Hello, the SI prefix for 'a thousand' is a lowercase k and not an uppercase K [1].
The default behavior of numfmt with '--from=si' option is therefore contrary to expectation: $ numfmt --from=si 500k # Should be accepted as valid SI numfmt: invalid suffix in input: ‘500k’ $ numfmt --from=si 500K # Should result in error, 'K' stands for kelvin unit. 500000 $ numfmt --version numfmt (GNU coreutils) 8.26 Packaged by Cygwin (8.26-2) Copyright (C) 2016 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>. This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Written by Assaf Gordon. Best regards, Daniel. [1] Page 143 of https://www.bipm.org/en/publications/si-brochure/