Circle calculator (area, circumference, radius)
Enter the radius, diameter, circumference or area and get all the other circle measures.
Area is in units², and the circumference is in the same unit as the radius.
How the calculation works
Every measure of a circle derives from the radius (center to edge):
- Diameter:
d = 2·r - Circumference (the outline):
C = 2·π·r - Area (the surface):
A = π·r²
Enter another measure and we find the radius first. E.g., from area, r = √(A ÷ π).
Step-by-step example (radius 5)
- Diameter: 10
- Circumference: 2 × π × 5 ≈ 31.42
- Area: π × 5² ≈ 78.54
Area vs circumference
Area is in squared units; circumference in the radius's unit. Doubling the radius doubles the circumference but quadruples the area.
Examples
- Radius 10: circumference ≈ 62.83; area ≈ 314.16.
- Area 50 → radius = √(50/π) ≈ 3.99.
Frequently asked questions
What is the value of π?
About 3.14159. We use the computer's full π for maximum precision.
Radius vs diameter?
The diameter crosses the whole circle through the center; the radius is half of it (center to edge).
How to find the radius from the circumference?
Divide the circumference by 2π: r = C ÷ (2 × 3.14159).
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Updated on June 30, 2026 · by Rafael Rossi · Methodology & sources