@don_crissti's first Answer is good, but can be even shorter using Here StringsHere Strings, e.g.
$ numfmt --to=iec-i <<< "12345" 13Ki $ numfmt --to=iec-i --suffix=B <<< "1234567" 1.2MiB or even
$ numfmt --from=iec-i --to=iec-i --suffix=B <<< "12345Ki" 13MiB if <<< is not available you can use e.g.
$ echo "1234567" | numfmt --to=iec-i --suffix=B 1.2MiB