Why a 1 TB drive can appear as about 931 GiB
The missing-looking space usually starts with a unit mismatch. Drive labels commonly use decimal terabytes. Many software displays historically use binary-sized units or binary values labeled in a confusing way.
TB to TiB/GiB converter
Convert a manufacturer-style decimal terabyte value into binary units.
Decimal and binary prefixes are not the same
The SI prefix giga means 10⁹ and tera means 10¹². NIST explicitly notes that SI prefixes are powers of ten and should not be used to represent powers of two.
For binary multiples, the IEC introduced separate prefixes:
| Unit | Bytes | Basis |
|---|---|---|
| 1 GB | 1,000,000,000 | 10⁹ |
| 1 GiB | 1,073,741,824 | 2³⁰ |
| 1 TB | 1,000,000,000,000 | 10¹² |
| 1 TiB | 1,099,511,627,776 | 2⁴⁰ |
The 1 TB calculation
Start with the manufacturer's decimal capacity:
1,000,000,000,000 ÷ 1,073,741,824 ≈ 931.3 GiB
The hardware did not lose roughly 69 “gigabytes.” The number changed because the size of the unit changed. A GiB is larger than a GB.
Why the final usable space can be lower still
The decimal-versus-binary conversion explains a large part of the apparent difference, but it is not the only factor. After a drive is partitioned and formatted, some capacity can be used by filesystem structures, partition metadata, recovery partitions, snapshots, reserved space, or vendor/software features.
That means the converter above should be treated as the theoretical unit conversion, not a promise of the final free-space figure shown by a particular computer.
Common nominal capacities converted
| Label capacity | Approx. GiB | Approx. TiB |
|---|---|---|
| 500 GB | 465.7 GiB | 0.455 TiB |
| 1 TB | 931.3 GiB | 0.909 TiB |
| 2 TB | 1,862.6 GiB | 1.819 TiB |
| 4 TB | 3,725.3 GiB | 3.638 TiB |
| 8 TB | 7,450.6 GiB | 7.276 TiB |
MB/s and Mb/s are another unit trap
Storage and networking specifications add a second common source of confusion: uppercase B means byte, while lowercase b means bit. One byte contains eight bits.
So a 1,000 Mb/s link is not a theoretical 1,000 MB/s file transfer. Ignoring overhead, 1,000 megabits per second divided by eight equals 125 megabytes per second. Real transfers are usually lower for additional reasons. See MB/s versus Mb/s for the unit conversion, then use the file transfer time guide to turn a size and sustained rate into a lower-bound time estimate.
How to compare storage products cleanly
- Compare manufacturer capacities using the same decimal unit: GB with GB, TB with TB.
- When checking operating-system values, identify whether the software is displaying GB/TB or GiB/TiB conventions.
- Separate raw capacity from usable free space after formatting.
- Keep capacity units separate from transfer-speed units.
Primary sources
- NIST — Binary prefixes
- NIST SP 330 — Decimal multiples and binary-prefix guidance
- NIST — Metric (SI) prefixes
Bottom line
If a 1 TB drive shows a number near 931 in a binary-style capacity display, the drive is not necessarily short by 69 GB. You are mostly looking at the same bytes measured with a larger unit.