How to convert Kilobytes to Megabytes or gigabytes through terminal?

The shell does fixed-width integer arithmetic with no check for overflow. So, when doing a calculation that might involve either large numbers or fractions, bc is a good choice. To get megabytes:

$ echo "scale=2; $(sudo fdisk -s /dev/sda6) / 1024" | bc
13641.75

To get gigabytes:

$ echo "scale=2; $(sudo fdisk -s /dev/sda6) / 1024^2" | bc
12.70

The assignment scale=2 tells bc to display two decimal places.

numfmt (part of GNU Coreutils) can be used here:

$ sudo fdisk -s /dev/sda | numfmt --to=iec-i --suffix=B --format="%.2f"
931.52MiB

In awk

To find the size of the disk in Megabytes,

$ sudo fdisk -s /dev/sda | awk '{$1=$1/1024; print $1,"MB";}'
953870 MB

To find the size of the disk in Gigabytes,

$ sudo fdisk -s /dev/sda | awk '{$1=$1/(1024^2); print $1,"GB";}'
931.513 GB