Circle Calculator
Get a circle’s diameter, circumference and area from its radius in one step.
How it works
d = 2r · C = 2πr · A = πr²
Everything about a circle follows from its radius. The diameter is twice the radius, the circumference is π times the diameter, and the area is π times the radius squared. π ≈ 3.14159 is the same for every circle — it is literally defined as circumference divided by diameter.
Worked example
A circle with radius 5: diameter = 10, circumference = 2π × 5 ≈ 31.4159, and area = π × 5² ≈ 78.5398 square units.
Frequently asked questions
I only know the circumference — how do I get the radius?
Divide by 2π: r = C ÷ 6.2832. A 31.42-inch circumference means a radius of about 5 inches, and from there the area follows.
I only know the area — how do I get the radius?
Reverse the area formula: r = √(A ÷ π). An area of 78.54 gives r = √25 = 5.
Why does area use square units?
Area measures surface, so a radius in inches gives an area in square inches. Mixing that up is the most common circle mistake — circumference is a length, area is not.
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This calculator is for educational purposes. Double-check important results before acting on them.