How do I find the area from the diameter?

Area = π(d/2)² = πd²/4. For example, a circle with diameter 10 has area = π × 25 ≈ 78.54 square units.

What is the circumference?

The circumference is the total length around the circle (the perimeter). C = 2πr. For a circle with radius 5: C = 2 × 3.14159 × 5 ≈ 31.42 units.

Pi (π) is the ratio of a circle's circumference to its diameter. It is an irrational number approximately equal to 3.14159265358979... Its digits never repeat or terminate.

Circle Calculator — Area, Circumference & Diameter

The Circle Calculator computes the area, circumference, and diameter of a circle from the radius. All circle measurements follow from a single value — the radius — through the constant π (pi ≈ 3.14159). This calculator is useful for geometry problems, engineering design, construction, and everyday applications.

Radius

How to Use This Calculator

1
Enter the radius of the circle.
2
Click Calculate to see the area (πr²).
3
Circumference = 2πr = 2 × 3.14159 × radius.
4
Diameter = 2 × radius.

Frequently Asked Questions

Area = πr². Circumference = 2πr = πd. Diameter = 2r. Where r = radius, d = diameter, π ≈ 3.14159265.

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