Storage Capacity Measurement Standards

Source

Discrepancy Between Reported Capacity and Actual Capacity

Many customers are confused when their operating system reports, for example, that their new ST3500620AS 500-GB hard drive is reporting only 465.76 GB in usable capacity. Several factors can come into play when you see the reported capacity of a hard drive. Unfortunately there are two different number systems used to express units of storage capacity:

• Binary, which says that a kilobyte is equal to 1024 bytes; and
• Decimal, which says that a kilobyte is equal to 1000 bytes.

The storage industry standard is to display capacity in decimal. Even though in binary you have more bytes, the decimal representation of a gigabyte shows greater capacity. In order to accurately understand the true capacity of your hard drive, you have to know which base unit of
measure (binary or decimal) is being used to represent capacity.

Another factor that can cause misrepresentation of the size of a hard drive is BIOS limitations. Many older BIOS are limited in the number of cylinders they can support.

Motivation for Proposed Prefixes for Binary Multiples
Once upon a time, computer professionals noticed that 1024 or 2¹⁰ (binary) was very nearly equal to 1000 or 10³ (decimal) and started using the prefix "kilo" to mean 1024. That worked
well enough for a decade or two because everybody who talked kilobytes knew that the term implied 1024 bytes. But almost overnight a much more numerous "everybody" bought computers, and the trade computer professionals needed to talk to physicists and engineers and even to ordinary people, most of whom know that a kilometre is 1000 metres and a kilogram is 1000 grams.

Two Different Measurements Systems
Name	Ab- bre- via- tion	Bi- na- ry Po- wer	Binary Value (represented in Decimal)	Deci- mal Power
kilo- byte	KB	210	1,024	103
mega byte	MB	220	1,048,576	106
giga- byte	GB	230	1,073,741,824	109
tera- byte	TB	240	1,099,511, 627,776	1012

Often,
when two or more people begin discussing storage capacity, some will refer to binary values and others will refer to decimal values without making a distinction between the two. This has caused much confusion in the past. In an effort to end this confusion, all major hard drive manufactures use decimal values when discussing storage capacity.

How Operating Systems Report Drive Capacity
Click on image to see larger version.

Windows Vista
Open
the Start menu on the bottom left of the screen (it may simply be a Windows logo). Open Computer, right-click on the drive in question, choose Properties.

Windows XP / 2000 / NT
From Windows Explorer, right click on a drive letter, then click on Properties. This shows capacities in bytes, MB, and GB.

Windows 98 / Me
From Windows Explorer, right click on a drive letter, then click on Properties. This shows bytes, MB, and GB.
DOS Prompt ? CHKDSK shows bytes
DOS Prompt ? FDISK shows MB

Mac OSX
From the Desktop, click Go on the top menu bar, choose Utilities, choose Disk Utility. Then hold down the Control key and click on the hard drive in question on the left, then choose Information (see the 1st image). The total capacity is listed as bytes and in GB, calculated as binary (see the 2nd image, and note the capacity circled in red).

Image 1


Image 2

Please also see the Seagate Knowledge Base article dealing with this issue.

Much of this information is available from the National Institute of Standards and Technology at http://physics.nist.gov/cuu/Units/binary.html