with the diameter and circumference.

**Diameter** Which is the circle's 'width'. The diameter is two times the radius.
See diameter of a circle

**Circumference**
The circumference is the distance around the edge of the circle. See
Circumference of a Circle for more.

- In the figure above, click 'reset' and drag the orange dot. Notice that the radius is the same length at any point around the circle.
- Click on "show diameter". Drag either orange dot at the ends of the diameter line. Note how the radius is always half the diameter.
- Uncheck the "fixed size" box. Repeat the above and note how the radius is always half the diameter no matter what the size of the circle.

- Circle definition
- Radius of a circle
- Diameter of a circle
- Circumference of a circle
- Parts of a circle (diagram)
- Semicircle definition
- Tangent
- Secant
- Chord
- Intersecting chords theorem
- Intersecting secant lengths theorem
- Intersecting secant angles theorem
- Area of a circle
- Concentric circles
- Annulus
- Area of an annulus
- Sector of a circle
- Area of a circle sector
- Segment of a circle
- Area of a circle segment (given central angle)
- Area of a circle segment (given segment height)

- Basic Equation of a Circle (Center at origin)
- General Equation of a Circle (Center anywhere)
- Parametric Equation of a Circle

- Arc
- Arc length
- Arc angle measure
- Adjacent arcs
- Major/minor arcs
- Intercepted Arc
- Sector of a circle
- Radius of an arc or segment, given height/width
- Sagitta - height of an arc or segment

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