Can I convert hex to decimal using the calculator app?
You can use Ubuntu's default calculator in programming mode.
Open the dash and search for Calculator, then select : Mode > Programming Mode.
Enter the value to convert, then press equal =. The entered value will get bolded. Then you may select the target Base from the drop-box to have the value converted.
And if you want something you can do from the command line, you can use trusty old bc
echo "obase=16; 255" | bc
produces FF
When I need to convert to hex from command line, I do this:
printf "%x\n" 255
and when I need to convert from hex, it get simpler:
echo $((0xff))