Why a 1 TB drive shows 931 GB

You buy a 1 TB drive, plug it into a Windows PC, and it reports 931 GB. A 256 GB phone shows less free space than you expected. A 500 GB SSD comes up as 465 GB. It looks like the manufacturer shorted you by about 7 to 9 percent. They didn’t. Every byte is there; it’s the counting that differs.

Two meanings of “gigabyte”

In the metric system, kilo means 1,000, mega a million and giga a billion. Storage makers use exactly those values:

1 kilobyte (kB) is 1,000 bytes.
1 megabyte (MB) is 1,000,000 bytes.
1 gigabyte (GB) is 1,000,000,000 bytes.
1 terabyte (TB) is 1,000,000,000,000 bytes.

Computers, though, address memory in powers of two, and early software borrowed the metric names for the nearest powers of two: 1,024 bytes became a “kilobyte”, 1,048,576 bytes (1,024²) a “megabyte”, and so on. For small numbers the difference hardly mattered. It grows with every step up:

At kilo, 1,024 is 2.4% more than 1,000.
At mega, the binary unit is 4.86% larger.
At giga, 7.37% larger.
At tera, 9.95% larger.

So the larger the drive, the bigger the apparent shortfall.

The binary units have their own names

To end the ambiguity, the International Electrotechnical Commission introduced separate names for the binary units in 1998, now part of the IEC 80000-13 standard:

kibibyte (KiB) = 1,024 bytes
mebibyte (MiB) = 1,024 KiB = 1,048,576 bytes
gibibyte (GiB) = 1,024 MiB = 1,073,741,824 bytes
tebibyte (TiB) = 1,024 GiB = 1,099,511,627,776 bytes

With those names the drive mystery disappears: 1 TB = 931.32 GiB. Windows does that arithmetic correctly, then labels the result “GB” instead of “GiB”. That label is the whole problem.

Where the 931 comes from

A 1 TB drive holds 1,000,000,000,000 bytes. Divide by the bytes in a gibibyte:

1,000,000,000,000 ÷ 1,073,741,824 = 931.32

The same calculation for other common sizes:

128 GB = 119.21 GiB
256 GB = 238.42 GiB
500 GB = 465.66 GiB
2 TB = 1.82 TiB, which Windows shows as 1.81 TB because it rounds down
4 TB = 3.64 TiB

The GB to GiB converter handles any size, in either direction.

Which systems count which way

Windows (File Explorer, Disk Management) counts in binary units and labels them KB, MB, GB and TB.
macOS has counted in decimal units since Mac OS X 10.6 Snow Leopard, so a 1 TB drive shows as 1 TB in Finder.
iPhone and Android storage screens generally count in decimal, matching the advertised capacity, though the system itself takes a share.
Linux depends on the tool: GNOME’s file manager uses decimal units, while df -h and ls -lh use binary multiples with single-letter suffixes (G, T). df -H switches to decimal.
RAM is the exception everyone agrees on: memory chips come in powers of two, so “16 GB” of RAM is really 16 GiB, and every system reports it that way.

When the same drive shows different sizes on a Mac and a PC, this is why. Neither is wrong; they are using different units under the same name.

The space you really lose

After the unit difference, a few things do take real space:

File system overhead. Formatting reserves room for the tables that track files. On NTFS and APFS volumes this is small. Linux’s ext4 also reserves 5% of the space for the system by default, which shows up as space you can’t use.
Hidden partitions. A new laptop’s drive often carries a recovery partition and an EFI system partition, together from a few hundred MB to tens of GB.
The operating system. On phones and on Macs with a single system volume, the OS and its updates are counted as used space from day one.
Spare area on SSDs. SSDs carry extra flash beyond the stated capacity for wear leveling. That area is never part of the advertised number, so it doesn’t reduce what you see.

So when Windows reports a new 512 GB laptop drive as 476 GB in total, that’s entirely the unit difference. When it shows much less free space after setup, the rest is the operating system and the recovery partition.

Is it false advertising?

Courts have looked at this. In the United States, hard drive and flash makers settled class actions in the mid-2000s, and their packaging now usually carries a footnote such as “1 GB = 1 billion bytes; actual formatted capacity will be less”. Decimal prefixes are the internationally standard meaning of kilo, mega and giga, so the drive label is the one using the units correctly. The operating system is the one borrowing a decimal name for a binary unit.

Working out what you’ll actually get

To predict what Windows will show: multiply the advertised GB by 0.9313 (or TB by 931.32 to get GiB).
To size a cloud volume or a virtual machine disk: check whether the provider says GB or GiB. Several major cloud platforms bill block storage in GiB, so a “100 GB” disk may really be 100 GiB, which is 107.37 GB.
To compare a file size with free space: use the same system for both. File sizes in Explorer are binary too, so they’ll match the free space it shows.

For quick conversions between decimal sizes, use GB to TB, GB to MB, MB to GB or KB to MB. For the binary units, use the GB to GiB converter.