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Hard disk capacity is the amount of data a drive can store, measured in bytes. A drive labeled 1 TB can appear as about 931 GB on a computer because manufacturers commonly use decimal units while some computer interfaces show binary-scaled values using the familiar GB label. That difference is usually a unit conversion, not lost capacity.
What does hard disk capacity mean?
Hard disk capacity is the quantity of data a hard disk drive can store. Operating systems also use capacity to describe storage media; Microsoft documents Capacity as a storage-media property. The number is meaningful only when you know both what is being counted—bytes, a partition, or free space—and which unit convention is being used.
Why does a 1 TB drive appear as about 931 GB?
Drive makers commonly use decimal units for labels: 1 TB means 1,000,000,000,000 bytes. A binary gigabyte (GiB) is larger than a decimal gigabyte (GB): it contains 1,073,741,824 bytes. Dividing the labeled byte count by that binary unit gives about 931.3 GiB. Some interfaces call this binary-scaled amount “GB,” which is why a 1 TB drive may appear to have roughly 931 GB. Seagate explains this label-versus-display difference in its guidance on why a hard drive reports less capacity than its label.
Compare the byte count when possible: it avoids confusion caused by different labels for decimal and binary units. The conversion alone does not mean the drive is defective or that data is missing.
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Decimal and binary capacity units
Decimal units scale by 1,000; binary units scale by powers of 1,024. The technically distinct binary prefixes are GiB and TiB, although some software retains the labels GB and TB for binary-scaled figures. Apple’s explanation of storage capacity distinguishes these unit conventions.
| Unit | Bytes | Convention |
|---|---|---|
| 1 GB | 1,000,000,000 | Decimal; commonly used for drive labels, according to Seagate’s capacity measurement guidance. |
| 1 TB | 1,000,000,000,000 | Decimal; commonly used for drive labels, according to Seagate’s capacity measurement guidance. |
| 1 GiB | 1,073,741,824 | Binary; Apple’s stated value. |
| 1 TiB | 1,099,511,627,776 | Binary; Apple’s stated value. |
Capacity on the label, in the system, and available to use
These figures can describe different things:
- Labeled capacity: the drive’s stated total, commonly expressed in decimal GB or TB.
- Reported capacity: what the operating system or another interface displays, potentially using binary scaling while showing GB or TB.
- Partition capacity: the space assigned to a particular partition, rather than the whole physical drive.
- Free space: capacity not currently occupied by files. It is not the same as total drive capacity.
Formatting, partition layout, firmware or controller limits, and platform support can also affect what is available or shown. The decimal-to-binary conversion does not explain every discrepancy. For example, Microsoft’s guidance for Windows scenarios involving disks over 2 TB says the disk must be initialized with GPT for Windows to fully support it.
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How to check an unexpectedly small capacity
- Compare bytes, not just the unit labels. Convert the labeled decimal capacity to bytes, then compare it with the system’s reported total. A 1 TB decimal label corresponds to about 931.3 binary GiB.
- Check what the interface is reporting. Confirm whether the number is for the physical disk, one partition, or free space.
- Inspect the partition layout and formatting. These can explain why a partition or available-space figure is lower than the drive’s full reported capacity.
- Check platform and controller support. If a drive larger than 2 TB is not fully recognized in a Windows scenario covered by Microsoft’s guidance, verify whether GPT initialization and system support requirements are met.
If the byte count or total reported capacity remains far below what the unit conversion predicts, do not assume the difference is normal; investigate the partitioning and system support before drawing a conclusion.
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