bash - check free available space against requered

in bash i will run a script that take a look in the folder folder123/ to know how much space is requered for the files they are in there. but this requered value will i multiplie with 1,5 and than subtract from the space they are available is at . if there are enough available space than go on in the script, but if not exit with an error. for checking how much space is requered i try this du -b folder123/ | tail -n 1 | awk '{print $1}' for checking how much space is available, i find out this df --output=avail -B 1 "$PWD" |tail -n 1 but how to multiplie the requered with 1,5 and than to subtract from the available, in bash-script? :edit if i use something like avail=$(df --output=avail -B 1 . | tail -n 1) req=$(( $(du -sb tempdir/ | cut -f1) * 3 / 2)) sum=$(printf '%d\n' "$((avail - req))") but how than to go on to check if $sum are oke?