Discrepancies in available capacity of Macbook Air SSD

I bought a MBA with an SSD advertised to be 256GB.

The actual unformatted capacity, according to the system info tool, is only 251000193024 bytes.

This is only 233 Gibi (2^30) bytes, or 251 Giga (10^9) bytes. Regardless of whether Apple means Giga or Gibi in their spec sheets, this capacity is still outrageously lower than advertised.

Is a part of the SSD reserved for some hidden function and is subtracted from the size that appears in the system info?

Or is Apple consciously cheating its customers?

It is interesting to note that Hard Drive Manufacturers use the correct meaning of "Gigabytes" when referring to hard drives, and not what most people think of as a GB which is actually a Gibibyte:

Gigabyte = 10 to the 9 bytes = 1000000000bytes = 1000 megabytes

Gibibyte = 2 to the 30 bytes = 1073741824bytes = 1024 mebibytes

Since Mac OS X also switched to this "proper" definition for storage units such as Gigabytes as of Snow Leopard, then the size of the drive should be understood in these terms, noting that on a Mac the OS and the Hard Drive will be talking the same language, there is no need to do the conversion from Gibibytes to Gigabytes.

The given size should be correct and accurate as reported on a Mac as they have a common understanding of what a Gigabyte is, and incorrect on Windows which uses the commonly misused Gibibyte structure on it's filesystems whilst users incorrectly assume it is measured in Gigabytes.

Technically manufacturers should use GiB not GB when marketting and packaging their drives, but they don't/won't, and use Gigabyte which they know practically everyone uses incorrectly.

In your case the available unformatted capacity is indeed low even for using the correct maths, but as referenced in another answer the size of 256Gb refers to the total storage capacity of the drive, and you have to not only accept that formatted capacity is lower, but also that the capacity that is available to format can also be lower, if portions are reserved for the purposes of replacing bad blocks through the lifecycle of the drive.

Lengthy comment follows after the first 2 comments below - too long for a comment tho, but not really part of the answer other than background info perhaps

The space reservation thing is not something I feel I can be canonical about, I know the theory, I can't really backup it up once and for all, especially because this is an Apple non-consumer drive. However, almost all drives over provision, providing more space than they state in order to have capacity for replacing worn areas.

It's not 100% clear, but it certainly appears to be the case, that in the case of these Apple supplied drives, the provision of "backup" blocks is taken from the stated capacity from the off. I think this is how most manufacturers do it, hence the fact that you see way more 60Gb drives than 64Gb etc. The implication being that there is 64Gb of storage capacity, and 60 of it us user addressable. Whether you call that a 60 or a 64Gb drive could potentially be taken either way. I would call it a 60, but from a legal perspective I would imagine both cases are arguable accurate. As for the instances you see where the available capacity is equal to or greater than the nominal capacity (in the case of the 128Gb drives you see) then it could simply be the case that some manufacturers are either more generous, or more worried about failure, than others and genuinely put in more chips worth, say 132Gb for example.

So in you case, you likley have a 256Gb drive in terms of chips, the fact that the user addressable space is lower, even before formatting is likely just how it is. I have an Apple supplied 128Gb Air SSD which shows much the same behaviour:

APPLE SSD TS128C:

Capacity: 121.33 GB (121,332,826,112 bytes) Model: APPLE SSD TS128C

No its a thing that has been going on since are drives were first introduced (when I was young and 20Mb Hard Drives were the whizz bang and DOS 4 was hot, it wasn't such a big deal)

Hard drive manufactures quote their hard drive sizes in thousands, millions, billions, just like cash, in base 10 (GB)

People who use computers calculate size in multiples of 2^10 bytes (1024 bytes)

The difference mostly is the conversion, plus some that is left over from partitioning that cannot be partitioned due to the way that partitioning works, and quite probably the Lion Recovery Partition.

Finally, in answer to your question, does Apple engage in false advertising. No, it does specifically state on all its product pages where appropriate to the device, that the HD capacity is measured in GB.

1GB = 1 billion bytes; actual formatted capacity less.

You should also be aware that some SSD's come with a default space used for formatting, and redundancy for wear levelling and can be up to 5% of the nominal capacity of the SSD drive. This area is managed by the SSD firmware and is never seen by the OS and the OS reports a lower size than the actual SSD size.

As has you have guessed, this does not have to do with how decimal and binary numeral systems measure a GB.

Apple has released a knowledge base article (see last paragraph) where they mention briefly why the capacity in Solid State Drives is smaller:

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

What's surprising is that Apple talks about Solid State Drives and not Flash Memory even though they are basically the same.

I had thought that this is because this article is about SSDs in general. But this can't be true because not all third-party SSDs which you can use in your Macbook Pro use that much space for wear-leveling for example. I have used a 128GB SSD Crucial m4 SSD in my Macbook Pro and the capacity reported by Disk Utility is 128GB.

Technical details about the management of the flash memory in the SSD differ depending on the manufacturer. Therefore I'll think this article refers to Apple SSDs only (which does not mean that some of the points raised here aren't true for other SSDs as well).