Circle Calculator

Area = π × r², where r is the radius. For a circle with radius 5, the area is π × 25 ≈ 78.540 square units. The unit follows from the radius: if r is in inches, area is in square inches.

Circumference = 2 × π × r, equivalently π × d where d is the diameter. For radius 5, the circumference is 10π ≈ 31.416 units.

From diameter, divide by 2: r = d ÷ 2. From circumference, divide by 2π: r = C ÷ (2π). From area, take the square root of A divided by π: r = √(A ÷ π). The calculator runs whichever rearrangement matches your input.

π is the ratio of a circle's circumference to its diameter, the same value for every circle. By definition C ÷ d = π, so C = πd = 2πr. Area follows from integrating concentric rings: A = πr². π is irrational, so every numeric answer involving a circle is a rounded approximation.

It uses JavaScript's Math.PI, which is correct to about 15 significant digits. Results are displayed with up to 6 decimal places, more than enough for typical homework or planning math. If you need symbolic answers, leave one term written as π and substitute at the end.

No. A circle is a 2D shape with area and circumference; a sphere is the 3D version, with surface area and volume. For sphere math, use the surface area calculator or volume calculator. The radius is the same concept in both.