The radius is the distance from a circle's center to any point on its edge. It is half the diameter. Every radius of a circle has the same length. The plural is radii.

The radius appears in formulas for circumference (2πr), area (πr²), and volume of spheres and cylinders. In a sphere, the radius extends from center to surface. In a cylinder, it is the radius of the circular base.

In geometry, the radius defines the size of a circle. In physics, it describes orbits, atomic models, and turning radii. In engineering, it specifies gears, pipes, and wheels. The concept is simple but appears in countless formulas and applications.

• Distance from center to edge
• Half the diameter
• Used in circumference, area, and volume formulas
• Defines circle size and appears in many applications