Circle & Geometry Calculator

A circle's area is A = π × r² and its circumference is C = 2 × π × r, where r is the radius and π is about 3.14159. For a radius of 5 m the area is π × 25 = 78.54 m² and the circumference is 2 × π × 5 = 31.42 m. If you know the diameter, halve it to get the radius. Because area depends on the radius squared, doubling the radius makes the area four times larger, while the circumference only doubles. This matters when you order concrete for a round slab, paint for a circular wall, or fencing for a circular enclosure, so always check which measurement you have before you calculate.