Circle Calculator
Calculate the area, circumference, diameter, and radius of a circle from any one known value. Instant results with formulas shown — useful for geometry, engineering, and design.
Radius
5
Diameter
10
Circumference
31.415927
Area
78.539816
How to use the circle calculator
Enter any one of radius, diameter, circumference, or area — the other three compute instantly.
Formula & explanation
C = 2πr. A = πr². d = 2r. r = √(A/π) when starting from area.
Examples
Radius 5 → diameter 10, circumference 31.416, area 78.54.
Frequently asked questions
- What is the relationship between radius and diameter?
- Diameter is exactly twice the radius. If you know one, you know the other. Radius is the distance from the center to the edge; diameter is the full width through the center.
- How is circumference calculated from radius?
- Circumference = 2πr, where π ≈ 3.14159. For a circle with radius 5, the circumference is 2 × π × 5 ≈ 31.416.
- How do I find the radius if I only know the area?
- Rearrange A = πr² to get r = √(A / π). Select 'area' as your input and type the area — the calculator does this automatically.
- What units does this calculator use?
- The calculator is unit-agnostic. If you enter the radius in centimeters, the area comes out in cm² and the circumference in cm. Just be consistent with your inputs.
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