def _si(number):
    """Format a number using base-2 SI prefixes"""
    prefixes = ['', 'K', 'M', 'G', 'T', 'P', 'E', 'Z', 'Y']
    while number > 1024:
        number /= 1024.0
        prefixes.pop(0)
    return '%0.2f%s' % (number, prefixes.pop(0))
