The circle is the shape that gives you the most area for the least perimeter — which is why pizzas, manholes, and storage tanks are all round — and its area formula is the most famous in geometry: A = πr². Just the radius, squared, times pi. The catch is that problems rarely hand you the radius directly; sometimes you get the diameter (halve it), sometimes the circumference (divide by 2π), and mixing those up is the number-one error. This calculator accepts the Radius (r) in your choice of units and returns three results at once: the Area (A = πr²), the Circumference (C = 2πr), and the Diameter (d = 2r). A radius of 5 meters gives an area of about 78.54 m², a circumference of about 31.42 m, and a diameter of 10 m. Gardeners sizing circular beds, painters estimating round ceilings, and students checking geometry homework all land here. Remember that area units are square — square meters, square feet — because you multiplied a length by a length.

With the pre-filled defaults — Radius (r) 5 — the Area of Circle Calculator returns Area 78.539816 m², Circumference 31.415927 m, Diameter 10.000000 m. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.