Why does a 256GB drive show only about 238GB after formatting?

Question

Weirdo · Accepted Answer

Makers count in decimal gigabytes (1 GB = 10⁹ bytes). Many operating systems count in binary gibibytes (1 GiB = 2³⁰ bytes). 256×10⁹ bytes is about 238.4 GiB, then the filesystem takes a little more.

Drive labels use SI decimal units: 1 gigabyte = 1,000,000,000 bytes. NIST lists a separate binary prefix: 1 gibibyte (GiB) = 2³⁰ = 1,073,741,824 bytes. Windows and similar systems divide capacity by 2³⁰ and often still label the result GB. A sticker that says 256 GB (256,000,000,000 bytes) therefore shows up as about 238 GB. 256 × 10⁹ ÷ 2³⁰ ≈ 238.4. Those 18 GB were not stolen; the unit changed.

Partition tables, filesystem metadata, and reserved areas consume extra bytes, so usable space sits a bit below 238.4 GiB. Western Digital’s support note is blunt: the gap is mostly decimal versus binary counting. The bytes on the platter are still there.

Why does a 256GB drive show only about 238GB after formatting?

The GB on the box is not the GB in the OS

Formatting trims a little more