You bought your new laptop but the SSD disc of the 500GB you were promised, it's only 476,8GB.
It's a surprise that most of us have faced. Below we'll see what's going on with the missing disk.
Installed software
The first and most obvious reason why you do not have access to all of your hard disk space is that there is already a software installed on it: your operating system.
Computer makers like HP or Dell are using Microsoft Windows, while Apple is launching its own macOS. There are also some companies that have computers with Linux or Android.
Regardless of the platform running on your computer, the operating systems apparently occupy one of the storage. For example, a new Windows installation requires more than 15 GB.
The manufacturer of course can also install additional software on the 500GB that promised you. This may include utilities, such as applications that keep the computer drivers up to date. Many times we also come across trial versions of applications known as bloatware or applications that come without being part of the operating system (eg MacOS Photos and Garage Band applications).
Apps allow users more functionality when they start their new computer, provided they need it. Otherwise, they simply take up disk space. Sometimes you can uninstall them, just as with default Linux applications.
Separate hard drive
Another reason that can "cut" space from your disk is the partitions that have been created by the manufacturer. In the past, computers came with an installation CD or DVD that contained the operating system. Today computers come with the operating system installed along with a recovery partition. In case of damage to your operating system, you could restore your computer to the way it was before.
For this, manufacturers split your disk into two partitions: the standard partition that contains the OS and one equivalent to these old CDs or DVDs and is used to recover a system.
You may also notice one or more even smaller partitions on your disk. These may belong to bootloaders. The bootloader UEFI, creates a space at the beginning of the drive, but also on the OS disk where it stores the information of various operating systems that exist on the machine. But tracks on the disk are very small and do not contribute much to the "incomplete" space of your disk. In any case, do not try to delete any of them because you will end up with a system that will not be able to boot.
Bytes and Gigabytes
Another reason why your drive looks smaller than you expected to be is has to do with capacity measurement.
We will look at two different things: how the total capacity of the disk is calculated and how this capacity is transmitted to you, the end user.
Calculating disk capacity
Ο υπολογισμός της ποσότητας των στοιχείων που μπορεί να κρατήσει ο δίσκος σας είναι θέμα ενός απλού πολλαπλασιασμού. Ένας παραδοσιακός σκληρός δίσκος κατασκευάζεται από μαγνητικούς δίσκους κατανεμημένους σε κομμάτια (tracks), τα οποία με τη σειρά τους χωρίζονται σε τομείς (sectors). Οι discs διαθέτουν επίσης διαβάθμιση και για την πυκνότητα του χώρου (το μέγεθος των δεδομένων που μπορεί να αποθηκεύσει σε έναν τομέα). Οι σύγχρονοι δίσκοι συνήθως χρησιμοποιούν τη Σύνθετη Μορφή (Advanced Format), η οποία επιτρέπει έως και 4096 byte ανά τομέα. Αυτό σημαίνει ότι η χωρητικότητα μιας δεδομένης μονάδας δίσκου είναι:
[Density bytes per sector] x [Number of sectors per track] [Density bytes per sector] x [Number of tracks per disk] [Density bytes per sector] x [Number of disks]Note that SSDs do not have drives or tracks, but you can assume a price for both, and that type still works. On Mac or Linux, the fdisk command is a great way to identify the exact partitions on your disk. 
Of course, who can easily understand a 12-digit number? This is how the prefix is used: "kilo-" for the thousands, "mega-" for the millions and "giga-" for the billions.
But we will mention another specificity: other sciences operate in decimal numbers, ie the numbering system based on 10 that we see every day. Computers only understand binary or zero and aces. So the definition of these prefixes is not based on thousands but in groups of 1.024. So one kilobyte is 1024 bytes, one megabyte is 1.024 kilobytes, and so on. The formula for calculating gigabyte capacity is shown below:
Total bytes
1.024 bytes per kilobyte
1.024 kilobytes per megabyte
1.024 megabytes per gigabytes
By taking the action 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 given to us by the fdisk command (as shown in the image below).
This discrepancy, in how the computer counts on how humans calculate, creates the surprise we described at the beginning of the article.
After all, if you are still worried, you should know that your operating system is technically correct and in terms of capacity.
However, we probably never see a 500GB SSD disc on a showcase advertised with the 476,8 GB capacity.
