Two ways to group the same bytes
Storage manufacturers commonly use decimal units: 1 GB is 1,000,000,000 bytes and 1 TB is 1,000,000,000,000 bytes. Binary units use powers of two. One gibibyte, written GiB, is 1,073,741,824 bytes. Both describe data, but they group bytes differently.
Some software labels make the comparison confusing. Windows may display a binary calculation with a GB label. Exact wording varies between tools and operating systems, so check the byte count or documented units when comparing two screens. Different numbers can still describe the same physical drive.
Work through the conversion
Divide one trillion bytes by the bytes in a GiB: 1,000,000,000,000 ÷ 1,073,741,824 = about 931.32 GiB. No files have been created in that calculation. The apparent reduction comes from using the larger binary unit.
The conversion applies at other capacities too. Compare the expected ratio rather than expecting every screen to show a round marketing number. Rounding, formatting and other uses of the drive can make the final displayed amount differ slightly from this simple calculation.
| Label on the drive | Approximate binary capacity |
|---|---|
| 500 GB | 465.66 GiB |
| 1 TB | 931.32 GiB |
| 2 TB | 1,862.65 GiB |
| 4 TB | 3,725.29 GiB |
Formatting is a separate question
A file system needs structures to keep track of files. A system drive may also have recovery, boot or other partitions. These uses are separate from the decimal-to-binary conversion, and can affect how much of the device is available to the volume you are viewing.
That distinction helps troubleshooting. A 1 TB drive showing about 931 GiB of total volume capacity may be ordinary. A 1 TB device with only a 32 GB volume deserves a look at its partition layout. Neither observation tells you how much space your documents occupy: total capacity and free capacity are different measurements.
Check the whole device before changing a volume
Start with the drive model and capacity in the operating system’s disk-management utility. Examine all volumes or partitions on that device. Use a view showing the whole disk, not only a removable-drive icon or one mounted folder.
If you find unallocated space, pause before changing it. Partitions may exist for another system, an installer or recovery. Record what the utility shows and confirm which physical device you selected. Back up important contents before modifying the layout. Do not format a drive simply to make a number look tidier.
Keep backup calculations in matching units
If a file manager reports GiB and a calculator asks for decimal GB, convert or choose a matching unit. For example, 400 GiB is about 429.50 decimal GB. Treating it as 400 decimal GB understates the input by about 29.50 GB before adding any growth allowance.
Use bytes when a tool exposes them, or note your unit convention and maintain it throughout the estimate. Leave room for new files and older backup versions rather than aiming to fill the destination exactly. The same arithmetic should describe the source files and the destination capacity.
Recognise an unusual result
A gap far larger than the expected conversion can have another explanation: a small partition, existing files, reserved storage or a device with an unexpected capacity. Investigate the specific device and volume information rather than assuming every shortfall has the same cause.
If a new drive repeatedly disconnects, reports errors or behaves inconsistently, stop relying on it for the only copy of important data. Keep the purchase details and use the manufacturer’s support route. A normal 931 GiB display has an arithmetic explanation; unreliable behaviour needs a separate diagnosis.
Sources & further reading
Seagate — Why a drive reports less capacity than its label ↗Apple — How storage capacity is measured ↗This guide is based on the sources linked above, with original explanations and examples. Software interfaces and service features can change. A documentation-based guide is not a hands-on product review. Read our editorial policy or report a correction.