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Circle Calculator

Use our free circle calculator to find the area, circumference, radius, and diameter of a circle. Enter any one known value and instantly get all the other measurements.

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Formula

A circle is the set of all points equidistant from a center point. The distance from the center to any point is the radius (r). This calculator computes the area (A = πr²) and circumference (C = 2πr) from the radius.

A = πr², C = 2πr

Percentage: The percentage value you want to apply

Number: The original number or value

Result: The calculated result

How It Works

1Enter the radius of the circle in the input field.
2Click Calculate to compute the area and circumference.
3Area is calculated as π times the radius squared.
4Circumference is calculated as 2π times the radius.
5All results are shown with up to 6 decimal places.

Examples

Circle with radius 5

1.Enter radius = 5.
2.Area = π × 5² = 25π ≈ 78.539816 units².
3.Circumference = 2 × π × 5 = 10π ≈ 31.415927 units.

Result: Area: 78.539816, Circumference: 31.415927

Circle with radius 10

1.Enter radius = 10.
2.Area = π × 10² = 100π ≈ 314.159265 units².
3.Circumference = 2 × π × 10 = 20π ≈ 62.831853 units.

Result: Area: 314.159265, Circumference: 62.831853

Frequently Asked Questions

The area of a circle is A = πr², where r is the radius. If you have the diameter (d), use A = π(d/2)².

The circumference is C = 2πr, where r is the radius. Alternatively, C = πd, where d is the diameter.

This calculator uses JavaScript's built-in Math.PI, which is approximately 3.141592653589793.

Divide the circumference by 2π: r = C / (2π). For example, if C = 31.416, then r = 31.416 / (2 × 3.14159) = 5.

Yes. The diameter is twice the radius: d = 2r. So if r = 5, then d = 10.

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CalculateMe Team

Last updated: 2026-07-13

Circle Calculator - Area, Circumference, Radius, Diameter | CalculateMe | CalculateMe