Why Does a 1 TB Drive Show Only 931 GB? The 30‑Year KB vs KiB Mix‑Up Explained

When a 1 TB hard drive is connected to a computer, the operating system often reports about 931 GB. The discrepancy originates from two different sizing conventions.

Decimal definitions used by manufacturers

1 KB = 1000 B

1 MB = 1000 KB = 1 000 000 B

1 GB = 1000 MB = 1 000 000 000 B

1 TB = 1000 GB = 1 000 000 000 000 B

Binary definitions used by most operating systems (e.g., Windows)

1 KiB = 1024 B

1 MiB = 1024 KiB = 1 048 576 B

1 GiB = 1024 MiB = 1 073 741 824 B

1 TiB = 1024 GiB = 1 099 511 627 776 B

The manufacturer’s 1 TB equals 1 000 000 000 000 bytes, while the OS counts 1 TiB as about 1.099 × 10¹² bytes. The ratio 1 TB / 1 TiB ≈ 0.931, i.e., a ~7 % reduction, which explains the 931 GB display.

Why the decimal standard?

In the International System of Units (SI), the prefixes k, M, G, T denote multiples of 1000. Applying the same rule to storage is logically consistent with other SI‑based measurements.

Why the binary standard?

Computer hardware operates on binary logic. Because 2¹⁰ = 1024 is close to 1000, early programmers used “K” to represent 1024, and the convention spread throughout the industry.

Terminology clash and IEC 1998 resolution

Both sides use the same abbreviations (KB, MB, GB, TB) but with different meanings. The International Electrotechnical Commission (IEC) introduced distinct symbols in 1998:

Decimal: kB, MB, GB, TB (× 1000)

Binary: KiB, MiB, GiB, TiB (× 1024)

Most software, including Windows, still labels GiB as “GB”, and many users are unaware of the KiB family.

Same effect on other storage media

16 GB USB flash drive → ~14.9 GB reported

128 GB smartphone → ~112 GB usable (excluding system partitions)

64 GB SD card → ~59 GB usable

Apple’s iOS and macOS display capacities using decimal units, so a device labeled 128 GB also reports 128 GB, providing a rare example of consistency.

Larger units

1 PB = 1024 TB ≈ 2 × 10⁸ phone photos

1 EB = 1024 PB ≈ global social‑media image/video volume generated in a year

1 ZB = 1024 EB ≈ global data produced in a year

For most consumers, TB is the practical ceiling; PB and larger units are primarily relevant to data‑center and cloud environments.

Non‑standard “GT”

The term “GT” occasionally appears online but is not an official unit. The standard sequence is KB → MB → GB → TB → PB → EB → ZB → YB.

Conclusion

Manufacturers’ decimal labeling and operating systems’ binary calculations are each correct within their own conventions. The lack of a unified standard creates the perception of missing capacity, typically around 7 % of the advertised size.

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