Some may have noticed that the new external hard disk drive (HDD) or solid state drive (SSD) that we have just bought from our favorite does not exactly have the amount of space available that we paid for. Ever wonder why?

First, it is important to understand how data is stored on such storage devices. All computer data is stored in a binary format, called a bit, represented as either a 1 or 0. 8 bits is equal to 1 byte. Storage capacity is measured by a prefix to the B (byte) to represent a specific amount, such as:

• K (Kilobyte)
• M (Megabyte)
• G (Gigabyte)
• T (Terabyte)
• etc.

Since all computers are based on the binary system, these prefixes represent base 2 amounts. Each level is an increment of 2 to the 10th power or 1,024.

The common prefixes are as follows:

• Kilobyte (KB) = 1,024 Bytes
• MegaByte (MB) = 1,024 Kilobytes or 1,048,576 Bytes
• Gigabyte (GB) = 1,024 Megabytes or 1,073,741,824 Bytes
• Terabyte (TB) = 1,024 Gigabytes or 1,099,511,627,776 Bytes

Long ago, manufacturers decided to rate most drive capacities based on the standard base 10 numbers we are all familiar with, since it's easier than for most people to think in base 2 mathematics. Therefore, one Megabyte equals one million bytes while one Gigabyte equals one billion bytes. This isn't too much of a problem with fairly small numbers such as a Kilobyte, but each level of increase in the prefix also increased the total discrepancy of the actual space compared to the advertised space.

Here is a quick reference to show the amount that the actual values differ compared to the advertised for each common referenced value:

• Megabyte Difference = 48,576 Bytes
• Gigabyte Difference = 73,741,824 Bytes
• Terabyte Difference = 99,511,627,776 Bytes

Based on this, for each Gigabyte that a drive manufacturer claims, they are over reporting the amount of disk space by 73,741,824 Bytes or roughly 70.3 MB of disk space. So, if a manufacturer advertises an 80 GB (80 billion bytes) hard drive, the actual disk space is around 74.5 GB of space, roughly 7% less than what they advertise.

Below is quick table referencing some common advertised disk sizes vs actual sizes.

Credits: Information above gathered from http://www.ussscctv.com/hdccalc.html.