Why Two "GB" Numbers Never Quite Match
The decimal-vs-binary gigabyte mismatch behind "missing" drive space, why internet speeds use a completely different unit, and how to avoid the confusion.
Published March 16, 2026
Buy a "1TB" drive, plug it in, and the operating system reports something like 931GB. Nothing is missing, and nothing is broken. Two different, both technically legitimate, definitions of "gigabyte" are colliding, and almost nothing in the buying or setup process warns you it's about to happen.
The two definitions
Storage manufacturers use decimal units: 1 kilobyte = 1,000 bytes, 1 megabyte = 1,000,000 bytes, and so on, powers of 1,000, the same convention as the metric system generally. Operating systems, on the other hand, typically report storage in binary units: 1 kilobyte = 1,024 bytes, because computers fundamentally work in powers of 2, and 1,024 is 2 to the 10th power.
The gap compounds at every step: 1,000 vs. 1,024 is a small difference for a kilobyte, but by the time you're at terabyte scale, that compounding gap is large enough to be very noticeable, which is exactly the "1TB drive shows as 931GB" experience.
The technically-correct fix nobody uses
There's an official standard for this: kibibyte (KiB), mebibyte (MiB), and gibibyte (GiB) specifically mean the binary (1,024-based) units, leaving KB, MB, and GB free to mean the decimal (1,000-based) ones without ambiguity. In practice almost nobody uses KiB/MiB/GiB outside of a few technical contexts (Linux tools are a notable exception), so "GB" ends up doing double duty for both meanings depending on who's using it, which is exactly the source of the confusion.
Why internet speed adds a second layer of confusion
Internet service providers advertise speed in megabits per second (Mbps), while file sizes and download progress bars are almost always shown in megabytes (MB). A byte is 8 bits, so a "100 Mbps" connection tops out around 12.5 MB/s in practice, not 100. This is an entirely separate mismatch from the decimal/binary gigabyte issue, but it compounds with it, and it's the actual reason a fast-sounding internet plan can feel slower than expected when you're watching a download's MB-per-second speed.