Circle Calculator
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Area (πr²) and circumference (2πr) from a radius or diameter.
Circle dimension
Area
78.5398
Circumference: 31.4159
- Circumference
- 31.4159
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Circle area and circumference
A circle's area is π times the radius squared (A = πr²), and its circumference (the distance around it) is 2 times π times the radius (C = 2πr), or equivalently π times the diameter (C = πd). The radius is always half the diameter.
π (pi) is an irrational constant, approximately 3.14159, representing the ratio of a circle's circumference to its diameter for every circle regardless of size.
Example: a circle with radius 2 has area π × 2² ≈ 12.57 square units and circumference 2π × 2 ≈ 12.57 units — it's a coincidence at r = 2 that area and circumference match numerically, since they use different exponents of r.
Common questions
What is the difference between radius and diameter?
The radius is the distance from the center to the edge; the diameter is the full width across the circle through its center — exactly twice the radius.
How precise is π in this calculator?
This calculator uses JavaScript's built-in double-precision value of π (about 15 significant digits), far more precise than typical classroom approximations like 3.14.