Converting between different units of temperature
Numerical consts are double, either add them eg as 5.0f or use double.
Define a constants for 0°C = 273.15K and also for Fahrenheit to Celsius conversation (avoid repeated magic constants 273.15, 9.0 / 5.0, 32.0).
In check the distance from 273.15 is not symmetric, probably a typo.
The name check doesn't imply what is checked, meaningful names not limited to this function would be a recommended.


A modified version could include:

#include &lt;math.h&gt;

const double Celsius2KelvinShift = 273.15;
const double Fahrenheit2KCelsiusShift = 32.0;
const double Fahrenheit2CelsiusFactor = 5.0 / 9.0;

/** check if kelvin temperature is celsius 0 **/
int checkIf0Celsius(double N)
{
    const double tol = 0.001; /** I had to set this limit. There is no other way. **/
    return fabs(N - Celsius2KelvinShift) &lt; tol;
}

/*** Fahrenheit ***/
double Fahrenheit2Kelvin(double F) 
{
    return Fahrenheit2CelsiusFactor * (F - Fahrenheit2KCelsiusShift) + Celsius2KelvinShift;
}

double Fahrenheit2Celsius(double F) 
{
    return Fahrenheit2CelsiusFactor * (F - Fahrenheit2KCelsiusShift);
}

/** Celsius **/
double Celsius2Kelvin(double C) 
{
return C + Celsius2KelvinShift;
}

double Celsius2Fahrenheit(double C) 
{
    return C / Fahrenheit2CelsiusFactor - Fahrenheit2KCelsiusShift;
}

/*** Kelvin ***/
double Kelvin2Celsius(double K) 
{
    return K - Celsius2KelvinShift;
}

double Kelvin2Fahrenheit(double K) {
    return (K - Celsius2KelvinShift) / Fahrenheit2CelsiusFactor - Fahrenheit2KCelsiusShift;
}