A drive labeled 1 TB contains 1,000,000,000,000 bytes. That same amount is about 931.3 GiB, and some software labels that binary-sized figure “GB.” So a 1TB drive showing roughly 931 “GB” is usually displaying the same bytes in different units—not reporting that 69 GB has vanished. Formatting and system storage can reduce the space available for your files further.
Why does a 1TB drive show about 931 GB?
Drive makers use decimal units: 1 TB is 1,000,000,000,000 bytes. Binary units count in powers of 1,024, so 1 GiB is 1,073,741,824 bytes. Divide the drive’s byte count by the size of a GiB and the result is about 931.3 GiB.
Some operating-system displays call that binary-sized quantity “GB” rather than “GiB.” Seagate’s support example lists a decimal 1 TB drive as 931 GB in Windows’ binary-output column. It is a rounded example, not a promise that every current operating-system version or formatted drive will show exactly that number. Seagate explains the capacity conversion.
TB and TiB are different units
| Unit | Bytes | Meaning |
|---|---|---|
| 1 TB | 1,000,000,000,000 | Decimal: 1,000 to the fourth power |
| 1 TiB | 1,099,511,627,776 | Binary: 1,024 to the fourth power |
| 1 GiB | 1,073,741,824 | Binary: 1,024 cubed |
In precise usage, TB and GB are decimal units, while TiB and GiB are binary units. The IEC approved the binary-prefix names in December 1998; NIST’s reference lists Ki, Mi, Gi, and Ti and their powers of 1,024. NIST’s binary-prefix reference provides the definitions.
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The labels matter because “931 GB” can be ambiguous if the software is actually counting GiB. Comparing exact byte counts avoids that ambiguity. Apple Support likewise defines 1 GB as 1 billion bytes and 1 TB as 1 trillion bytes. Apple’s explanation of storage capacity also notes that formatted capacity is lower.
Why usable space is lower still
The unit conversion explains the familiar drop from 1,000 decimal GB to about 931 binary GiB; it does not mean that formatting removed those bytes. Available file space is a separate figure. A useful way to interpret a capacity readout is to distinguish:
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- Device capacity: the total byte count represented by the drive’s rating.
- Displayed capacity: that byte count converted and labeled according to the operating system’s unit convention.
- Formatted or available capacity: the space left for files after formatting and any operating-system storage.
- Space used by files: the capacity consumed by saved data, best compared using the same units or exact bytes.
Apple says formatting uses some storage and the operating system uses additional storage. The amount depends on the device and its configuration; the available sources do not establish one percentage that applies to every drive. Keep this distinct from the decimal-to-binary conversion: they are separate effects.
How to check whether the difference is expected
- Find the most exact capacity figure available. If your system or device documentation shows a byte count, use it as the baseline rather than comparing rounded labels.
- Identify the units in the display. The product rating is normally decimal TB. Check whether the operating system is displaying binary-sized values and labeling them “GB,” or explicitly using GiB/TiB.
- Check what the number describes. A raw device, a partition, and a formatted volume with operating-system storage are not the same measurement.
- If the figures still do not reconcile, consult documentation for the drive and system. Seagate lists older BIOS limitations as one possible reporting factor; that is not a diagnosis for every modern computer.
Will reformatting restore the missing capacity?
No. Reformatting does not change how the same byte count converts from decimal units to binary units. It can also erase data, so it is not a way to recover the apparent 1TB-to-931GB difference. Treat formatting as relevant to available space, not as the cause of the unit conversion.
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How should you compare file sizes with drive capacity?
Compare like with like. A file manager’s “GB” label may follow a different convention from a drive’s decimal TB rating, so a total that appears not to add up may be a labeling mismatch. Use exact byte counts where available, or make sure both figures are expressed using the same decimal or binary units. If the capacity is already a formatted-volume figure, account separately for storage used by the operating system and existing files.
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