Surely you have ever wondered why SSD hard drives they come in different sizes than HDD hard drives; 120, 240 or 480 GB SSD disk, while HDDs are found at 250GB, 500GB or even 1 or 2 TB.
The first answer is pure marketing And it could be compared as the gigabytes used by a computer geek versus the GB used by the rest of the world. But there is a reason for this as we will try to explain below to solve that mystery.
RAM or random access memory is built and accessed in multiples and powers of two. When you buy a computer, the memory modules are 512 MB or 2 GB. Those aren't really megabytes and megabytes because in the early days of computers, metric terms like kilos were used, which meant exactly 1.000 and were used as a shorter way of referring to 1024. Even today, some operating systems and programs use the 'wrong' value of 1024 when calculating both computer and storage memory.
When hard drives were invented, there were no electronic structures created in powers of two, so selling those electronic gadgets used "correct" multiples of a thousand when it came to storage capacity. This also had the effect of appearing to be "older" and marketing always likes anything bigger. It is that marketing that likes round figures, so hard drives began to be advertised in terms of 250GB or 2TB.
Solid hard drives are structured in such a way that they look more like computer memory than mechanical hard drives (you can see it in the image below). A 240GB SSD probably has a more raw storage capacity at 256x1024x1024x1024 bits, so why not label it 256GB or even 275GB? Again, we go back to marketing where people like round numbers.
Another interesting point for this change is that there are many who are used to the fact that hard drives never have the full capacity that has been said. In mechanical disks it was due to the difference between the binary / decimal measurement. With SSDs, it is mainly due to something called "Over-provisioning". In this way, the marketers of the product take a round figure that has to do more or less with the real space of the hard disk.
Over-provisioning is when the storage capacity of an SDD is marked as out of bounds. Your 256GB SSD has 16GB set aside for future use, so it stays at 240 GB. If the electronic controls of the SSD realize that sector 18 is about to receive a fault, it will copy the information to a new sector in the 16 GB one. That bad sector 18 will be permanently disabled and the new sector 18 will be marked in use.
We are left with a 480 GB SSD, to give another example, which surely it is 512GB but maintains 32GB for over-provisioning. So the doubt about those rounder figures and how SSD disks play to save a little memory to avoid problems.