Flash Memory Actual Capacity Estimates

Discussion in 'Microsoft Surface FAQ & Guides' started by J515OP, Jan 29, 2013.

  1. J515OP

    J515OP Super Moderator

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    Here is a handy chart I Googled from a flash memory manufacturer for anybody who wants a reference. Keep this in mind when you see articles proclaiming how much space is taken by apps or OSes or by manufacturers claiming the capacity of their devices.

    Source: USB Storage | Idol Memory

    The manufacturer did not specify the method used to calculate usable capacity but here are the numbers according to standard calculations.

    First a key to the numbers.

    Decimal (used my manufacturers):
    1 G = 10^9 = 1,000,000,000
    1 M = 10^6 = 1,000,000
    1 K = 10^3 = 1,000

    These are the correct prefixes under the SI standards.

    Binary (used by the OS for bytes):
    1 GiB = 2^30 bytes = 1,073,741,824 bytes
    1 MiB = 2^20 bytes = 1,048,576 bytes
    1 KiB = 2^10 bytes = 1,024 bytes

    These are the correct units for binary though by convention we drop the "i" and use the SI names which leads to the confusion.

    The formula to figure actual capacity in GiB is: GB listed * 10^9 / 2^30 = calculated capacity

    Calculated capacity - overheard = usable capacity

    Note: overhead is inaccessible space on a formatted drive and will very slightly but can be as much as 100 MB

    128 GB * 10^9 / 2^30 bytes = ~119.21 GiB
    256 GB * 10^9 / 2^30 bytes = ~234.42 GiB

    Summary

    Using the manufacturer's numbers and the calculated numbers gives us:

    1 GB : 0.94 GB stated usable capacity : ~0.93 GiB calculated capacity
    2 GB : 1.86 GB stated usable capacity : ~1.86 GiB calculated capacity
    4 GB : 3.79 GB stated usable capacity : ~3.73 GiB calculated capacity
    8 GB : 7.51 GB stated usable capacity : ~7.45 GiB calculated capacity
    16 GB : 15 GB stated usable capacity : ~14.9 GiB calculated capacity
    32 GB : 29.7 GB stated usable capacity : ~29.8 GiB calculated capacity
    64 GB : 59.53 GB stated usable capacity : ~59.60 GiB calculated capacity
    128 GB : no stated usable capacity : ~119.21 GiB calculated capacity
    256 GB : no stated usable capacity : ~234.42 GiB calculated capacity
     
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  2. Frederuco

    Frederuco Super Moderator Staff Member

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    I may have to steal this blurb...
     
  3. J515OP

    J515OP Super Moderator

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    Maybe you should borrow it. That way it will still be here for members to see ;)
     
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