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Hard drive capacity is the amount of data a drive can store, expressed in units such as bytes, gigabytes (GB), or terabytes (TB). A drive labeled 1 TB contains 1,000,000,000,000 bytes. The number shown by a computer may be lower because it may express those same bytes using binary units, and formatting and system files can reduce the space available for your files.
What hard drive capacity measures
Capacity describes how much data a storage device can contain. The definition in the IEC 80000-13:2025 catalog expresses it as an amount of data represented by a number of specified data elements. In everyday use, capacity is stated in bytes or larger byte units.
A drive’s capacity is not the same as the space currently free for your files. It helps to distinguish three figures:
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- Advertised capacity: the decimal byte total stated on the drive’s packaging or specifications.
- Displayed capacity: the number and unit convention shown by an operating system or storage app.
- Formatted usable space: the space left for files after formatting and any operating-system use.
Why the number on your computer can look smaller
Drive manufacturers commonly state capacity using decimal units. In decimal measurement, each step is a factor of 1,000: 1 GB is 1,000,000,000 bytes, and 1 TB is 1,000,000,000,000 bytes. Seagate’s explanation of capacity measurement and Apple Support’s explanation of storage capacity describe this convention.
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Binary units use powers of 1,024 instead. NIST’s definitions of the IEC binary prefixes distinguish, for example, gigabytes (GB) from gibibytes (GiB), and terabytes (TB) from tebibytes (TiB).
| Unit | Definition |
|---|---|
| 1 byte (B) | 8 bits |
| 1 GB | 1,000,000,000 bytes |
| 1 TB | 1,000,000,000,000 bytes |
| 1 GiB | 230 bytes, or 1,073,741,824 bytes |
| 1 TiB | 240 bytes, or 1,099,511,627,776 bytes |
As a result, the 1,000,000,000,000 bytes on a manufacturer-labeled 1 TB drive equal about 931 GiB when divided into units of 230 bytes. The bytes have not disappeared; the smaller figure reflects a different unit size. Some software displays binary-based quantities with GB or TB labels, so the label alone may not tell you which convention is being used.
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- To get set up, connect the portable hard drive to a computer for automatic recognition software required
- This USB drive provides plug and play simplicity with the included 18 inch USB 3.0 cable
- The available storage capacity may vary.
Why formatted space can be lower still
Formatting prepares a drive for use, and the operating system may also use some of its storage. Apple Support notes that a device’s actual formatted capacity is lower than its stated capacity because of formatting and operating-system use. The usable amount varies by drive and computer configuration, so there is no single exact formatted-space figure that applies to every 1 TB drive.
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- To get set up, connect the portable hard drive to a computer for automatic recognition no software required
- This USB drive provides plug and play simplicity with the included 18 inch USB 3.0 cable
- The available storage capacity may vary.
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- Easily store and access 1TB to content on the go with the Seagate Portable Drive, a USB external hard drive.Specific uses: Personal
- Designed to work with Windows or Mac computers, this external hard drive makes backup a snap just drag and drop. Reformatting may be required for Mac
- To get set up, connect the portable hard drive to a computer for automatic recognition no software required
- This USB drive provides plug and play simplicity with the included 18 inch USB 3.0 cable
- The available storage capacity may vary.
How to interpret a capacity figure
- If a computer reports a smaller number but the difference is consistent with converting decimal bytes to binary units, it is a unit-conversion difference rather than evidence that the drive lost capacity.
- If you need precise comparisons, check whether each figure uses GB/TB or GiB/TiB rather than treating those units as interchangeable.
- For the space available to save files, use the operating system’s available-space figure; advertised capacity describes the drive’s stated byte total, not necessarily formatted free space.
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