Why do SSDs have sizes like 240 GB or 120 GB rather than the normal 256 GB or 512 GB?
Those numbers make much more sense than 240 or 120.
Answer
While a lot of modern SSDs like the 840 EVO series do provide the sizes you’re used to like, the mentioned 256GB, manufacturers used to preserve a bit of storage for mechanisms fighting performance drops and defects.
If you—for example—bought a 120GB drive, you can be pretty sure that it’s really 128GB internally. The preserved space simply gives the controller/firmware room for stuff like TRIM, Garbage Collection and Wear Leveling. It has been common practice to leave a bit of space unpartioned—on top of the space that has already been made invisible by the controller—when SSDs first hit the market, but the algorithms have gotten significantly better, so you shouldn’t need to do that anymore.
EDIT: There have been some comments regarding the fact that this phenomenon has to be explained with the discrepancy between advertised space, stated in GigaBytes (e.g. 128x 10^9 Bytes) versus the GibiByte value the operating system shows, which is—most of the time—a power of two, calculating to 119.2 Gibibyte in this example.
As for as I know, this is something that comes on top of the things already explained above. While I certainly can’t state which exact algorithms need most of that extra space, the calculation stays the same. The manufacturer assembles an SSD that indeed uses a power of two number of flash cells (or a combination of such), though the controller does not make all that space visible to the operating system. The space that’s left is advertised as Gigabytes, netting you 111 Gibibyte in this example.
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