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C Program: Area and Circumference of a Circle

Introduction

A circle is a closed 2D shape where every point on the boundary is equidistant from the center.
Two important properties of a circle are its area (the space enclosed) and circumference (the distance around it).

The formulas are:

Area=π×r²  

Circumference=2×π×r

Where:

• r = radius of the circle
• π(pi) ≈ 3.14159

In this program, we take the radius of a circle from the user and compute both the area and circumference.

C Program: Area and Circumference of a Circle

 
#include <stdio.h>   // Standard I/O header
#define PI 3.14159   // Defining constant value of PI
 
int main() {
    float radius, area, circumference;
 
    // Input from user
    printf("Enter the radius of the circle: ");
    scanf("%f", &radius);
 
    // Calculations
    area = PI * radius * radius;
    circumference = 2 * PI * radius;
 
    // Output results
    printf("Area of the circle          = %.2f\n", area);
    printf("Circumference of the circle = %.2f\n", circumference);
 
    return 0; // Successful termination
}

 
OUTPUT 1 :
Enter the radius of the circle: 7
Area of the circle          = 153.94
Circumference of the circle = 43.98

OUTPUT 2 :
Enter the radius of the circle: 3.5
Area of the circle          = 38.48
Circumference of the circle = 21.99

Explanation :

1. #define PI 3.14159
   • Defines a constant value for π (pi).
2. Variable declaration
   • float radius, area, circumference;
3. Input
   • User enters the radius of the circle.
4. Formulas applied
   • area = PI * radius * radius;
   • circumference = 2 * PI * radius;
5. Output
   • Results displayed with %.2f → two decimal places.