Hard disk capacity is the amount of data a drive can store, measured in bytes and larger units such as gigabytes (GB) and terabytes (TB). A drive labeled 1 TB may appear as about 931 GB on a computer because manufacturers commonly use decimal units, while some operating systems display capacity using binary-sized units but keep the GB label. That difference is a conversion, not evidence by itself that the drive is defective or data is missing.
What does hard disk capacity mean?
Hard disk capacity is the drive’s total storage size: the number of bytes it can hold. Operating systems also use capacity as a property of storage media; Microsoft’s System.Capacity documentation describes it as indicating the capacity of a storage device.
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To compare capacity figures, first identify the unit convention. As Seagate explains in its storage capacity measurement standards, the byte count is the clearest basis for understanding a drive’s stated and displayed size.
Why does a 1 TB drive show about 931 GB?
Drive manufacturers commonly use decimal units on labels: 1 TB is 1,000,000,000,000 bytes. A binary gigabyte is larger—1,073,741,824 bytes—so dividing those same bytes by the binary unit gives about 931.3. Seagate describes this familiar difference in its explanation of why a drive may report less capacity than its label.
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The underlying capacity has not changed: the figures count the same bytes using different-sized units. Seagate notes that “In order to accurately understand the true capacity of your disk drive, you need to know which base unit of measure (binary or decimal) is being used to represent capacity.”
Decimal and binary storage units
Decimal units scale by 1,000. Binary units scale by 1,024 and have distinct names—kibibyte, mebibyte, gibibyte, and tebibyte—abbreviated KiB, MiB, GiB, and TiB. Apple’s explanation of how storage capacity is measured gives the binary GB- and TB-sized values below.
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| Unit | Bytes | Convention |
|---|---|---|
| 1 GB | 1,000,000,000 | Decimal; commonly used on drive labels |
| 1 TB | 1,000,000,000,000 | Decimal; commonly used on drive labels |
| 1 GiB | 1,073,741,824 | Binary |
| 1 TiB | 1,099,511,627,776 | Binary |
Some software displays a binary-scaled number while labeling it GB or TB rather than GiB or TiB. For that reason, the displayed unit name alone may not tell you which convention the interface uses.
Capacity is not the same as free space
A drive label, an operating system’s reported capacity, a partition’s size, and the amount of free space answer different questions:
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- Drive-label capacity: the manufacturer’s stated total, commonly expressed in decimal bytes.
- Reported capacity: the amount the system or interface recognizes, displayed according to its unit convention.
- Partition capacity: the space assigned to a particular volume or partition.
- Free space: the portion of a volume not currently occupied by files or other data.
Formatting and partition layout can affect how space is allocated or shown. System firmware, controllers, or operating-system support can also matter; a decimal-to-binary conversion does not account for every possible discrepancy.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What to check if the difference seems too large
If the displayed size is far below the roughly 931 GB expected for a decimal 1 TB drive shown in binary-sized GB, check the actual byte count and how the storage is configured before treating the difference as ordinary unit conversion.
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- Check the exact byte count. Compare the drive’s stated capacity with the system’s reported total, not just rounded GB or TB figures.
- Check partitioning and formatting. Look for unallocated space, smaller-than-expected partitions, or a volume that does not use the whole device.
- Check system and controller support. Some older or particular system configurations may not recognize all of a drive’s capacity. Microsoft’s guidance on Windows support for hard disks exceeding 2 TB says GPT initialization is required for full support above 2 TB in the scenarios it covers; this is not a blanket rule for every drive or computer.
Seagate and SanDisk also discuss label-versus-reported capacity differences in their support guidance: SanDisk’s available-capacity explanation is another reference for interpreting a smaller displayed figure.
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