Data Storage Converter

Why your 1 TB drive shows 931 GB

Storage manufacturers use SI prefixes, where a terabyte is 1000⁴ = 1,000,000,000,000 bytes. Windows reports capacity in powers of 1024 but labels the result “GB”. One trillion bytes divided by 1024³ is 931.32, so the drive is honestly 1 TB and honestly 931 GiB at the same time. Nothing is missing; two different units share one abbreviation.

macOS switched to decimal counting in 2009, which is why the same drive reports as 1 TB on a Mac and 931 GB on a Windows PC. Linux tooling varies: ls -h uses 1024 with SI labels, while ls --si uses 1000.

The binary prefixes exist to end this argument

IEC 80000-13 defines kibi, mebi, gibi and tebi for the powers of 1024. A gibibyte (GiB) is unambiguously 1,073,741,824 bytes. They are correct, they are standard, and roughly nobody says them out loud — but they appear in RAM specifications, Linux tools and technical documentation, and using them removes the ambiguity entirely.

Bits versus bytes: the internet-speed trap

Network speeds are quoted in bits per second; file sizes are in bytes. There are 8 bits in a byte, so a 100 Mb/s connection downloads at a maximum of about 12.5 MB/s. An ISP advertising “100 Mbps” and a download manager showing “12 MB/s” are reporting the same speed. The capital B versus lowercase b is the only thing distinguishing them, which is why this confusion will never go away.

Where the missing capacity actually goes

Beyond the prefix confusion, a formatted drive really does lose a little space: the filesystem needs room for its own metadata, journals and allocation tables. On a modern filesystem that is typically well under 1% — nothing like the 7% gap the prefix mismatch creates.

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