Geometry
Area, circumference, and diameter of a circle from its radius — in whatever unit you enter, out in the same unit.
Enter the radius in any unit — metres, centimetres, inches — and the results come back in that unit (square units for area). Results update as you type. All calculations run locally in your browser.
—
—
Area = π × radius². Circumference = 2 × π × radius. Diameter = 2 × radius. π is used at full precision and results are rounded to two decimals for display.
Radius 5: area = π × 5² = π × 25 = 78.54. Circumference = 2 × π × 5 = 31.42. Diameter = 10. If the radius was 5 cm, the area is 78.54 cm² and the circumference is 31.42 cm.
This is a perfect circle: one radius, one centre. Ellipses, arcs, sectors, and rings need their own formulas and are not covered here. The page does not convert units — a radius in inches gives an area in square inches; convert first if you need another unit.
Radius: the distance from the centre to the edge. If you only know the diameter (edge to edge through the centre), halve it first — or read the diameter chip above in reverse.
Because area covers a surface. A radius of 5 cm gives 78.54 cm², not 78.54 cm. Mixing the two is the most common circle mistake.
Full double-precision pi (3.141592653589793…), not 3.14. The difference only matters past the second decimal, but there is no reason to round early.
With the square: doubling the radius quadruples the area. A radius-10 circle has area 314.16, exactly four times the radius-5 area of 78.54.
No. A sphere's surface area is 4πr² and its volume is (4/3)πr³ — different formulas for a 3D object.
Divide it by 2π to get the radius, then enter that here. For a circumference of 31.42, the radius is 31.42 ÷ 6.2832 = 5.
Results are estimates only and do not constitute professional advice.