The area is calculated using the formula A = π × r², where r is the radius. For example: a circle with radius 5 cm has area π × 5² = 78.54 cm².

The circumference is calculated using the formula C = 2 × π × r, where r is the radius. For example: a circle with radius 5 cm has circumference 2 × π × 5 = 31.42 cm.

The radius is the distance from the center to the edge. The diameter is the distance across the circle through the center. Diameter = 2 × radius.