Why a 1 TB drive shows only 931 GB — and other storage numbers explained

You buy a 1 TB drive, plug it in, and your computer says 931 GB. The missing 69 GB was never there to lose — the box and the computer are simply using different units under the same name.

Two ways to count bytes

• Decimal units — kB, MB, GB, TB — go up in steps of 1,000. Drive and phone makers use these: 1 TB is 1,000,000,000,000 bytes.
• Binary units — KiB, MiB, GiB, TiB — go up in steps of 1,024. This is how computers actually measure memory and storage.

Windows adds to the confusion by measuring in binary units but labelling them with decimal names. What it calls “931 GB” is really 931 GiB.

The maths

1,000,000,000,000 bytes ÷ 1,024 ÷ 1,024 ÷ 1,024 = 931.32 GiB. Same drive, same number of bytes, a bigger unit.

The gap grows with the size of the unit:

The right-hand column is what Windows reports, except that it labels the figures GB and TB and shows fewer digits. A kilobyte and a kibibyte differ by 2.4%, a megabyte and a mebibyte by 4.9%, a gigabyte and a gibibyte by 7.4%, and a terabyte and a tebibyte by nearly 10%. On a new phone or laptop the operating system and pre-installed apps take a further share, so the free space you see is lower still.

Why are there two systems at all?

Computers work in powers of two, and 2¹⁰ happens to be 1,024 — close enough to 1,000 that early engineers borrowed the prefix “kilo” for it. When memory was measured in kilobytes, the 2.4% difference did not matter. As sizes grew, the gap grew with them. In 1998 the International Electrotechnical Commission introduced the separate names kibibyte, mebibyte and gibibyte so that kilo, mega and giga could go back to meaning exactly a thousand, a million and a billion. The new names are correct, but they never fully caught on in everyday use, which is why both meanings are still in circulation.

Which system shows which

• Windows measures in binary units and labels them GB and TB. This is where the 931 figure comes from.
• macOS has used decimal units since 2009, so a 1 TB drive is shown as about 1 TB.
• Drive, phone and memory-card packaging uses decimal units.
• RAM is the exception among hardware: memory is sold in binary units, so 16 GB of RAM really is 16 GiB.
• Linux tools vary, and many let you choose. Where you see KiB, MiB or GiB written out, the figure is binary.

So the same drive can honestly be described as 1 TB on a Mac and 931 GB on a Windows PC sitting beside it.

Where the rest of the space goes

The unit difference explains most of the gap, but not always all of it. A little space is used by the file system itself, which keeps the index of where every file lives. Computers and external drives may also ship with a hidden recovery partition or bundled software. And every file occupies whole blocks on the drive, so thousands of tiny files take up more room than their sizes add up to — which is why Windows shows both “size” and “size on disk” in a file's properties.

The ladder: MB, GB, TB, PB

In decimal units each step is 1,000 times the one before: 1 GB = 1,000 MB, 1 TB = 1,000 GB, 1 PB = 1,000 TB. In binary units each step is 1,024: 1 GiB = 1,024 MiB, 1 TiB = 1,024 GiB. To turn bytes into megabytes, divide by 1,000,000 (MB) or by 1,048,576 (MiB).

What a terabyte actually holds

Rough figures help when choosing a drive. At about 4 MB each, 1 TB holds around 250,000 phone photos. At about 4 GB each, it holds around 250 feature films in high definition. Video recorded on a recent phone at its highest resolution can use a few hundred megabytes a minute, which is what actually fills phones and drives. Work out your own number from the sizes of the files you really keep — and leave headroom, because drives slow down and become awkward to manage when they are nearly full.

Mbps is not MB/s

Internet speeds are quoted in megabits per second (Mbps); download progress is usually shown in megabytes per second (MB/s). A byte is eight bits, so a 100 Mbps connection downloads at most 12.5 MB/s — before any overhead.

The capital letter is the clue: a lower-case b means bits, a capital B means bytes. To estimate a download, convert the speed to megabytes per second and divide. A 50 GB game is 50,000 MB; at 12.5 MB/s that is 4,000 seconds, or about 67 minutes on a 100 Mbps line running at full speed.

The Data Storage Converter keeps decimal and binary units side by side, and includes bits, so you can check any of these numbers for yourself.