How OS X and iOS report storage capacity

Learn why the storage capacity stated in a product's specification may be lower than what is reported by OS X or iOS. 

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), Flash Storage, or solid-state drive (SSD) (some MacBook models).

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, 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."

 Understanding storage capacity in iPad, iPhone, iPod

When you view the storage capacity of your 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.

For example: The way decimal and binary numeral systems measure a GB is what causes a 32 GB storage device to appear as approximately 28 GB when detailed by its operating system, even though the storage device still has 32 billion bytes (not 28 billion bytes), as reported.

You can see this difference if you look at how your computer summarizes the capacity 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 capacity in Mac OS X v10.6, OS X Lion, and OS X Mountain Lion 

In Mac OS X v10.6 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). 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 and Flash Storage

Storage capacity displayed in Disk Utility for Solid State Drives and Flash Storage will show a slightly smaller size. For example, a 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 and Flash Storage: 

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

Note: Mac OS X v10.5 and earlier, along with Microsoft Windows, use the Base 2 system.

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