Circle Calculator (A = πr²)

Enter any one known value — radius, diameter, circumference or area — and the rest follow through π. Worked example: r = 5 cm gives A = π × 25 = 78.54 cm² and C = 2π × 5 = 31.42 cm. If you know the diameter instead, halve it first: d = 10 cm means r = 5 cm.

1. 1

   Enter the known value

   Radius, diameter, circumference or area — any one of them.

2. 2

   Check the units

   Length in cm, m, ft or in; area comes out squared.

3. 3

   Read all results

   The remaining three circle properties appear instantly.

• 1

  π ≈ 3.14159; the calculator keeps full precision until the final rounding

• 2

  Circumference is the perimeter of the circle: C = πd works too

• 3

  Area scales with the square of the radius: double r, quadruple A

• Using the diameter where the radius belongs — r = d/2, so the area shrinks 4×

• Forgetting to square r in A = πr²

• Reporting area in linear units instead of square units

• Rounding π to 3.1 in precision work (use 3.14159 or better)