Why is my 1 TB drive only 931 GB?
Drive makers count in powers of ten and operating systems count in powers of two. Both are right; the labels disagree.
Almost exactly 1 GB (decimal)
1 GB is bigger than 4 of the 8 references on this page.
Reference figures are measurements and are rounded; the unit conversion behind them is exact.
| Reference | Size | Yours ÷ theirs | Note |
|---|---|---|---|
| 1 kB (decimal) | 1×10⁻⁶ GB | 1,000,000× | |
| 1 KiB (binary) | 1.02×10⁻⁶ GB | 976,563× | 2.4% larger |
| 1 MB (decimal) | 0.001 GB | 1,000× | |
| 1 MiB (binary) | 0.00105 GB | 954× | 4.9% larger |
| 1 GB (decimal) | 1 GB | 1× | |
| 1 GiB (binary) | 1.07 GB | 0.931× | 7.4% larger |
| 1 TB (decimal) | 1,000 GB | 1/1,000 | |
| 1 TiB (binary) | 1,100 GB | 1/1,100 | 10% larger |
Worth knowing
- The gap compounds: 2.4% at kilo, 4.9% at mega, 7.4% at giga, 10% at tera. A 16 TB drive "loses" 1.5 TB to the naming difference alone.
- The binary prefixes kibi, mebi and gibi were standardised by the IEC in 1998. Windows still ignores them; macOS and Linux mostly do not.
Frequently asked
Why does my 1 TB drive show as 931 GB?
Because the manufacturer sold you 10¹² bytes and Windows is dividing by 1024 three times and calling the result GB. 10¹² ÷ 1024³ = 931. Nothing is missing.
Which prefix should I use?
Decimal (GB) for storage and network speeds, binary (GiB) for memory, where powers of two are physically real. Being explicit costs one letter.