A line forming a closed loop, every point on which is a fixed distance from a center point.

Try this Drag an orange dot. The circle can be moved by dragging the center point
and resized by dragging the point on the circle.

A circle is a type of line. Imagine a straight line segment that is bent around until its ends join. Then arrange that loop until it is exactly circular - that is, all points along that line are the same distance from a center point.

There is a difference between a circle and a disk. A circle is a line, and so, for example, has no area - just as a line has no area. A disk however is a round portion of a plane which has a circular outline. If you draw a circle on paper and cut it out, the round piece is a disk.

Center | A point inside the circle. All points on the circle are equidistant (same distance) from the center point. |

Radius | The radius is the distance from the center to any point on the circle. It is half the diameter. See Radius of a circle. |

Diameter | The distance across the circle. The length of any chord passing through the center. It is twice the radius. See Diameter of a circle. |

Circumference | The circumference is the distance around the circle. See Circumference of a Circle. |

Area | Strictly speaking a circle is a line, and so has no area. What is usually meant is the area of the region enclosed by the circle. See Area enclosed by a circle . |

Chord | A line segment linking any two points on a circle. See Chord definition |

Tangent | A line passing a circle and touching it at just one point. See Tangent definition |

Secant | A line that intersects a circle at two points. See Secant definition |

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.

You can define a circle as the shape created when a plane cuts through a cone at right angles to the cone's axis. For more on this see Conic sections - circle.

A circle is the locus of all points a fixed distance from a given (center) point. This definition assumes the plane is composed of an infinite number of points and we select only those that are a fixed distance from the center. (See locus definition.)

In coordinate geometry, a circle can be described using sets of equations.

For more on this see
Equations of circles and ellipses.

- 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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