@don_crissti's first Answer is good, but can be even shorter using [Here Strings][1], 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

 [1]: https://stackoverflow.com/a/4775845/5477197