Hard drive capacity is the amount of data a drive can store, expressed in bits or bytes. A drive labeled 1 TB contains 1,000,000,000,000 bytes under the decimal convention used for consumer drive capacities. The number shown by a computer can look smaller because software may express those same bytes using binary units; formatting and system files can reduce the space available for your files further.
What capacity means
Capacity describes how much data a storage device can contain. The IEC defines storage capacity in terms of the amount of data a device can contain, expressed as a number of specified data elements. In everyday drive specifications, that amount is usually given in bytes or larger byte units.
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A byte is a unit of data equal to 8 bits. Capacity figures therefore describe a quantity of data, not a guarantee that every byte will be available for personal files after setup.
Why a drive labeled 1 TB may show about 931 GiB
Drive makers commonly state capacity in decimal units: 1 TB is 1,000,000,000,000 bytes. Binary units count bytes in powers of two, so 1 GiB is 1,073,741,824 bytes. Dividing the bytes in a decimal 1 TB drive by the bytes in a GiB gives about 931 GiB. The underlying byte count has not disappeared; it is being expressed using a different unit.
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| Unit | Definition | Use |
|---|---|---|
| GB | 1,000,000,000 bytes | Decimal |
| TB | 1,000,000,000,000 bytes | Decimal |
| GiB | 230 bytes, or 1,073,741,824 bytes | Binary |
| TiB | 240 bytes, or 1,099,511,627,776 bytes | Binary |
The distinction is reflected in the unit prefixes: kilo means 103, while kibi means 210. The same pattern applies to giga/gibi and tera/tebi. When exactness matters, GB and TB refer to decimal quantities, while GiB and TiB refer to binary quantities. Some software labels binary quantities as GB or TB, so a display’s unit label may not make its counting convention obvious.
Advertised, displayed, and usable capacity
Advertised capacity
This is the decimal byte total stated by the manufacturer. For example, 1 TB means 1,000,000,000,000 bytes.
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Displayed capacity
This is the number and unit shown by an operating system or storage app. It may be numerically lower if the same bytes are represented in binary units. Interfaces do not all label or report units the same way, so the figure alone may not identify the convention.
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Formatted usable space
Formatting uses some storage, and an operating system may also use space. As a result, room available for your files can be lower than either the advertised figure or the displayed total. The precise amount depends on the drive and computer configuration; there is no single usable-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.
How to interpret a lower-than-expected number
- If the displayed figure is around 931 GiB for a 1 TB drive, that is consistent with converting the advertised decimal byte count to binary units.
- If the interface says GB or TB, check whether that software is using decimal or binary quantities before comparing its number directly with the drive label.
- If you want to know how much room remains for files, look at the operating system’s available-space figure after setup; formatting and system use affect that amount.
Apple Support summarizes the decimal convention this way: “1GB = 1 billion bytes, and 1TB = 1 trillion bytes; actual formatted capacity less.” The quoted wording is from Apple Support’s article on storage capacity measurement.
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- This USB drive provides plug and play simplicity with the included 18 inch USB 3.0 cable
- The available storage capacity may vary.
Sources
- Seagate Support, “Storage capacity measurement standards” (accessed October 7, 2026).
- NIST, “Definitions of the SI units: The binary prefixes” (accessed October 7, 2026).
- Apple Support, “How storage capacity is measured on Apple devices” (published February 26, 2025; accessed October 7, 2026).
- IEC 80000-13:2025 catalog excerpt, “Quantities and units for information technology” (accessed October 7, 2026).
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