Asked by Shivam Storage device manufacturers usually assume 1 kilobyte to equal 1000 bytes, 1 megabyte to equal 1000 kb, and so on. On the other hand, windows assumes 1 kilobyte to mean 1024 bytes, and 1024 kb to mean 1 megabyte, and so on. Calculations would show that a pen drive advertised as 16 GB by the manufacturer are equal to 16*10⁹ bytes, which is 16 * 10⁹ / 2³⁰ = 14.9 GB in windows, which is pretty accurate based on what you experimentally observe.