Why storage devices show a less capacity than real value

Many of the Microsoft windows users complain their storage devices like hard disks and flash drives show a less capacity in windows operating systems than manufacturer mentioned capacity. For example 4 GB pen drive usually have a capacity of about 3.72 GB. 320 GB hard disk has about a capacity of about 300 GB. People give different opinions as reason for this. Some says windows allocate some space for back up process. Some accuse the manufacturer. But actually the mentioned capacity is there. Reason is a matter of using different unite.
The smallest unit we use to measure data is byte(B). Bit (b) is smaller than byte. But we usually don't use bit to measure size of stored data. Bit is used to measure data transfer rates. Size of stored data is measured in bytes. When numerical value of the bytes grows we use prefixes like Kilo, Mega, Giga and Tera. The problem starts from this point.

In system international (SI) units these prefixes are used for powers of 10.

• Kilo = 10³ = 1000
• Mega = 10⁶ = 1000 * 1000
• Giga = 10⁹ = 1000 * 1000 * 1000
• Tera = 10¹² = 1000 * 1000 * 1000 * 1000 

But in computer world this prefixes are used for powers of 2.

• Kilo = 2¹⁰ =  1024
• Mega =2²⁰ = 1024 * 1024  
• Giga = 2³⁰ = 1024 * 1024 * 1024
• Tera = 2⁴⁰ = 1024 * 1024 * 1024 * 1024

Storage device manufactures use SI unit system to mention the capacity of their storage devices. But computer displays the capacity of storage devices using binary prefixes (powers of 2). This is the reason for the mismatch between the mentioned capacity and displayed capacity.

SI: 10⁹ B = 1 GB 

Binary: 2³⁰ B = 1 GB

Therefore SI 1 GB = Binary (1/2³⁰ )*10⁹ = Binary 0.9313225 GB

SI 4 GB = 0.9313225 * 4 = Binary 3.725 GB

Now I think you understand the reason for showing a lesser capacity in Windows. But Mac operating systems show the same storage value as same as the mentioned value in the cover. The reason is Mac operating systems use SI system not binary system.