Why does my computer say my brand new hard drive has less capacity than was advertised?

It's because hard drive manufacturers define gigabytes and megabytes differently than some computer systems.

For simplicity, hard drive manufacturers define a megabyte as 1,000,000 bytes and a gigabyte as 1,000,000,000 bytes. This is a decimal system and seems familiar to anyone who worked with the metric system in school. (Here in the United States, we're introduced to the metric system in school, even if we don't use it very often.) It's the way hard drive manufacturers have always defined these terms.

Computers, however, sometimes define a megabyte as 1,048,576 bytes and a gigabyte as 1,073,741,824 bytes. This is called a binary measurement and it's the method historically used by computers.

Hard Drive manufacturer G-Tech likens this to the difference between 0 degrees Celsius and 32 degrees Fahrenheit: it's really the same thing, we just use different names.

Now, hard drive capacities are moving into 2, 4, 6 (and higher) Terabytes. A terabyte equals either 1,000,000,000,000 bytes or 1,099,511,627,776 bytes depending on what system you use:

Advertised Size (Decimal)	=	"Computer" Size (Binary)	=	Number of Bytes
1TB		931.10GB		1,000,000,000,000
2TB		1.82 TB		2,000,000,000,000
4TB		3.64TB		4,000,000,000,000
6TB		5.46TB		6,000,000,000,000
8TB		7.28TB		8,000,000,000,000

(I rounded the numbers after the second decimal place. )

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