Explore Radius on MathWorld

The radius of a circle is the distance from its center to its circumference or from the center of a sphere to its surface. The radius is equal to half the diameter.

Radius is an elementary-school-level concept. It is listed in the California State Standards for Grade 4.


Circle: A circle is the set of points in a plane that are equidistant from a given center point.
Perimeter: The perimeter is a length around the boundary of a closed two-dimensional region. The perimeter of a circle is called its circumference.
Sphere: A sphere is the set of all points in three-dimensional space that are located at a fixed distance from a given point.

Classroom Articles on Plane Geometry

  • Acute Angle
  • Hypotenuse
  • Altitude
  • Midpoint
  • Angle
  • Obtuse Angle
  • Area
  • Parallel
  • Circumference
  • Perpendicular
  • Collinear
  • Pi
  • Complementary Angles
  • Plane Geometry
  • Diameter
  • Point
  • Geometric Construction
  • Supplementary Angles
  • Golden Ratio
  • Triangle Inequality
  • Golden Rectangle

  • Classroom Articles on Geometry (Up to Elementary-School Level)

  • Cone
  • Pentagon
  • Cube
  • Polygon
  • Cylinder
  • Prism
  • Decagon
  • Pyramid
  • Dodecagon
  • Quadrilateral
  • Equilateral Triangle
  • Rectangle
  • Hendecagon
  • Regular Polygon
  • Heptagon
  • Right Triangle
  • Hexagon
  • Square
  • Isosceles Triangle
  • Surface Area
  • Nonagon
  • Triangle
  • Octagon
  • Volume
  • Parallelogram