Problem scenario: Using Bash you want to compute kB, MB, and GB with decimal values. But you only seem to be able to do arithmetic on integer values.
Root cause: Bash does not support "float" arithmetic with values to the right of the period. Decimal values cannot be calculated.
Solution: Install the bc utility/language (e.g. yum -y install bc). It is often used in Bash to augment the numeric processing features. It is technical its own processing language, but do not be intimidated by that.