C Program to Find Area and Circumference of a Circle

The area of a circle is A = π r² and the circumference is C = 2πr, where r is the radius. This program computes both from a user-supplied radius. Key points: use radius * radius instead of pow(radius, 2) (no need for the math library for a simple square), define π as a high-precision constant with #define, and validate the input (radius cannot be negative). The original post used conio.h, void main, clrscr(), and #define PI 3.142 (only 4 significant figures); this rewrite fixes all of those.

Radius	Area (π r²)	Circumference (2πr)
1	3.1416	6.2832
5	78.5398	31.4159
7	153.9380	43.9823
10	314.1593	62.8319

C Program: Area and Circumference of a Circle

/* Find the area and circumference of a circle given the radius
 * Compile: gcc -ansi -Wall -Wextra circle.c -o circle -lm */
#include <stdio.h>
#include <math.h>

#define PI 3.14159265358979323846

int main(void)
{
    double radius, area, circumference;

    printf("Enter the radius: ");
    if (scanf("%lf", &radius) != 1 || radius < 0.0) {
        printf("Enter a non-negative radius.\n");
        return 1;
    }

    area          = PI * radius * radius;
    circumference = 2.0 * PI * radius;

    printf("Radius        = %.4f\n", radius);
    printf("Area          = %.4f\n", area);
    printf("Circumference = %.4f\n", circumference);

    return 0;
}

How to Compile and Run

gcc -ansi -Wall -Wextra circle.c -o circle -lm
./circle

Note: -lm is not strictly needed here since we do not call any math functions, but it is included for completeness when the header is present. If the linker complains, remove -lm.

Sample Output

Enter the radius: 5
Radius        = 5.0000
Area          = 78.5398
Circumference = 31.4159

Enter the radius: 7
Radius        = 7.0000
Area          = 153.9380
Circumference = 43.9823

Enter the radius: 0
Radius        = 0.0000
Area          = 0.0000
Circumference = 0.0000

Code Explanation

PI as a #define constant — using a named constant instead of the literal 3.14159 everywhere means you change it in one place if you want more or less precision. The value here has 20 digits — well beyond what double can represent (about 15–17 significant decimal digits), so this is the most precise value possible in C’s double.
radius * radius instead of pow(radius, 2) — pow() is a general-purpose floating-point function that handles arbitrary real exponents. For a simple integer exponent like 2, multiplying directly is both faster and avoids linking the math library just for this purpose. Use pow() only when the exponent is non-integer or very large.
Input validation: radius < 0.0 — a negative radius has no geometric meaning. Checking after reading prevents computing a negative area, which would silently produce a wrong result. Zero radius is valid (degenerate circle) and outputs zeros correctly.
double instead of float — float has only about 6–7 significant digits of precision. For geometric calculations where rounding errors accumulate, double (15–17 digits) is the better default. The original post used float with %f; this rewrite uses double with %lf for scanf and %f/%.4f for printf.

What This Program Teaches

Named constants with #define — always name magic numbers. #define PI 3.14159265358979323846 is clearer than scattering 3.14 throughout the code and avoids subtle bugs where you type 3.1415 in one place and 3.14159 in another.
Choosing the right arithmetic — prefer simple multiplication over library functions when both are correct. r * r for squaring is idiomatic C. Reserve pow() for cases where the multiplication chain would be unwieldy (e.g., pow(x, 17)) or when the exponent is fractional (pow(x, 0.5) = √x).
double vs float — in modern C, double is the default floating-point type. float is used when memory is genuinely scarce (embedded systems, large arrays). For everyday scalar arithmetic, always use double.

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C Program to Find the Area of a Circle Given the Radius