A drive labeled 1 TB represents 1,000,000,000,000 bytes when the decimal unit is used. The same number of bytes is approximately 931.32 gibibytes (GiB). That numerical difference can arise before any files are saved. It is a change of unit, not evidence that 69 billion bytes disappeared.
GB and GiB have different denominators
| Unit | Bytes |
|---|---|
| 1 GB | 1,000,000,000 = 109 |
| 1 TB | 1,000,000,000,000 = 1012 |
| 1 GiB | 1,073,741,824 = 230 |
| 1 TiB | 1,099,511,627,776 = 240 |
NIST distinguishes the binary prefixes from decimal SI prefixes. A byte contains eight bits. Capitalization matters: GB and Gb refer to different units, and transfer-rate labels often use bits rather than bytes.
Work the 1 TB example from bytes
1,000,000,000,000 / 1,073,741,824 = 931.3225746 GiB
1,000,000,000,000 / 1,099,511,627,776 = 0.9094947 TiB
Both results describe the same raw byte count. A 2 TB decimal capacity is about 1,862.65 GiB; 500 GB decimal is about 465.66 GiB. These are unit conversions, not measurements of a particular product or guarantees of space available to an application.
Separate three capacity questions
- Device capacity: How many addressable bytes does the device report?
- Volume/partition capacity: How much of that device belongs to this volume, after the partition layout and filesystem's structures?
- Free space: How much space remains after files, snapshots and other allocations on that volume?
A difference caused by units is different from a recovery partition, a second volume, a snapshot or real stored files. Do not add a generic “formatting loss” percentage without inspecting the actual layout. Different interfaces can also use or label units differently, so compare the byte counts when possible.
When the difference deserves investigation
If the reported byte capacity is much smaller than the documented device capacity, inspect the exact model, enclosure/interface and partition layout. A small volume on a larger device may simply leave space unallocated. A counterfeit or failing device is another possibility, but a 931 GiB display for a decimal 1 TB drive is not, by itself, evidence of either.
Keep a backup before changing partitions or running any write-based capacity test. Choose a test appropriate to your situation; do not erase the only copy of your files to answer a unit question. This article explains interpretation and does not perform a drive test.
Use the same care for transfer speeds
A quoted 100 MB/s and 100 Mbit/s differ by a factor of eight before overhead. If a download application reports MiB/s, there is another binary/decimal distinction. Write down the complete unit, file byte count and elapsed time before comparing performance. See the Wi-Fi link-rate explanation for an end-to-end worked example.