Enter any one value
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cm
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r · d · C · A
A = πr². Square the radius, then multiply it by π. Example: radius 5 cm gives and value of π = 3.14159 (or 3.14 for basic math). then put the value in formula 3.14 × 5² = 78.50 cm².
Wrap a string once around the circle's edge and its length is the circumference: C = 2πr, or the same thing written as C = πd. A bicycle wheel with a 30 cm radius covers 2 × π × 30 ≈ 188.5 cm of ground with every full turn.
Radius is a single spoke from the center to the rim; diameter is two spokes laid end to end straight across, passing through the center. That relationship, d = 2r, holds for every circle no matter the size. if radius are 7cm, 2 × 7 = 14. so diameter are 14cm.
If you know the area of a circle and want to find its radius, start with the circle-area formula: A = πr², r = √(A ÷ π). Example: Suppose the area of a circle is 154 cm². Divide the area by π and then take the square root: r = √(154 ÷ 3.14), r ≈ √49.04, r ≈ 7 cm So, the radius of the circle is approximately 7 cm.