Why your 1 TB drive shows 931 GB
Nobody cheated you and no space is missing. The gap comes from the manufacturer and your operating system using two different definitions of the word "giga", and it widens as sizes grow.
You buy a 1 TB drive. You plug it in, open its properties, and it says 931 GB.
Sixty-nine gigabytes short. It feels like somebody took a slice along the way.
Nobody did. The drive holds exactly what it says. The word “giga” just means two different things depending on who’s using it, and nobody told you.
Two ways of counting
The metric system defines prefixes in powers of ten. Kilo is a thousand, mega a million, giga a billion. That’s how it works for metres, grams and watts.
But a computer doesn’t count in tens. It counts in bits, and each bit doubles the possibilities: 2, 4, 8, 16, 32… A computer’s round numbers are powers of two, not ten.
And as it happens, 2¹⁰ = 1,024, very close to 1,000. Close enough that in the 1960s it seemed reasonable to call 1,024 bytes a “kilobyte”.
That convenient approximation is what compounds:
| Unit | Decimal (manufacturer) | Binary (operating system) | Gap |
|---|---|---|---|
| Kilo | 1,000 | 1,024 | 2.4 % |
| Mega | 1,000,000 | 1,048,576 | 4.9 % |
| Giga | 1,000,000,000 | 1,073,741,824 | 7.4 % |
| Tera | 10¹² | 1,099,511,627,776 | 10.0 % |
| Peta | 10¹⁵ | 2⁵⁰ | 12.6 % |
Look at the last column. The gap grows at every step. With kilobytes it was 2 % and nobody noticed. With terabytes it’s already 10 %, and on large drives that becomes hundreds of “missing” gigabytes.
Where 931 comes from
The manufacturer sells 1 decimal TB:
1,000,000,000,000 bytesWindows divides by 1,073,741,824 to display “GB”:
1,000,000,000,000 ÷ 1,073,741,824 = 931.32And writes 931 GB.
The bytes are exactly the same. Only the ruler changed.
Who uses which
Decimal (powers of 10): hard drive and SSD manufacturers, telecom operators, memory card makers, macOS since 2009, iOS, Android and most modern Linux systems.
Binary (powers of 2): Windows, RAM (always, without exception), and disk sector sizes.
The curious consequence is that the same drive shows different sizes depending on the computer you plug it into. On a Mac it reads 1 TB; on Windows, 931 GB. Neither is lying.
A fix exists, and almost nobody uses it
In 1998 the International Electrotechnical Commission created prefixes specifically for powers of two:
| Name | Symbol | Value |
|---|---|---|
| Kibibyte | KiB | 1,024 bytes |
| Mebibyte | MiB | 1,024 KiB |
| Gibibyte | GiB | 1,024 MiB |
| Tebibyte | TiB | 1,024 GiB |
With that vocabulary the correct sentence would be: “a 1 TB drive holds 931 GiB”, and no confusion would be possible.
The problem is they sound awkward and nobody adopted them outside technical circles. Linux uses them, some tools do, and the rest of the world keeps saying “giga” for both.
Why a bit more is still missing
Beyond the units question, there’s space you genuinely can’t use:
The filesystem takes room. Formatting creates the structures that track where everything is: allocation tables, journals, indexes. On a large drive that can be a few gigabytes.
Reserved space. SSDs keep a hidden percentage — over-provisioning — to replace worn cells and to work internally. It’s what makes the drive last years, so it’s space well spent.
System partitions. A new laptop ships with recovery and boot partitions that take space and don’t show as available.
All of this adds to the unit difference, which is why a brand-new 1 TB drive usually shows a bit under 931 GB free.
Where else you’ll meet this
Your internet connection. There’s a second, worse trap here: providers advertise megabits per second (Mbps) while download tools show megabytes per second (MB/s). A byte is 8 bits, so a 300 Mbps connection downloads at about 37 MB/s at best. It isn’t a fault: they’re different units with nearly identical names, and the difference is eightfold.
Memory cards and USB sticks. Same story as drives: sold in decimal.
RAM. This is the interesting exception: RAM is manufactured and sold always in binary. An 8 GB module genuinely holds 8,589,934,592 bytes. That’s why RAM never “loses” space.
The short rule
To know how much space you’ll actually see in Windows, multiply by 0.93 for terabytes and 0.95 for gigabytes:
- 500 GB drive → about 465 GB
- 1 TB drive → about 931 GB
- 2 TB drive → about 1.81 TB
- 4 TB drive → about 3.63 TB
And for internet speed, divide megabits by 8 to get the megabytes per second you’ll see downloading a file.
Nothing is missing. There are just two ways to count, and marketing picked the one with the bigger number.