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Why a 1 TB Drive Shows 931 GB
A new 1 TB drive appears in Windows as 931 GB. A 100 Mbps connection downloads at about 12 MB/s. Nothing is missing and nobody is short-changing you: in both cases the number on the box and the number on the screen use different units that happen to share a name.
8 min read · updated 17 September 2026 · links to 7 tools
In short
Drive makers count storage in decimal units, where a gigabyte is 1,000,000,000 bytes and a terabyte is 1,000,000,000,000. Windows counts in binary units, where the same word means 1,073,741,824 bytes for a gigabyte, and still labels them “GB”. So a 1 TB drive holds exactly the 1,000,000,000,000 bytes it promises, and Windows divides that by 1,073,741,824 to show 931 GB. The gap is 6.9% at gigabyte scale and 9.1% at terabyte scale; the file system and any preinstalled software take a little more on top. Internet speeds have a different gap: they are sold in megabits per second, while downloads are shown in megabytes per second, and a byte is 8 bits. A 100 Mbps connection therefore moves at most 12.5 MB/s, and nearer 11 MB/s once network overhead is taken out.
Two meanings of the same word
Computers store data in powers of two, so early software counted memory in blocks of 1,024: a “kilobyte” of 1,024 bytes was close enough to a thousand that nobody minded. Everywhere else in science, kilo means exactly 1,000. At kilobyte scale the two meanings differ by only 2.3%, but the difference compounds with every step up.
In 1998 the International Electrotechnical Commission gave the binary units their own names — kibibyte, mebibyte, gibibyte, tebibyte, written KiB, MiB, GiB and TiB — so that KB, MB, GB and TB could mean powers of 1,000 without ambiguity. Drive makers, macOS and most network equipment use the decimal meaning. Windows still counts in binary and shows the old names.
| Step | Decimal (drive box) | Binary (Windows) | A drive looks smaller by |
|---|---|---|---|
| Kilobyte | 1,000 bytes | 1,024 bytes | 2.3% |
| Megabyte | 1,000,000 bytes | 1,048,576 bytes | 4.6% |
| Gigabyte | 1,000,000,000 bytes | 1,073,741,824 bytes | 6.9% |
| Terabyte | 1,000,000,000,000 bytes | 1,099,511,627,776 bytes | 9.1% |
The last column is how much smaller a drive looks when its decimal size is shown in binary units. The same gap measured the other way round — how much larger a binary unit is — is 7.4% for a gigabyte, because the two percentages have different bases.
What your new drive will actually show
Every drive loses the same share to the change of units at the same scale, so the “missing” space grows with the drive. The figures below are the binary sizes of drives sold in decimal units, before anything is installed.
| On the box | Bytes | In binary gigabytes (GiB) | Looks smaller by |
|---|---|---|---|
| 128 GB | 128,000,000,000 | 119.2 | 8.8 GB |
| 256 GB | 256,000,000,000 | 238.4 | 17.6 GB |
| 512 GB | 512,000,000,000 | 476.8 | 35.2 GB |
| 1 TB | 1,000,000,000,000 | 931.3 | 68.7 GB |
| 2 TB | 2,000,000,000,000 | 1,862.6 | 137.4 GB |
| 4 TB | 4,000,000,000,000 | 3,725.3 | 274.7 GB |
Where the rest of the space goes
The change of units explains the big gap, and it is not space you have lost. Two smaller things do take real space. Formatting a drive reserves some of it for the file system — the tables that record where each file lives. And a phone or laptop arrives with its operating system, recovery partition and preinstalled apps already occupying storage, which is why a 128 GB phone can offer far less than 119 GB to you.
A quick way to tell the two apart: if the total capacity shown is close to the binary figure in the table above, the drive is exactly as advertised and the difference is only units. If the free space is much lower than that, something is already stored on it.
Why Windows and a Mac disagree
The same drive plugged into a Mac and into a Windows PC shows two different sizes, because macOS switched to decimal units in 2009, with Mac OS X 10.6. A 1 TB drive is 1 TB on a Mac and 931 GB on Windows. Linux depends on the program: common file managers use decimal units, while the df -h command uses powers of 1,024.
Memory is the exception to the whole story. RAM is manufactured in powers of two, so a module sold as 16 GB really is 16 GiB, and both systems report it that way.
Internet speed: bits, not bytes
Connections are sold in bits per second, written with a small b: Mbps, or Mb/s. Browsers, game launchers and download managers usually show progress in bytes per second, written with a capital B: MB/s. There are 8 bits in a byte, so the download number is one eighth of the connection number before anything else is considered.
| Plan | Best possible download | With 10% overhead |
|---|---|---|
| 25 Mbps | 3.1 MB/s | 2.8 MB/s |
| 50 Mbps | 6.3 MB/s | 5.6 MB/s |
| 100 Mbps | 12.5 MB/s | 11.3 MB/s |
| 300 Mbps | 37.5 MB/s | 33.8 MB/s |
| 500 Mbps | 62.5 MB/s | 56.3 MB/s |
| 1,000 Mbps | 125 MB/s | 112.5 MB/s |
Overhead is the share taken by protocol headers, encryption and resent packets. Ten percent is the transfer time calculator's default and a reasonable figure for a clean wired link; Wi-Fi, a busy network or a slow server can cost much more.
How long a download really takes
Put the two conversions together to estimate a real download. A 50 GB game is 400,000,000,000 bits. At a perfect 100 Mbps that takes 4,000 seconds, just under 1 hour 7 minutes; with 10% overhead it takes about 1 hour 14 minutes. The file transfer time calculator does the whole calculation for any size and speed, and shows the same file over common connection types.
If a download is far slower than the table suggests, the limit is usually somewhere other than your plan: the Wi-Fi link between your device and the router, other devices using the connection, or the server sending the file.
Reading any size or speed correctly
- Check the letter case: a small b is bits, a capital B is bytes. Divide bits by 8 to get bytes.
- Assume a drive, a download size or a network speed is decimal unless it says KiB, MiB or GiB.
- Assume a size shown by Windows is binary, even though it says GB.
- To compare a drive with what Windows shows, divide its size in bytes by 1,073,741,824.
- Expect a real download to run about 10% below the converted figure on a good wired connection, and lower on Wi-Fi.
Frequently asked questions
Why does my 1TB hard drive only show 931GB?
The drive holds 1,000,000,000,000 bytes, which is 1 TB in the decimal units drive makers use. Windows divides by 1,073,741,824 bytes per gigabyte instead of 1,000,000,000, so it shows 931 GB. No space is missing.
What is the difference between GB and GiB?
A gigabyte (GB) is 1,000,000,000 bytes. A gibibyte (GiB) is 1,073,741,824 bytes, which is 2 to the power of 30. A GiB is 7.4% larger, so the same drive shows a smaller number in GiB.
Why is my download speed lower than my internet speed?
Internet plans are sold in megabits per second and downloads are shown in megabytes per second. There are 8 bits in a byte, so 100 Mbps is at most 12.5 MB/s, and about 11 MB/s after network overhead.
How many MB/s is 100 Mbps?
12.5 MB/s at best, because 100 divided by 8 is 12.5. Protocol overhead usually brings a real download to around 11 MB/s on a good wired connection.
Does a Mac show the same drive size as Windows?
No. macOS has used decimal units since 2009, so a 1 TB drive shows as 1 TB on a Mac and 931 GB on Windows.
Is RAM measured the same way as storage?
No. Memory is made in powers of two, so 16 GB of RAM really is 16 GiB, and it is reported as 16 GB on both Windows and macOS.
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