Difference between nominal capacity and actual capacity:
    Since the nominal capacity of storage devices is usually calculated according to 1GB=1000MB, 1MB=1000KB, 1KB=1000B, while the computer system recognizes the storage capacity according to 1GB=1024MB, 1MB=1024KB, 1KB=1024B, the actual usable capacity of USB flash drives is slightly smaller than the nominal capacity. Therefore, the actual usable capacity of USB flash disk will be slightly smaller than the nominal capacity. Generally speaking, the actual usable capacity of a USB flash drive with a nominal capacity of 16GB is around 14.5GB to 15GB.
System Occupancy and Hidden Partition:
    Some USB flash drives have built-in system files or hidden partitions for storing device drivers, encryption software, etc., which will further reduce the actual usable capacity. For example, some encrypted USB flash drives may take up hundreds of MB or even more than 1GB of space for their encryption software.