Why Does My SSD Show up as Smaller than Advertised?

Your drive shows up smaller than advertised because storage drive capacity is calculated and reported slightly differently than other capacities in computing. If you look at the specifications of any storage device, you will see a note that says something along the lines of ”1 GB = 1 billion bytes. Actual usable capacity may vary.” In other words, the drive capacity is reported on the assumption that 1GB is 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. The Windows® OS on the other hand 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.