Circle Calculator
Solve any circle dimension from any other — radius, diameter, circumference or area. Just enter what you know and get the rest.
How to use the Circle Calculator
- Enter your inputs into the Circle Calculator above.
- Results update instantly as you type — no submit button needed.
- Adjust any value to see how the result changes in real time.
The circle formulas
d = 2r · · · C = 2πr = πd · · · A = πr²
All four quantities are related through the radius. The constant π ≈ 3.14159 appears in every formula because it's the defining ratio of a circle.
Worked example
A circle with radius 7 cm: diameter = 14 cm, circumference = 2π × 7 ≈ 43.98 cm, area = π × 49 ≈ 153.94 cm². Given area = 100 m²: radius = √(100/π) ≈ 5.64 m.
Frequently asked questions
Why does π appear in every formula?
π is defined as the ratio of any circle's circumference to its diameter. So any measurement of a circle's size in terms of its center-to-edge distance involves π.
What is the formula for sector area?
Area of a sector with angle θ in radians: ½r²θ. In degrees: (θ/360) × πr². A semicircle (180°) is half the full area.
How accurate is π?
For all practical purposes, 3.14159 (5 decimals) is plenty. Engineering uses up to 8–12 places; pure math has computed π to trillions of digits with no practical application beyond benchmarking.