Why does my SSD storage show up as smaller than advertised?

Your drive shows up smaller than advertised because of different ways hardware storage manufacturers and software companies like Microsoft/Windows define capacities differently. All hardware manufacturers define 1GB = 1 billion (1,000,000,000 bytes. A 480GB SSD is, in other words, actually 480,000,000,000 bytes; these are what we call decimal bytes, and it has been an industry standard to use them when advertising storage space.

A Unix® based operating system like macOS X® or Linux® uses decimal bytes when reporting storage space, so a 480GB SSD will show up as 480GB in Mac Disk Utility for instance. However, Windows OS uses binary bytes, so 1,024 bytes per Kilobyte, 1,024 KB per Megabyte, and so on. This means that when you install a 480,000,000,000 bytes storage drive into a Windows® computer, that computer converts the number of bytes into gigabytes by dividing by 1024 all the way up through the scale, not by dividing by 1,000. Doing the math, this is what we end up with:

480,000,000,000 Bytes / 1,024 = 468,750,000 actual Kilobytes

468,750,000 KB / 1,024 = 457,764 actual Megabytes

457,764 MB / 1,024 = 447 actual Gigabytes

This is why a 480GB SSD will be correctly reported by a Windows computer as 447GB. The larger the numbers are, the larger the discrepancies will be. On an 8GB USB drive the difference between the advertised capacity and the actual capacity is about half a gigabyte, while in our example above the difference is a very noticeable 33GB. It is important to understand that these 33GB aren’t lost. The drive is 480,000,000,000 bytes in capacity, and after 480,000,000,000 bytes have been converted by a Windows computer into Gigabytes, the total capacity comes to 447 GB. Below are some conversions for standard drive sizes.

The file system overhead is another thing that makes an SSD have less space than a hard drive. When you format an SSD, the working device makes a document device that organises and handles the data on the drive. This method only uses a small amount of the total space needed to store important device papers, metadata, and indexes.

Depending on the file system you use, such as FAT32 or NTFS on Windows or APFS on macOS, the amount of space the file system uses can vary. Also, the cluster size picked during formatting can affect how well storage space is used, which could cause small files to waste space. Even though these overheads are necessary for the file to work correctly on device, they take up room on the SSD.

Operating systems often create hidden machine files and partitions on hard drives for various reasons. Most people don’t see these files and walls, but they still occupy some of the SSD’s space. The Windows Recovery Partition, the macOS Recovery Partition, the sleep record, and the page record are good examples.

Device recovery, caching, and managing virtual memory all depend on these files and folders. But they add to the feeling that your SSD has less room than you thought it did. You can use disk management tools or check the power houses on your system to see if hidden device files are taking up a lot of room.

Page File or virtual memory is a system file that the computer uses to make up for low RAM (random access memory) when running low. The page report can take up a lot of storage space on your SSD. 

References

Crucial: Why Does My SSD Show up as Smaller than Advertised?

https://www.crucial.com/support/articles-faq-ssd/ssd-showing-smaller-than-advertised

CPUGPU Nerds: Why does my SSD show less space?
https://cpugpunerds.com/why-does-my-ssd-show-less-space/