How OS X and iOS report storage capacity

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Note: This article applies to any kind storage media that an Apple product ships with, such as a hard drive (most Macs and some earlier iPods), flash drive (iPad, iPhone, most iPods), or solid-state drive (SSD) (some MacBook models).

Understanding storage capacity in Mac OS X v10.5 (or earlier), iPad, iPhone, iPod

Capacity stated on product packaging

Storage device manufacturers measure 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 media in your Mac (Mac OS X v10.5 or earlier), iPad, iPod, iPhone and other Apple hardware is measured using this decimal system. This is detailed on product packaging and online through the statement "1 GB = 1 billion bytes."

Capacity stated in Mac OS X or iOS

When you view the storage capacity of your Mac (Mac OS X v10.5 or earlier), iPod, iPhone, iPad, or other electronic devices within its operating system, the capacity is reported using the 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 32 GB storage device to appear as about 28 GB when detailed by its operating system, even though the storage device still has 32 billion bytes, as reported. You will see this difference if you look at how your computer summarizes the capacity of the computer’s storage, or of your iPod, iPad, or iPhone’s storage when the device is connected to your computer. You will also see this difference in the About menu on your iPod, iPad, or iPhone. The important point to understand is that the available storage capacity is the same no matter which system (decimal or binary) is used. Nothing is missing.

The storage media in your Apple product, like all storage devices, uses some of its capacity for formatting, so actual capacity available for applications and files 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.
 

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

In Mac OS X v10.6 Snow Leopard and later, storage capacity is displayed as per product specifications using the decimal system (base 10). A 200 GB drive shows 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). For example, if you upgrade from an earlier version of OS X, your drive may show more capacity than it did in the earlier OS X version.

The storage drive in your computer with Mac OS X v10.6 and later, 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.

Understanding storage capacity in Solid State Drives

Storage capacity displayed in Disk Utility by for Solid State Drives will show a slightly smaller size. For Example, the 256 GB Solid State Drive (SSD) should have a total of approximately 250 GB.  

These items may account for the additional space used in your Solid State drive:

• EFI Partition
• Restore Partition 
• Wear-leveling blocks
• Write-buffer area
• Metadata
• Spare blocks
• Grown bad blocks
• Factory bad blocks