Your 1 TB drive isn’t missing space. Drive makers count in decimal units, so 1 TB means 1,000,000,000,000 bytes. Windows counts in binary units, dividing by 1,024 at each step instead of 1,000, and still labels the result “GB.” One trillion bytes divided by 1,024³ is 931.32, so that’s what Windows shows. macOS and iOS count in decimal, so the same drive shows close to 1 TB there, minus whatever formatting and the system use.
What’s the difference between GB and GiB? #
There are two families of prefixes for bytes:
| Decimal (SI) | Bytes | Binary (IEC) | Bytes |
|---|---|---|---|
| kilobyte (KB) | 1,000 | kibibyte (KiB) | 1,024 |
| megabyte (MB) | 1,000,000 | mebibyte (MiB) | 1,048,576 |
| gigabyte (GB) | 1,000,000,000 | gibibyte (GiB) | 1,073,741,824 |
| terabyte (TB) | 1,000,000,000,000 | tebibyte (TiB) | 1,099,511,627,776 |
The binary names were approved by the International Electrotechnical Commission in December 1998, as NIST explains: “one gibibyte: 1 GiB = 2³⁰ B = 1 073 741 824 B” and “one gigabyte: 1 GB = 10⁹ B = 1 000 000 000 B.” NIST adds that “the confusion is real.”
The confusion comes from software that divides by 1,024 but prints “GB” instead of “GiB.” The number is a gibibyte count wearing a gigabyte label.
Why does the gap grow with bigger drives? #
Each prefix step compounds the 2.4% difference between 1,000 and 1,024:
| Level | Binary ÷ decimal | Apparent shortfall |
|---|---|---|
| Kilo | 0.9766 | 2.3% |
| Mega | 0.9537 | 4.6% |
| Giga | 0.9313 | 6.9% |
| Tera | 0.9095 | 9.1% |
At the terabyte level, the same drive reads about 9% smaller in binary. That’s why the gap was barely noticeable on old floppy disks and is obvious on today’s drives.
What will my drive show? #
| Advertised size | Bytes | Shown in binary “GB” (GiB) | Shown in binary “TB” (TiB) |
|---|---|---|---|
| 64 GB | 64 billion | 59.6 | 0.06 |
| 128 GB | 128 billion | 119.2 | 0.12 |
| 256 GB | 256 billion | 238.4 | 0.23 |
| 512 GB | 512 billion | 476.8 | 0.47 |
| 1 TB | 1 trillion | 931.3 | 0.91 |
| 2 TB | 2 trillion | 1,862.6 | 1.82 |
| 4 TB | 4 trillion | 3,725.3 | 3.64 |
The number you actually see will be a little lower still, because formatting the drive with a file system takes some space, and a computer’s system drive also holds recovery partitions and the operating system.
Which operating systems use which? #
- Windows reports sizes in binary units but labels them KB, MB, GB and TB. That’s the source of the 931.
- macOS and iOS report sizes in decimal. Apple’s own support article on how iOS and macOS report storage capacity says that with its decimal system, “1GB = 1 billion bytes, and 1TB = 1 trillion bytes.” It notes that iOS 10 and earlier, and OS X Leopard and earlier, used binary.
- Linux varies by tool. Some show GiB with the correct label, others show decimal.
So the same 1 TB external drive can read 931 GB on a Windows PC and about 1 TB on a Mac. Both are correct. They’re counting in different units.
Where else do GB and GiB matter? #
Memory (RAM) is built in powers of two, so a “16 GB” stick of RAM holds 16 GiB, and everyone reports it that way.
Network speeds use decimal bits, not bytes. A 100 Mbps connection moves 100 million bits per second, which is 12.5 MB/s (divide by 8). A 4 GB download at a steady 100 Mbps takes 4,000 MB × 8 ÷ 100 = 320 seconds, or about 5 minutes and 20 seconds, in the best case.
Phone storage is advertised in decimal. On an iPhone, which also reports in decimal, the gap between advertised and available space is mostly the operating system and preinstalled apps, not unit math.
Worked examples #
A 256 GB USB stick shows 238 GB in Windows. 256,000,000,000 ÷ 1,073,741,824 = 238.4 GiB. Nothing’s wrong.
A 2 TB backup drive shows 1.81 TB. 2 trillion bytes is 1.82 TiB, and formatting takes a little more.
A file is 1,500 MiB. Will it fit on a 1.5 GB card? 1,500 MiB is 1,572,864,000 bytes, which is 1.57 GB in decimal. It won’t fit.
Converting data units on your phone #
Convert Units Calculator has a Data category with bits, bytes, kilobytes through petabytes in decimal, and, from version 1.0.5, the binary kibibytes, mebibytes, gibibytes and tebibytes as separate units. Type 1 TB and the GiB figure (931.32) appears in the list underneath, so you can see exactly what Windows will report. Version 1.0.5 also adds a Bandwidth category for converting Mbps to MB/s.
The decimal prefixes here are the same kilo, mega, giga and tera covered in how the metric system works. For the general method behind conversions like these, see how to convert units.
Frequently asked questions #
Is 1 TB 1,000 GB or 1,024 GB? #
In decimal, which drive makers and Apple use, 1 TB is 1,000 GB. In binary, 1 TiB is 1,024 GiB. A 1 TB drive is 1,000 GB, which is 931.3 GiB.
Why does my new drive show less space than advertised? #
Mostly because of units. The box counts in decimal, and Windows counts in binary but labels it GB. Formatting and any preinstalled files take a bit more.
What is a GiB? #
A gibibyte: 2³⁰ bytes, or 1,073,741,824 bytes. It’s about 7.4% larger than a gigabyte (1,000,000,000 bytes).
How many bytes are in a gigabyte? #
1,000,000,000 bytes, by the SI and IEC definitions. Software that uses the older binary meaning of “gigabyte” means 1,073,741,824 bytes, which is properly called a gibibyte.
Is RAM measured in GB or GiB? #
RAM comes in powers of two, so its “GB” is really GiB. A 16 GB memory module holds 16 × 1,073,741,824 bytes.