how compiler decide the range of real numbers in C...

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Say 16 bit compiler (for ex Turbo C) uses 2's compliment to find the range of signed integer.
Say for 16 bit n=16. Now applying 2's compliment equation : -2^(n-1)  to 2^(n-1) -1
so for n=16 : The range is -32768 to 32767

This is ok. The question arises when it tell about real constant's range.
For 16 bit compiler the range of real constant is : -3.4e38 to 3.4e38

How they arrived to this real constant range?

Can any one share the mathematical calculation behind it?

thnx,
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