Why Your 1 TB Drive Shows 931 GB: Binary vs Decimal Storage Units
You buy a 1 TB drive, plug it in, and Windows reports 931 GB. Nothing has gone wrong, and no space is lost to formatting. The drive maker and your operating system are using two different definitions of "gigabyte".
Two kinds of gigabyte
The SI prefixes kilo, mega, giga and tera mean powers of 1,000: 1 GB = 1,000,000,000 bytes. Computers address memory in powers of two, so for decades software used the same words to mean powers of 1,024: 1 "GB" = 1,073,741,824 bytes. To end the confusion, the IEC introduced binary prefixes (standardised today in IEC 80000-13): KiB, MiB, GiB, TiB — kibibyte, mebibyte, gibibyte, tebibyte — for the powers of 1,024. Windows still labels GiB as "GB".
| Decimal (SI) | Bytes | Binary (IEC) | Bytes |
|---|---|---|---|
| 1 kB | 1,000 | 1 KiB | 1,024 |
| 1 MB | 1,000,000 | 1 MiB | 1,048,576 |
| 1 GB | 1,000,000,000 | 1 GiB | 1,073,741,824 |
| 1 TB | 1,000,000,000,000 | 1 TiB | 1,099,511,627,776 |
The gap grows with each step: about 2.4% at kilo, 4.9% at mega, 7.4% at giga and 10% at tera.
What you actually get
| Advertised | Bytes | Windows shows (GiB/TiB) |
|---|---|---|
| 128 GB | 128,000,000,000 | 119.2 GB |
| 256 GB | 256,000,000,000 | 238.4 GB |
| 512 GB | 512,000,000,000 | 476.8 GB |
| 1 TB | 10¹² | 931.3 GB |
| 2 TB | 2 × 10¹² | 1.82 TB (1,862.6 GB) |
| 4 TB | 4 × 10¹² | 3.64 TB (3,725.3 GB) |
A phone's usable space is smaller still, because the operating system and preinstalled apps take some of it. That's a separate issue from units.
Who uses which
- Decimal: drive and SSD makers, macOS (since 10.6), iOS, Android storage screens, most Linux file managers, and mobile data plans.
- Binary: Windows Explorer, RAM sizes (a "16 GB" RAM stick really holds 16 GiB), and many programming tools.
Our MB to GB, GB to MB and TB to GB converters show both answers side by side.
Internet speeds are different again
Network speeds use decimal bits, not bytes: 100 Mbps = 100,000,000 bits per second = 12.5 MB per second. So a 1 GB (decimal) download takes at least 80 seconds on a 100 Mbps line, before overheads. Convert speeds with Mbps to Gbps, and file sizes with the unit converter.