You bought your new laptop but the 500GB SSD drive you were promised is only 476.8 GB.
It's a surprise that most of us have encountered. Below we'll see what exactly happens with the missing disc.
Installed software
The first and most obvious reason you don't have access to all the space on your hard drive is that there's already software installed on it: your operating system.
Computer manufacturers like HP or Dell use Microsoft's Windows, while Apple promotes its own macOS. There are also some companies that offer computers with Linux or Android.
Regardless of the platform your computer runs on, operating systems obviously take up some storage space. For example, a fresh installation of Windows requires over 15 GB.
The manufacturer may of course install additional software on top of the 500GB that they promised you. This may include utilities, such as applications that keep your computer's drivers up to date. We often come across trial versions of applications known as bloatware, or applications that come without being part of the operating system (e.g. the macOS Photos and Garage Band applications).
Applications allow users to do more things when they boot up their new computer, provided they need them. Otherwise, they just take up disk space. Sometimes you can uninstall them, just like the default applications in Linux distributions.
Partitioned hard drive
Another reason that can “cut” space from your disk is the partitions created by the manufacturer. In the past, computers came with an installation CD or DVD that contained the operating system. Today, computers have the installed operating system along with a recovery partition. In case your operating system is damaged, you could restore your computer to the way it was before.
For this reason, manufacturers divide your disk into two partitions: the standard partition that contains the OS and an equivalent to those old CDs or DVDs and serves for system recovery.
You may also notice one or more even smaller partitions on your disk. These may belong to bootloaders. The UEFIcreates a space at the beginning of the drive, but also on the OS disk where it stores information about various operating systems on the machine. But the disk partitions are very small and do not contribute much to your “missing” disk space. In any case, do not try to delete any of them because you will end up with an unbootable system.
Bytes and Gigabytes
Another reason why your drive seems smaller than you expected it to be has to do with capacity measurement.
We'll look at two different things: how the total disk capacity is calculated and how that capacity is passed on to you, the end user.
Disk capacity calculation
Figuring out how much data your drive can hold is a matter of simple multiplication. A traditional hard drive is made of magnetic disks arranged in tracks, which are in turn divided into sectors. Drives are also rated for their density (the amount of data they can store in a sector). Modern drives typically use Advanced Format, which allows up to 4096 bytes per sector. This means that the capacity of a given drive is:
[Bytes per sector density] x [Number of sectors per track]
[Bytes per sector density] x [Number of tracks per disc]
[Bytes per sector density] x [Number of discs]
Note that SSDs don’t have disks or tracks, but you can assume a value for both, and the above formula still works. On Mac or Linux, the fdisk command is a great way to determine the exact partitions on your drive. 
Of course, who can easily understand a 12-digit number? That’s why the prefix is used: “kilo-” for thousands, “mega-” for millions, and “giga-” for billions.
But we will mention another peculiarity: other sciences work with decimal digits, that is, the base-10 numbering system that we see every day. Computers only understand binary or zeros and ones. So the definition of these prefixes is not based on thousands, but on groups of 1,024. So a kilobyte is 1024 bytes, a megabyte is 1,024 kilobytes, and so on. The formula for calculating capacity in gigabytes is shown below:
Total bytes
1,024 bytes per kilobyte
1,024 kilobytes per megabyte
1,024 megabytes per gigabyte
Performing the operation from the information we got from fdisk, we get:
256,060,514,304/1,024/1,024/1,024 = 238,474937439 GB
The first number is the first value that the fdisk command gave us (as shown in the image below).
This discrepancy in how the computer calculates compared to how a human calculates creates the surprise we described at the beginning of the article.
After all this, if you are still worried, you should know that your operating system is technically correct in reporting the capacity.
However, we will probably never see a 500GB SSD advertised in a storefront with a capacity of 476.8 GB.
