Why a 1 TB drive shows as 931 GB
Nothing is missing. The maker counts in powers of 10: 1 TB = 1,000,000,000,000 bytes. Some software counts in powers of 2 while still writing “GB”: to it, those 1,000,000,000,000 bytes are 931.32 gibibytes (GiB), hence the “931 GB” on screen.
Updated November 6, 2026 · Sources
Two ways to count bytes
Kilo, mega, giga and tera are prefixes of the International System of Units: they mean powers of 10. So 1 kB = 1,000 bytes and 1 GB = 1,000,000,000 bytes.
Computers address memory in powers of 2, which gave a second set of units: 1 KiB = 1,024 bytes and 1 GiB = 1,073,741,824 bytes.
To end the confusion, the International Electrotechnical Commission (IEC) approved distinct names for binary multiples in December 1998: kibi (Ki), mebi (Mi), gibi (Gi), tebi (Ti). These prefixes are not part of the International System of Units.
The gap grows with size
Each step widens the gap: about 6.9% at the giga scale and 9.1% at the tera scale. That is why the difference mostly shows on large drives.
| Decimal | Bytes | Binary | Bytes | Gap |
|---|---|---|---|---|
| 1 kB | 1,000 | 1 KiB | 1,024 | 2.3 % |
| 1 MB | 1,000,000 | 1 MiB | 1,048,576 | 4.6 % |
| 1 GB | 1,000,000,000 | 1 GiB | 1,073,741,824 | 6.9 % |
| 1 TB | 1,000,000,000,000 | 1 TiB | 1,099,511,627,776 | 9.1 % |
What about internet speeds?
Speeds are given in bits per second, and it takes 8 bits to make a byte. A 100 Mbit/s connection therefore moves at best 12.5 MB per second.
A 5 GB file needs at least 6 min 40 s at 100 Mbit/s. In practice it takes a little longer: protocols and the network take their share.
How not to get it wrong
- Check the symbol: GB and TB are decimal, GiB and TiB are binary.
- A “GB” shown by software may be a GiB: compare the byte counts, they never lie.
- For a speed, divide Mbit/s by 8 to get MB/s.