If you use busybox's "du" in emebedded system, you can not get a exact bytes with du, only Kbytes you can get.
BusyBox v1.4.1 (2007-11-30 20:37:49 EST) multi-call binary
Usage: du [-aHLdclsxhmk] [FILE]...
Summarize disk space used for each FILE and/or directory.
Disk space is printed in units of 1024 bytes.
Options:
-a Show sizes of files in addition to directories
-H Follow symbolic links that are FILE command line args
-L Follow all symbolic links encountered
-d N Limit output to directories (and files with -a) of depth < N
-c Output a grand total
-l Count sizes many times if hard linked
-s Display only a total for each argument
-x Skip directories on different filesystems
-h Print sizes in human readable format (e.g., 1K 243M 2G )
-m Print sizes in megabytes
-k Print sizes in kilobytes(default)
