C Program to find area of a circle with given radius

October 20, 2023, Learn eTutorial

For a better understanding of this C program example, we always recommend you to learn the basic topics of C programming listed below:

How to find the area of a circle given radius

In this C program, we need to find the area of a circle with the radius given. A circle is a shape that has no corners, and the area of the circle is defined as the region that is inside that closed shape.

In a circle, we have two important measurements which are

  1. Radius r: Radius is the length of the line that joins any point on the circle and its center.
  2. Diameter d: The diameter of a circle can be defined as the double of the radius, that is a line that connects two points on the circle which passes through the center.
find area of a circle with given radius

The area of a circle is calculated using the formula,  Π r2, where, r is the radius.

Let's take an example of a circle with a diameter of 4. which means the circle has a 4 cm length between the longest points. then its radius will be 2.

Now we can calculate the area by using the formula Π r2

  • "Π * 2 * 2"
  • that is "Π* 4"
  • The value of Π is 3.14
  • then the final area will be 3.14 * 4 = 12.56.

How we calculate the area of the circle with #define using the C program

  • Let's check how we implement the above logic in the C program.
  • We are defining the value of 'pi' as 3.14 as it is a constant value.
  • To apply the formula that use to find the ara of the circle we are using pow function which is defined in the math library for calculating the square of the radius, and then we are using precision to the float value to print only two decimals after the dot.


STEP 1: Import the header file libraries into the C program to use built-in functions such as stdio, conio, and math.h.

STEP 2: Define the value of 'pi' as '3.14' using C programming syntax.

STEP 3: Open the main() to start the program in the C language.

STEP 4: After initializing the variables for radius and area using float data type, accept the value of radius from the user using printf and scanf functions in C.

STEP 5: Calculate the area of the circle using the formula 'pi * pow (radius,2)'.

STEP 6: Print the result using printf statement, Here we use 5.2f which adds Precision in the float data type. which means it prints only that much decimal value as result.

C Source Code

                                          #include <stdio.h>
#include <math.h>
#define PI 3.142

void main()
     float radius, area;
     printf("Enter the radius of a circle\n");
     scanf ("%f", & radius);
     area = PI * pow (radius,2);
     printf ("Area of a circle = %5.2f\n", area);       


Enter the radius of a circle
Area of a circle = 32.17

Enter the radius of a circle
Area of a circle = 113.11