How to calculate the range of maximum float...???

Question

which is 3.4e-38 to 3.4e+38

Answer 1

The difference between the maximum float value and the minimum float value (?). You may need to elaborate a little more on your query.

Answer 2

First include limits.h :
#include <limits.h>

Then you have access to the macro FLT_MAX.
As the minimum value of float is -FLT_MAX, you only need one macro.