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Circle Radius, Diameter & Area Calculator

Solve all circle properties (radius, diameter, circumference, area) from any single input.

Quick Answer:Select which circle dimension you know (radius, diameter, area, circumference), enter the value, and the solver calculates all other metrics.

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How to Use the Circle Calculator: Radius, Diameter, Circumference & Area

1Select your known dimension from the dropdown (Radius, Diameter, Area, or Circumference).
2Enter the numerical measurement.
3Instantly view the calculated diameter, radius, perimeter circumference, and total area.
4Use the exact metrics for pipe sizing, circular patios, or round tables.

Formula & How It Works

Fundamental Circle Geometry

d = 2r; C = 2πr = πd; A = πr² = π(d/2)²

Applies standard relations governed by mathematical constant π (3.14159...).

Worked Examples

Circle with Radius 5 cm

Input: Radius = 5

Explanation: d = 2(5) = 10; C = 2π(5) ≈ 31.42; A = π(25) ≈ 78.54 square units.

Result: Diameter = 10, Circumference = 31.4159, Area = 78.5398

Frequently Asked Questions

What is the relationship between radius and diameter?▼

The diameter of a circle is always exactly twice the length of its radius (d = 2r).

How do you find the radius if you only know the area?▼

Divide the area by π and take the square root of the result: r = √(Area / π).

How do you calculate circumference from diameter?▼

Multiply the diameter directly by π: Circumference = π × d.

What is the value of pi used in this calculator?▼

This calculator uses the full IEEE 64-bit precision of Math.PI (approximately 3.141592653589793).

Quick Reference & Conversion Table

Radius (r)	Diameter (d)	Circumference (C)	Area (A)
1.00	2.00	6.283	3.142
2.50	5.00	15.708	19.635
5.00	10.00	31.416	78.540
10.00	20.00	62.832	314.159

⚠️ Common Pitfalls to Avoid

•Squaring the diameter instead of the radius when calculating circle area.
•Truncating π to 3.14 on large architectural projects, which causes measurable dimensional error.