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Free online circle calculator to find area, circumference, radius & diameter instantly. Enter any one value and get all circle properties with step-by-step solutions.
Calculate area, circumference, radius & diameter - Enter any one value
A circle calculator is a mathematical tool that calculates all properties of a circle including radius, diameter, circumference, and area. You only need to enter ONE value, and the calculator automatically computes all other measurements using standard circle formulas.
Choose Input Method: Select whether you want to calculate from radius, diameter, circumference, or area
Select Unit: Choose your preferred measurement unit (mm, cm, m, inch, feet)
Enter Value: Input the known measurement (only ONE value needed)
Calculate: Click the "Calculate" button to get instant results
View Results: See all circle properties with detailed step-by-step solutions. Download or copy results as needed
Formula: A = πr²
Alternative: A = π(d/2)² or A = πd²/4
Example: If radius = 5 cm, Area = π × 5² = 78.54 cm²
Formula: C = 2πr
Alternative: C = πd
Example: If radius = 7 cm, Circumference = 2π × 7 = 43.98 cm
Formula: r = C/(2π)
Alternative: r = C/(2 × 3.14159)
Example: If C = 31.42 cm, Radius = 31.42/(2π) = 5 cm
Formula: r = √(A/π)
Steps: Divide area by π, then take square root
Example: If A = 78.54 cm², Radius = √(78.54/π) = 5 cm
Formula: d = 2r
Reverse: r = d/2
Example: If radius = 6 cm, Diameter = 2 × 6 = 12 cm
Formula: d = C/π
Steps: Divide circumference by π
Example: If C = 31.42 cm, Diameter = 31.42/π = 10 cm
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Learn circle geometry, solve homework problems, and understand π. Practice mathematical concepts with real examples.
Area of circle = πr² where r is the radius. For example, if radius = 5 cm, then area = π × 5² = 78.54 cm². You can also use diameter: A = π(d/2)² or circumference: A = C²/(4π).
Circumference = 2πr or πd, where r is radius and d is diameter. For example, if radius = 7 cm, circumference = 2 × π × 7 = 43.98 cm. If diameter = 10 cm, circumference = π × 10 = 31.42 cm.
Radius = C/(2π) where C is circumference. For example, if circumference = 31.42 cm, then radius = 31.42/(2π) = 5 cm. You can also find diameter first: d = C/π, then r = d/2.
Diameter = 2 × radius, or radius = diameter/2. The diameter is the longest distance across a circle passing through the center, while radius is the distance from center to edge.
Area = π(d/2)² or πd²/4, where d is diameter. For example, if diameter = 10 cm, area = π × (10/2)² = π × 25 = 78.54 cm². First find radius (d/2), then use A = πr².
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Calculate from radius, diameter, circumference, or area - enter any ONE value
Detailed calculation steps with formulas for educational value
Get radius, diameter, circumference, and area in one calculation
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