Why Your Drive Shows Less Space Than Advertised

You buy a 1 TB drive, plug it in, and Windows reports 931 GB. Nothing is missing, nothing is faulty, and no one has taken a cut. The drive holds exactly what it says — the two numbers are the same quantity described in different units.

Two definitions of the same word

A byte is unambiguous. Everything above it is not.

Prefix	Decimal (SI)	Binary (IEC)
kilo	1,000 bytes = 1 kB	1,024 bytes = 1 KiB
mega	1,000,000 = 1 MB	1,048,576 = 1 MiB
giga	1,000,000,000 = 1 GB	1,073,741,824 = 1 GiB
tera	1,000,000,000,000 = 1 TB	1,099,511,627,776 = 1 TiB

Drive manufacturers use the decimal column, because that is what the prefixes mean everywhere else in science. Windows uses the binary column — but labels it GB rather than GiB, which is the entire source of the confusion.

A 1 TB drive holds 1,000,000,000,000 bytes. Divide that by 1,073,741,824 and you get 931.3. Windows writes "931 GB". It means 931 GiB. Both are correct; only one of them is on the box.

Convert either way with the GB to TB converter, or the MB to GB converter for smaller figures.

The gap grows at every step

The discrepancy is 2.4% per prefix, and it compounds:

Advertised	Shown in Windows	Apparent shortfall
500 GB	465 GB	7%
1 TB	931 GB	7%
2 TB	1.82 TB	9%
4 TB	3.64 TB	9%

Which is why nobody notices on a USB stick and everyone notices on a large drive.

macOS and Linux agree with the box

Apple changed macOS to report decimal gigabytes in 2009, and most Linux file managers do the same. So the identical drive reads 1 TB on a Mac and 931 GB on a Windows machine, with no difference whatsoever in what it stores.

If you have ever moved a drive between the two and thought the capacity changed, this is why.

Why does Windows do it?

History. Early computers addressed memory in binary, so 1,024 was the natural "round" number and engineers borrowed the metric prefixes loosely. That was harmless while kilobytes were the unit — a 2.4% error nobody could see.

The IEC defined unambiguous binary prefixes in 1998 — kibi, mebi, gibi, tebi — specifically to end this. They are used correctly in some Linux tooling and almost nowhere else. Windows kept the old labels, and changing them now would make every existing file size appear to shift.

Where the space really goes on a new machine

The unit confusion is only part of what you lose:

• The filesystem itself takes a small amount for its index and journal
• Windows and preinstalled software occupy 20–30 GB before you do anything
• The recovery partition is typically another 10–20 GB
• Hibernation and page files can be as large as your RAM

A laptop advertised with a 512 GB SSD commonly shows around 430 GiB of which perhaps 380 is free on first boot. The units explain the first gap; the operating system explains the second.

Phones work the same way

A 128 GB phone holds 128,000,000,000 bytes, which is 119 GiB. After the operating system and preinstalled apps, perhaps 105 GiB is yours. The storage was never 128 GiB and was never advertised as such.

RAM is the exception

Memory genuinely is sold in binary multiples. An 8 GB module really is 8 GiB — 8,589,934,592 bytes — because memory is physically addressed in binary and the chips double at each step.

That is why RAM comes in 8, 16 and 32 while drives come in 500, 1,000 and 2,000. The two follow different conventions for a real reason, and it is the one case where Windows' labelling happens to be right.

The short version

Nothing is missing. Your drive holds every byte you paid for, and your operating system is dividing by 1,024 while the box divided by 1,000. If the number bothers you, a Mac will show you the one you expected.