Tool to convert between human-readable byte sizes and bare byte counts

Can you recommend a Unix-style tool that converts between human-readable bytesizes (e.g., 10KiB->10240, 10240->10KiB)? By Unix-style I mean that it should do just this (i.e., be just a simple filter without any GUI or other cruft) and do it well (i.e., handle different units, behave intelligently by default while allowing optional settings, such as different interpretation of non-IT units (k==1000 OR 1024), optional custom rounding, optional conversion to units that aren't the most fitting ones, etc.).