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Why Your 1 TB Drive Shows Only 931 GB

No storage is missing. Drive makers count in powers of 1,000 and most operating systems count in powers of 1,024, so the same bytes get two different labels.

By Muhammad Ahmad. Published . 2 min read.

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You buy a 1 TB drive, plug it in, and your computer says it holds about 931 GB. It looks like 69 GB went missing. None did. The drive and the computer are using two different definitions of a gigabyte.

Two ways to count

Drive manufacturers use decimal units, where each step up is 1,000. A kilobyte is 1,000 bytes, a megabyte is 1,000 kilobytes, and so on. By that definition, 1 TB is exactly 1,000,000,000,000 bytes.

Windows, and many other tools, use binary units, where each step up is 1,024. A kilobyte is 1,024 bytes, a megabyte is 1,024 kilobytes, and a gigabyte is 1,024 × 1,024 × 1,024 bytes.

1 TB from the box, in the units your computer uses

  1. Bytes on the drive

    1 TB (decimal)equals1,000,000,000,000 B

  2. One binary gigabyte

    1,024 × 1,024 × 1,024equals1,073,741,824 B

  3. Shown by your computer

    1,000,000,000,000 ÷ 1,073,741,824equals931.32 GB

The gap grows with each step up. At the kilobyte level the two definitions differ by 2.4%. At the gigabyte level the gap is about 7%, which is exactly the difference between 1,000 and 931. Measured in terabytes it is about 9%, because the 1,024 factor has been applied four times.

KiB, MiB and GiB

To end the confusion, standards bodies introduced separate names for the binary units: kibibyte (KiB), mebibyte (MiB), gibibyte (GiB) and tebibyte (TiB). Under those names, the drive holds 1 TB, which is 931.32 GiB. Both statements are correct.

In practice, the old names stuck. Most people and a lot of software still write GB when they mean GiB, which is why the confusion continues.

Where you will see each system

  • Decimal (1,000): drive and USB stick packaging, and network speeds, which are also counted in bits rather than bytes.
  • Binary (1,024): memory (RAM) sizes, and file and disk sizes in Windows.
  • It varies: macOS has used decimal units for disk sizes since 2009, so the same drive can look bigger on a Mac than on a Windows PC.

A 500 GB drive shows as about 465.66 GB on a system that counts in binary. On top of that, formatting the drive and any recovery partition use some space, so the free space you see can be lower still.

Tip: Network speeds are quoted in bits, not bytes. Divide megabits per second by 8 to get megabytes per second.

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