T(°C) = (T(°F) - 32) × 5/9

// C Program to convert
// Fahrenheit to Celsuis
#include <stdio.h>
 
// Function to convert Degree
// Fahrenheit to Degree Celsuis
float fahrenheit_to_celsius(float f)
{
  return ((f - 32.0) * 5.0 / 9.0);
}
 
// Driver code
int main()
{
  float f = 40;
 
  // Passing parameter to function
  printf("Temperature in Degree Celsius : %0.2f",
          fahrenheit_to_celsius(f));
  return 0;
}

Temperature in Degree Celsius : 4.44