C sin() Function

Beginner
5 min read
Updated: Nov 2026
3 examples
math.h
Decorative image with the word SIN

Definition

In C, sin() returns the sine of an angle. The angle is in radians. sin(0) is 0.

You pass one double. The result is a double from -1 to 1. Include <math.h>.

Syntax

Pass the angle in radians. Start with this call.

C
sin(0.0);
CallWhat it does
sin(0.0)Returns 0.

Include <math.h>. On GCC, link with -lm. Convert degrees with rad = deg * acos(-1.0) / 180.0.

Library declarations
C
double sin(double x);
float sinf(float x);
long double sinl(long double x);

Parameters

ParameterDescription
xRequired. The angle in radians.

Return Value

Return ValueDescription
SineA double from -1 to 1.

C Examples

Example 1 — Common Radian Values

Use a portable π and print classic unit-circle results.

C
#include <math.h>
#include <stdio.h>

int main(void) {
    const double PI = acos(-1.0);

    printf("sin(0)    = %.4f\n", sin(0.0));
    printf("sin(PI/6) = %.4f\n", sin(PI / 6.0));
    printf("sin(PI/2) = %.4f\n", sin(PI / 2.0));
    printf("sin(PI)   = %.4f\n", sin(PI));

    return 0;
}

How It Works

  1. Get π. acos(-1.0) is a portable constant for π.
  2. Pass radians. PI/6 is 30°; PI/2 is 90°.
  3. Known results. sin(π/6) = 0.5, sin(π/2) = 1.
  4. Link libm. On GCC, remember -lm.

Example 2 — Convert Degrees Before sin

Never pass degree numbers directly—convert first.

C
#include <math.h>
#include <stdio.h>

int main(void) {
    double deg = 30.0;
    double pi = acos(-1.0);
    double rad = deg * pi / 180.0;

    printf("%.0f deg = %.4f rad\n", deg, rad);
    printf("sin(30 deg) = %.4f\n", sin(rad));

    return 0;
}

How It Works

  1. Start with degrees. Humans often think in 30°, 45°, 90°.
  2. Scale to radians. Multiply by π / 180.
  3. Call sin. Now the argument is correct → 0.5.
  4. Avoid the trap. sin(30) without conversion is 30 radians.

Example 3 — sin with cos

On a unit circle, sin² + cos² = 1. Pair with cos to check.

C
#include <math.h>
#include <stdio.h>

int main(void) {
    double rad = acos(-1.0) / 3.0;  /* 60° */
    double s = sin(rad);
    double c = cos(rad);

    printf("sin = %.4f\n", s);
    printf("cos = %.4f\n", c);
    printf("sin^2 + cos^2 = %.4f\n", s * s + c * c);

    return 0;
}

How It Works

  1. Pick an angle. π/3 is 60°.
  2. Compute both. sin ≈ 0.866, cos ≈ 0.5.
  3. Check identity. Squares sum to about 1.
  4. Unit circle. sin is the y-component; cos is the x-component.

Frequently Asked Questions

sin() returns the sine of an angle in radians. sin(0) is 0.
x is required. It is the angle in radians.
A double from -1 to 1.
Include <math.h>. On GCC, link with -lm.
No. Convert degrees to radians first. 180 degrees is π radians.

Next: sinh()

Learn the hyperbolic sine—similar name, different math.

sinh() tutorial →

About the author

Mari Selvan M P
Mari Selvan M P 🔗

Developer, cloud engineer, and technical writer

  • Experience 12 years building web and cloud systems
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I write practical tutorials so students and working developers can learn by doing—from databases and APIs to deployment on AWS.

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