How Mac OS X reports drive capacity

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Understanding storage drive capacity in Mac OS X v10.0 through 10.5

Storage drive manufacturers measure storage drive capacity using the decimal system (base 10), so 1 gigabyte (GB) is calculated as exactly 1,000,000,000 bytes. The capacity of the storage drive in your Mac, iPod, iPhone and other Apple hardware is measured using the decimal system. We set this out on our product packaging and on our website through the statement "1 GB = 1 billion bytes."

Operating systems, including the operating system on your Mac, iPod, iPhone, or other electronic devices, use the binary system (base 2) of measurement. In binary, 1 GB is calculated as 1,073,741,824 bytes. This difference in how the decimal and binary numeral systems measure a GB is what causes a 4 GB storage drive to appear as 3.7 GB when detailed by an operating system, even though the storage drive still has 4 billion bytes, as reported. You will see this difference if you look at how your computer summarizes the capacity of the computer’s storage drive or of your iPod’s or iPhone’s storage drive when the device is connected to your computer. You will also see this difference in the "About" menu on your iPod or iPhone. The important point to understand is that the available storage capacity is the same no matter which system is used. Nothing is missing.

The storage drive in your Apple product, like all storage drives, uses some capacity for formatting, so actual capacity available for applications will be less. In addition, other factors, such as pre-installed systems or other software and media, will also use part of the available storage capacity on the drive.

Understanding storage drive capacity in Mac OS X v10.6 and later

In Mac OS X v10.6 Snow Leopard, storage capacity is displayed as per product specifications (base 10). A 200 GB drive show 200 GB capacity (for example, if you select the hard drive's icon and choose Get Info from the Finder's File menu, then look at the Capacity line). This means that, for example, if you upgrade from an earlier version of Mac OS X, your drive may show more capacity than in the earlier Mac OS X version.

The storage drive in your Apple product, like all storage drives, uses some capacity for formatting, so actual storage available for applications will be less. In addition, other factors, such as pre-installed systems or other software and media, will also use part of the available storage capacity on the drive.