STORAGE UNITS EXPLAINED
GB vs GiB: why 1 TB is about 931 GiB
Same bytes. Different measuring units. A smaller number does not necessarily mean storage has disappeared — but formatting, reserved storage and partitions do take real space, and this guide keeps the two apart.
Sources checked Sep 21, 2026 – Sep 23, 2026
TRY YOUR OWN CAPACITY
GB to GiB, without the guesswork.
Enter the capacity on the label, or convert back from a binary unit.
SAME CAPACITY, DIFFERENT UNIT
1 TB × 1,000,000,000,000 ÷ 1,073,741,824
1,000,000,000,000 bytes. The byte count has not changed.
Results are rounded for display. This is a unit conversion, not your drive’s free space. It does not subtract files, partitions, or filesystem overhead.
Common drive sizes, converted
| Label capacity | Equivalent in GiB |
|---|---|
| 256 GB | ≈ 238.42 GiB |
| 512 GB | ≈ 476.84 GiB |
| 1 TB | ≈ 931.32 GiB |
| 2 TB | ≈ 1,862.65 GiB |
| 4 TB | ≈ 3,725.29 GiB |
Thousands versus 1,024s
A gigabyte (GB) is 1,000 × 1,000 × 1,000 bytes. A gibibyte (GiB) is 1,024 × 1,024 × 1,024 bytes. The binary step, 1,024, equals 2 to the power of 10.
- 1 GB = 1,000,000,000 bytes. NIST gives this as 109 bytes.
- 1 GiB = 1,073,741,824 bytes. NIST gives this as 230 bytes.
NIST records that the binary prefixes — kibi (Ki), mebi (Mi), gibi (Gi), tebi (Ti), pebi (Pi) and exbi (Ei) — were adopted by the IEC as Amendment 2 to international standard IEC 60027-2, with a publication date of 1999-01. It states directly that a gibibyte is approximately 7 % larger than a gigabyte. Counting the same amount of storage in larger units gives a smaller number.
Convert one terabyte
1 TB = 1,000 GB = 1,000,000,000,000 bytes.
Divide by 1,073,741,824 bytes per GiB: 1 TB ≈ 931.32 GiB. The byte count is unchanged. Seagate publishes the same arithmetic in a table: a 500 GB drive shows as 465 GB in Windows, a 1 TB drive as 931 GB, a 5 TB drive as 4.54 TB and a 16 TB drive as 14.55 TB. Western Digital documents a 1 TB drive as 1,000,000,000,000 bytes, displayed as 931.30 GB in binary units, and says the drive's full space available for storage can be used.
This is not a free-space measurement
The conversion does not account for files or other storage uses on a real drive. It is not a cleanup technique. If the drive is genuinely full, the fix is in the files, not the units — see what to move before deleting files.
TWO CONVENTIONS, ONE DRIVE
Why drive makers and operating systems count differently
Neither side is inventing a number. They are using different multipliers for the same byte count, and the label and the screen were written by different people.
- 01
The maker counts in thousands. Seagate states that hard drive manufacturers market drives in decimal capacity, where one gigabyte equals 1,000,000,000 bytes and one terabyte equals 1,000,000,000,000 bytes, and says all major disk drive manufacturers use decimal values when discussing storage capacity. Apple prints the same convention in its specifications: 1GB = 1 billion bytes and 1TB = 1 trillion bytes.
- 02
Some software counts in 1,024s. Seagate states that programs such as FDISK, system BIOS, Windows and older versions of macOS use the binary numbering system, where one gigabyte equals 1,073,741,824 bytes. Western Digital says the same of Windows and Linux, and that macOS uses decimal.
- 03
The label on screen often stays “GB”. That is the part that misleads. When a system divides by 1,073,741,824 and still writes GB, the reader is comparing a decimal number on the box with a binary number on the screen. NIST's names for the binary units exist precisely to remove that ambiguity.
- 04
Formatting takes a further slice. Apple states that the storage device's formatting uses some of this storage space, and that additional storage space is used by factors such as the device's operating system — which is why its specifications add “actual formatted capacity less”. That is a separate effect from the unit conversion, and it is small next to it.
What each system reports, and in which unit
Where the documentation we read does not state the unit a current version uses, this table says “check” rather than guessing. The safest way to compare two numbers is to convert both to bytes.
Scroll sideways to see all columns →
| System | Where to look | Unit in the documentation we read |
|---|---|---|
| Windows | Start → Settings → System → Storage. | Seagate and Western Digital both state that Windows uses the binary system, where one gigabyte is 1,073,741,824 bytes. Seagate's table shows a 1 TB drive reported as 931 GB in Windows. |
| macOS | Apple menu → System Settings → General → Storage. | Seagate's table shows a 500 GB drive as 500 GB and a 1 TB drive as 1 TB on macOS; Western Digital states that macOS uses decimal. Apple adds that OS X Leopard and earlier reported capacity using the binary system. |
| iPhone and iPad | Settings → General → iPhone Storage (or iPad Storage). | Apple's specifications use 1GB = 1 billion bytes, and it notes that iOS 10 and earlier reported storage capacity using the binary system. Apple does not restate the unit for current iOS on the page we read, so compare byte counts if the difference matters. |
| Android | Settings → Storage, then select a category. Google notes settings can vary by phone. | Check. Google's storage help page we read does not state which unit the figures use. |
Formatting, recovery partitions and unallocated space
Once the units are settled, three documented things can still separate the number on the box from the free space in front of you. They are real, and unlike the unit conversion they do consume capacity.
Reserved storage and system files
Microsoft states that Windows reserves a portion of storage space on your device for use by temporary files, caches and other files, so that the device can update successfully, and that when the device is low on space Windows will clear reserved storage so it can be used for other processes, like a Windows update. To see it, Microsoft says to go to the Storage settings and select Show more categories.
Two hidden files are often the largest single items. Microsoft says the Hiberfil.sys hidden system file sits in the root folder of the drive where the operating system is installed, is reserved by the Windows Kernel Power Manager when you install Windows, and that the size of this file is approximately equal to how much RAM is installed on the computer. For the page file, Microsoft says system-managed page files automatically grow up to three times the physical memory or 4 GB, whichever is larger, but no more than one-eighth of the volume size, when the system commit charge reaches 90 % of the system commit limit.
Partitions the drive was shipped with
A drive can hold more than one partition, and only the ones with a drive letter show up in the usual storage view. Microsoft documents viewing the rest in Disk Management: right-click Start, select Disk Management, and unallocated space on a disk drive appears as a black bar with the label Unallocated.
Unallocated space is not automatically usable by an existing drive letter. Microsoft states that the unallocated space must be located immediately after the volume that you want to extend, that it must be on the same disk drive, and that a volume can be extended only if it is formatted with the NTFS or ReFS file system. If another volume sits in between, Microsoft says you cannot extend the volume. It also warns that a disk larger than 2 terabytes must use the GPT partitioning scheme.
Recovery and system partitions hold the files a PC needs to start and to repair itself. Removing them is not a space-saving step; if a partition is unexplained, ask the PC maker what it is before touching it.
What the numbers look like together
For a 1 TB drive: 1,000,000,000,000 bytes on the label, about 931.32 GiB counted in binary units, then whatever formatting, reserved storage, the hibernation file and any recovery partition take out of that. The first step is a unit conversion and costs nothing. The rest are real space, and the documentation above says where to look at each one.
What this does not explain
A drive that is genuinely smaller than advertised. Unit conversion is predictable: roughly 7 % at the GB/GiB step, about 9 % at TB/TiB. A gap far larger than that, on a drive with nothing installed, is not the units. Compare the byte count the system reports against the byte count on the label, and take it up with the seller or the maker.
Space held by snapshots. On a Mac this can look like storage that is used and free at the same time. Apple says Time Machine also stores some of its backups on your Mac, that these are local snapshots, that your Mac counts the space used by snapshots as available storage, and that Time Machine automatically deletes snapshots as they age or as space is needed for other things. Apple also describes System Data (or Other) as a general category measuring the storage space used by all Apple and third-party files that do not belong to any more specific category.
Files you cannot see. Cloud placeholders, other user accounts and hidden system folders all count in a total without appearing in a folder listing. That is a different investigation, and it starts in your system's own storage view rather than in a unit converter.
A GOOD REASON TO LOOK FURTHER
When the difference is worth investigating
- The shortfall is far more than the roughly 7 % the GB-to-GiB conversion accounts for, and the converter above does not close the gap.
- A brand-new external drive reports a byte count well below the byte count printed on its label or box.
- Disk Management shows a large Unallocated block, or a partition you cannot account for, on a drive you expected to be one volume.
- Free space keeps falling while you are not saving anything — that is a files question, not a units question.
Watch the explanation
Watch the Kambas GB vs GiB video.
For actual space management, see what to move before deleting files, or browse our video guides.