I have bought 160GB USB Drive, but why I only get a disk size of 149 GB, where is the remaining 11 GB gone ?

Well, the answer lies in how the bytes are interpreted. Disk Manufacturers use "Decimal" as a base for calculation (i.e Base 10).

10^3 = 1000 bytes = 1 KiloByte , 10^6 = 1000 KB = 1 MegaByte , 10^9 = 1000 MB = 1 GigaByte

Whereas, the actual utilization is calculated using "Binary" as a base for calculation (i.e Base 2).

2^10 = 1024 bytes = 1 KiloByte , 2^20 = 1024 KB = 1 MegaByte, 2^30 = 1024 MB = 1 GigaByte

Note: Hard Disk Manufacturers will use 1000, not 1024 as base.

In this case, my USB drive has total bytes equal to = 160,039,239,690 Bytes. And we know 1 KB = 1024 Bytes, 1MB = 1024 Kilobytes, 1GB = 1024 Megabytes.

Hence, the total actual capacity comes to = 160,039,239,690 / 1024 x 1024 x 1024 = 149 GB.

In general, the capacity of a hard disk can be calculated using this formula:

Total Size of the Disk (Bytes) = (Cylinders) X (Heads) X (Sectors) X (Bytes per Sector)