New air says only 251gb?!

Discussion in 'MacBook Air' started by askrib, Aug 6, 2011.

  1. askrib macrumors member

    Apr 15, 2011
    How come when I check my SDD on my new 256gb air it says max space is 251? Even through disk utility it only shows the space as 251....i thought i bought a 256gb sdd?!

  2. miles01110 macrumors Core


    Jul 24, 2006
    The Ivory Tower (I'm not coming down)
  3. askrib thread starter macrumors member

    Apr 15, 2011
    so does this mean when I want to clean out my HD i don't have to reinstall Lion? ....How does that work exactly....i don't see a partition.
  4. clyde2801 macrumors 601


    Mar 6, 2008
    In the land of no hills and red dirt.
    Reboot and hit the option key. It'll show up then.
  5. xraydoc macrumors demi-god


    Oct 9, 2005
    This is correct.

    First, all SSDs set aside a certain amount of space (on the 256GB models its around 3GB) to offset cell errors that creep in over time. Spinning hard drives do something similar for bad blocks, so this isn't out of the ordinary.

    Second, if you're using Lion, there's a small (650MB) partition used for recovery of of the OS should you need to reinstall. A 250MB EFI boot partition also is present if you're using Lion.

    Lastly, you'll see some "missing" space that's actually being used for your deep sleep memory image (equal to the amount of RAM in your machine) and a small partition for Time Machine backups (if you use Time Machine) for if/when your backup drive isn't available (say, a Time Capsule to which you're not presently connected).

    My MBA's 256GB SSD and my iMac's 256GB SSD also both show 251GB available.
  6. Mr. Retrofire macrumors 601

    Mr. Retrofire

    Mar 2, 2010
    In Disk Utility click on the device (on the left side) and click on the info button in the toolbar! This should show the full capacity in bytes of your SSD.
  7. askrib thread starter macrumors member

    Apr 15, 2011
    Thank you for the breakdown. This makes more sense.
  8. askrib thread starter macrumors member

    Apr 15, 2011
    Still says 251 total capacity when i do that.
  9. drxcm macrumors regular

    Nov 2, 2010
    That is all you get.

    "256GB" is the advertised size the drive, but the actual formatted capacity is less than 256. It even says this on the tech specs page of the MBA on Apple's website (footnote 2).

    Cut from another website:

    Determining drive capacity can be confusing at times because of the different measurement standards that are often used. When dealing with Windows and Mac based systems, you will commonly see both decimal measurements and binary measurements of a drive's capacity. In either case, a drive's capacity is measured by using the total number of bytes available on the drive. As long as the drive displays the correct number of bytes (approximate), you are getting the drive's full capacity.

    Decimal vs. Binary:
    For simplicity and consistency, hard drive manufacturers define a megabyte as 1,000,000 bytes and a gigabyte as 1,000,000,000 bytes. This is a decimal (base 10) measurement and is the industry standard. However, certain system BIOSs, FDISK and Windows define a megabyte as 1,048,576 bytes and a gigabyte as 1,073,741,824 bytes. Mac systems also use these values. These are binary (base 2) measurements.

    To Determine Decimal Capacity:
    A decimal capacity is determined by dividing the total number of bytes, by the number of bytes per gigabyte (1,000,000,000 using base 10).

    To Determine Binary Capacity:
    A binary capacity is determined by dividing the total number of bytes, by the number of bytes per gigabyte (1,073,741,824 using base 2).
    This is why different utilities will report different capacities for the same drive. The number of bytes is the same, but a different number of bytes is used to make a megabyte and a gigabyte. This is similar to the difference between 0 degrees Celsius and 32 degrees Fahrenheit. It is the same temperature, but will be reported differently depending on the scale you are using.


    Various Drive Sizes and their Binary and Decimal Capacities:

    Drive Size in GB Approximate Total Bytes Decimal Capacity
    Approximate Binary Capacity (bytes/1,073,724,841)
    10 GB 10,000,000,000 10 GB 9.31 GB
    20 GB 20,000,000,000 20 GB 18.63 GB
    30 GB 30,000,000,000 30 GB 27.94 GB
    40 GB 40,000,000,000 40 GB 37.25 GB
    60 GB 60,000,000,000 60 GB 55.88 GB
    80 GB 80,000,000,000 80 GB 74.51 GB
    100 GB 100,000,000,000 100 GB 93.13 GB
    120 GB 120,000,000,000 120 GB 111.76 GB
    160 GB 160,000,000,000 160 GB 149.01 GB
    180 GB 180,000,000,000 180 GB 167.64 GB
    200 GB 200,000,000,000 200 GB 186.26 GB
    250 GB 250,000,000,000 250 GB 232.83 GB
  10. KPOM macrumors G5

    Oct 23, 2010
    All SSDs do this. It's called "over provisioning." It sets aside a portion of the SSD that isn't written to. Over time, that portion is written to and a different portion is set aside. It does this to avoid writing to the same SSD cells over and over again, since there is a physical limit to how many times each cell can be written to. The 256GB drive has 251GB of usable capacity. The 128GB has 121GB of usable capacity, while the 64GB drive has about 59GB of usable capacity. See this article for further information.

    BTW, OS X measures 1GB as 1 billion bytes. Windows uses the binary measure or 1,073,741,824 bytes, so it will actually show the capacity to be about 234GB while OS X shows it to be 251GB.
  11. Cynicalone macrumors 68040


    Jul 9, 2008
    Okie land
    It is normal for an SSD. Mine says 251GB in Disk Utility too.

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