# A 256 GB drive, asked another way, says 238. Where did 18 GB go?

- Series: [How Computers Work](https://miu.fm/howcomputerswork.md)
- Published: 2026-10-20
- Page: https://miu.fm/howcomputerswork/a-256-gb-drive-asked-another-way-says-238-where-did-18-gb-go

## Story

A 256 GB drive, asked another way, says 238. Where did 18 GB go?

Take a laptop with a 256 GB drive (our example): it holds 256 billion bytes. Ask one way how big it is and the answer is 256; ask another way and it's 238. Same drive, same bytes, two answers.

The second answer counts the way memory does. Memory finds each byte by its address, a row of ones and zeros, and every extra digit doubles the places an address can name: 10 digits name 1,024 places, 30 digits name 1,073,741,824. So memory comes in twos, and 1,024 was close enough to a thousand that, as NIST tells it, "computer professionals noticed that 2^10 was very nearly equal to 1000" and called it kilo. A gigabyte of memory is 1,073,741,824 bytes.

Drives are sold in tens. Apple's own definition: "1GB = 1 billion bytes". So the same word means two sizes, and the gap grows with every step: 2.4 % at kilo, 7.4 % at giga, 10 % at tera. A terabyte, counted in twos, is 931 (our arithmetic). Our 256 GB example is 256 counted in tens and 238 counted in twos. Nothing is gone. In December 1998 the IEC gave the twos their own names: kibibyte, mebibyte, gibibyte (KiB, MiB, GiB).

A new drive also loses a little to formatting ("actual formatted capacity less", as Apple puts it), but here all 18 of the missing gigabytes are the counting.

The laptop, the drive and the address grid are our own simplified drawings, not real products.

Which number does your computer show for its drive?

Sources: NIST, Prefixes for binary multiples (IEC 60027-2 Amendment 2, December 1998); Apple, How storage capacity is measured on Apple devices (2025); our arithmetic.
Sounds and music: made by us.

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