Temperature conversion application
UI experience

Your menue text is hard to read and not intuitively clear. I'd prefer to see something like that:

     Console.WriteLine(
@"Choose to convert temperature values, enter a key please:\n
1 - Fahrenheit to Celsius
2 - Fahrenheit to Kelvin
3 - Celsius to Fahrenheit
4 - Celsius to Kelvin
5 - Kelvin to Fahrenheit
6 - Kelvin to Celsius, Press 6
&gt; ");


Also outputting the text that way would make the code more readable.

Taking input from console

Your statements

 var YInput = Console.ReadLine();
 int iInput = Convert.ToInt32(YInput);


can be simplified to

 int iInput = Convert.ToInt32(Console.ReadLine());


consider to catch exceptions or use a String and TryParse() to check for invalid input.1

Rather use switch than if else cascades

You should replace 

 if (iInput == 1) 
          {
              var aAnswer = FtoCelsius(dInput);

              Console.WriteLine("The value of {0} degrees Fahrenheit is equall to {1} degrees Celsius", dInput, aAnswer);
          }
 else if (iInput == 2)
          {
              var bAnswer = FtoKelvin(dInput);

              Console.WriteLine("The value of {0} degrees Fahrenheit is equall to {1} degrees Kelvin", dInput, bAnswer);
          }


with

switch(iInput) {
case 1:
    var aAnswer = FtoCelsius(dInput);
    Console.WriteLine("The value of {0} degrees Fahrenheit is equall to {1} degrees Celsius", dInput, aAnswer);
    break;
case 2:
    var bAnswer = FtoKelvin(dInput);
    Console.WriteLine("The value of {0} degrees Fahrenheit is equall to {1} degrees Kelvin", dInput, bAnswer);
    break;
}


That makes your code more readable and concise.

Write generic code, if it's prone to be extended

At least I'd try to make all of that more generic, using a 

public delegate double ConversionDelegate(double x); 
Dictionary&lt;Tuple&lt;string,string&gt;,ConversionDelegate) = {
    { {"Fahrenheit","Celsius"}, FtoCelsius } ,
    { {"Fahrenheit","Kelvin"}, FtoKelvin} ,
    // ...
    { {"Kelvin", "Celsius" }, KtoCelsius ,
};


dictionary pair. Where the tuple key serves the possible combinations of Fahrenheit to Celsius, Kelvin to Fahrenheit, etc. You can use the functions from Dictionary to match the key, and call the delegate and render the output accordingly.

The command line input should look like this then

&gt; Enter temperature and &lt;unit&gt; &lt;conversion unit&gt;
&gt; 39.5 celsius kelvin
&gt; The value of 39.5 degrees celsius is equal to 312,65 degrees Kelvin




1)BTW you rather want to deal with double values IMO.