Why Does a 1 TB Drive Show Only 931 GB?
You buy a 1 TB drive, plug it into a Windows computer, and it shows about 931 GB. A 256 GB phone shows roughly 238 GB total. It is not a fault and nothing has been stolen: the drive maker and the operating system are counting with different units that share the same name. Here is what happens and how much space you should expect.
Two kinds of gigabyte
- Decimal (SI) units: 1 kilobyte = 1,000 bytes, 1 GB = 1,000,000,000 bytes, 1 TB = 1,000,000,000,000 bytes. Drive makers, phone makers and network speeds use these.
- Binary units: computers naturally work in powers of two, so 1,024 bytes, 1,048,576 bytes (220) and so on. Since 1998 the IEC has named these kibibyte (KiB), mebibyte (MiB), gibibyte (GiB) and tebibyte (TiB).
Windows calculates sizes in binary units but labels them "KB", "MB", "GB" and "TB". A drive with exactly 1,000,000,000,000 bytes therefore appears as 1,000,000,000,000 ÷ 1,073,741,824 ≈ 931.32 "GB", which is really 931.32 GiB. macOS (since 2009) and many Linux tools use decimal units, so the same drive shows close to 1 TB there.
Expected size for common drives
| Advertised size | Shown in binary units |
|---|---|
| 128 GB | 119.21 GiB |
| 256 GB | 238.42 GiB |
| 500 GB | 465.66 GiB |
| 1 TB | 931.32 GiB (0.91 TiB) |
| 2 TB | 1,862.65 GiB (1.82 TiB) |
| 4 TB | 3,725.29 GiB (3.64 TiB) |
The gap grows with size: about 2.4% at the kilobyte level, 6.9% for gigabytes and 9.1% for terabytes. Convert any value with the TB to TiB or GB to GiB converter.
Other things that use space
- The file system needs a little room for its own records.
- Recovery partitions on new computers can take several gigabytes.
- The operating system and pre-installed apps on phones and laptops reduce the free space further; a phone's "system" category can be large.
- SSDs keep some capacity in reserve for wear-levelling; the advertised capacity usually already excludes this.
Memory is different
RAM is built in binary sizes, so "16 GB of RAM" is really 16 GiB, or 17.18 decimal GB. This is the one place where the marketing number is actually the binary one.
Practical tips
- When planning backups, compare like with like: if your files total "931 GB" in Windows, that is 1 TB in decimal terms. The backup storage calculator shows both.
- Leave 10–20% of a drive free; full drives slow down, and SSDs especially benefit from free space.
- When a cloud plan says "2 TB", it usually means decimal terabytes, which your computer will show as about 1.82 TB.
Summary
A 1 TB drive does have a trillion bytes. Windows counts them in binary units (GiB) while calling them GB, which makes the drive look about 7% smaller. The rest of the "missing" space is used by the system and pre-installed software.