Decimal vs binary units
Drive manufacturers use decimal units: 1 GB = 1,000,000,000 bytes. Windows and many tools use binary units, 1 GiB = 1,073,741,824 bytes, but often label them "GB". That is why a new 1 TB drive shows up as about 931 GB.
1 KB = 1,000 B 1 KiB = 1,024 B
1 GB = 10⁹ B 1 GiB = 2³⁰ B = 1.074 GB
1 TB = 10¹² B 1 TiB = 2⁴⁰ B = 1.0995 TB
What a drive really shows
| Advertised | Shown in Windows (GiB/TiB) |
|---|---|
| 256 GB | 238 GiB |
| 512 GB | 477 GiB |
| 1 TB | 931 GiB |
| 4 TB | 3.64 TiB |
| 10 TB | 9.09 TiB |
Frequently asked questions
Why is my 1TB drive only 931GB?
The drive has 1,000,000,000,000 bytes. Windows divides by 1,024³ and reports 931 GiB, but labels it GB.
Is a gigabit the same as a gigabyte?
No. A gigabit (Gb) is 1/8 of a gigabyte (GB). Network speeds use bits; file sizes use bytes.
Which unit does macOS use?
macOS uses decimal units since version 10.6, so a 1 TB drive shows as 1 TB.