UnitBench

Data sizes, both definitions

SI and binary side by side — which is why a 1 TB drive shows up as 931 GB, and why nothing is actually missing.

Click any tile to make it the input — the value carries across.

The missing 69 gigabytes

You buy a 1 TB drive. Windows reports 931 GB. Nothing has been lost and nobody has cheated — two different definitions of the same prefix are being used at once.

Drive manufacturers use the SI meaning, where kilo is 1000, mega is 1000² and tera is 1000⁴. So 1 TB is exactly one trillion bytes. Operating systems, for historical reasons rooted in how memory is addressed, often compute in powers of 1024 — while still printing the label "GB". One trillion bytes divided by 1024³ is 931.3, and that is the number you see.

The gap widens as the units get larger: about 2.4% at kilobytes, 7.4% at gigabytes, and nearly 10% at terabytes. On a 4 TB drive that is roughly 370 "missing" gigabytes, which is why the question comes up more the bigger drives get.

The units that end the argument

KiB, MiB, GiB and TiB — kibibyte, mebibyte, gibibyte, tebibyte — were standardised for exactly this purpose. Each is unambiguously a power of 1024. When they are used, a 1 TB drive is 931 GiB, and both numbers describe the same capacity without contradiction.

Adoption is uneven. macOS and most Linux tools report SI units and match the box. Windows continues to display binary quantities with SI labels. Both sets are in the converter above so you can move between them without arithmetic.

Bits are not bytes

One more trap, and it costs people real money. Storage is sold in bytes; network speed is sold in bits per second. There are 8 bits to a byte, so a 100 Mbps connection tops out around 12.5 MB/s of actual download. Internet plans are advertised in the bigger-sounding unit, which is not an accident.

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Questions

Why does my 1 TB drive show as 931 GB?
Because two different definitions are in play and neither party is lying. The manufacturer sells 1 TB meaning one trillion bytes — the SI definition, where the prefixes mean powers of 1000. Windows then displays that capacity using powers of 1024 while still writing the label "GB". One trillion divided by 1024³ is 931, so you see 931 where you expected 1000. Nothing is missing.
What are KiB, MiB and GiB?
The unambiguous binary units: a kibibyte is exactly 1024 bytes, a mebibyte 1024², a gibibyte 1024³. They were standardised precisely to end this confusion. macOS and Linux generally report SI units correctly; Windows still shows binary quantities labelled with SI prefixes, which is the direct cause of the missing-space question.
Are network speeds the same units?
No, and this catches people out badly. Network speeds are quoted in bits per second, not bytes — and there are 8 bits in a byte. A 100 Mbps connection downloads at roughly 12.5 MB/s at best. If your download seems eight times slower than your plan, that is arithmetic rather than a fault.
Which should I use?
For storage and file sizes, say what you mean: use GiB when you mean 1024³ and GB when you mean 10⁹. For anything user-facing, SI units match what the hardware says on the box. The one thing to avoid is writing GB while calculating in 1024s, which is exactly what created the problem.