Why Your 1 TB Drive Shows Only 931 GB (and Where the Space Went)
You buy a 1 TB external drive, plug it in, and Windows reports 931 GB. Nothing is broken and nobody stole 69 gigabytes. The manufacturer and your operating system are simply counting with two different rulers — and both of them call their unit a “gigabyte”.
This article explains the two rulers, shows the exact math, and gives you a quick way to check any storage or internet-speed figure yourself.
Two meanings of “kilo”
Everywhere else in science, kilo means 1,000. A kilometer is 1,000 meters, a kilogram is 1,000 grams. Storage manufacturers follow this decimal (SI) convention:
| Unit | Bytes (decimal) |
|---|---|
| 1 kilobyte (kB) | 1,000 |
| 1 megabyte (MB) | 1,000,000 |
| 1 gigabyte (GB) | 1,000,000,000 |
| 1 terabyte (TB) | 1,000,000,000,000 |
Computers, however, address memory in powers of two. Early on, programmers noticed that 2¹⁰ = 1,024 is close enough to 1,000 and started calling 1,024 bytes a “kilobyte”. The habit stuck, and each step up multiplies by 1,024 instead of 1,000.
Because the gap grows with every step, the error that was only 2.4 % at the kilobyte level becomes almost 10 % at the terabyte level:
| Prefix | Decimal value | Binary value | Difference |
|---|---|---|---|
| kilo | 10³ | 2¹⁰ = 1,024 | 2.4 % |
| mega | 10⁶ | 2²⁰ = 1,048,576 | 4.9 % |
| giga | 10⁹ | 2³⁰ = 1,073,741,824 | 7.4 % |
| tera | 10¹² | 2⁴⁰ = 1,099,511,627,776 | 10.0 % |
The fix nobody uses: kibibytes
In 1998 the International Electrotechnical Commission (IEC) introduced separate names for the binary units to end the confusion:
- kibibyte (KiB) = 1,024 bytes
- mebibyte (MiB) = 1,048,576 bytes
- gibibyte (GiB) = 1,073,741,824 bytes
- tebibyte (TiB) = 1,099,511,627,776 bytes
Linux tools and many technical programs use these names. Windows, however, still calculates in binary units while labelling them GB. macOS switched to decimal units in 2009, which is why the same drive shows as 1 TB on a Mac and 931 GB on a PC.
The exact math for a 1 TB drive
The drive really contains 1,000,000,000,000 bytes. Windows divides by 1,073,741,824 (one gibibyte):
1,000,000,000,000 ÷ 1,073,741,824 = 931.32 GiB
That is the number Windows shows, just with “GB” written next to it. You can check any other size the same way, or use the terabyte to gibibyte converter:
| Advertised size | Shown by Windows |
|---|---|
| 128 GB | 119.2 GB |
| 256 GB | 238.4 GB |
| 512 GB | 476.8 GB |
| 1 TB | 931.3 GB |
| 2 TB | 1.82 TB |
| 4 TB | 3.64 TB |
A freshly formatted drive will usually show slightly less still, because the file system reserves some space for its own bookkeeping. Pre-installed recovery partitions on laptops take another bite.
RAM is the exception
Memory chips are built in powers of two, so RAM is sold in binary units. A “16 GB” memory module really holds 16 GiB (17,179,869,184 bytes), and your system will report the full 16 GB — minus any portion reserved for integrated graphics.
Bits versus bytes: the internet-speed trap
A second, even bigger mix-up hides in internet speeds. Network speeds are measured in bits per second, while file sizes are measured in bytes. One byte is 8 bits.
So a “100 Mbps” connection (100 megabits per second) transfers at most:
100 ÷ 8 = 12.5 megabytes per second
A 1 GB file therefore needs at least 80 seconds on that connection, not 10. Real downloads are usually a little slower again because of protocol overhead. The lower-case b in Mbps means bits; the upper-case B in MB/s means bytes. Our megabits per second to megabytes per second converter does the division for you.
| Advertised speed | Best-case download speed |
|---|---|
| 50 Mbps | 6.25 MB/s |
| 100 Mbps | 12.5 MB/s |
| 300 Mbps | 37.5 MB/s |
| 1 Gbps | 125 MB/s |
How to tell which unit you are looking at
A few rules of thumb:
- Drive packaging, SSD spec sheets, and cloud storage plans use decimal units (1 GB = 1,000,000,000 bytes).
- Windows Explorer uses binary units but labels them GB.
- macOS (since 10.6) and iOS use decimal units.
- Linux tools vary: some print
GiB, some printGmeaning binary, and many accept a flag to switch. - Internet plans and Wi-Fi speeds use bits, not bytes.
- RAM uses binary units.
If a number looks about 7 % or 10 % too small, you are almost certainly looking at the same quantity measured with the other ruler.
Key takeaways
- A 1 TB drive holds exactly one trillion bytes; Windows reports it as 931 GB because it divides by 1,024³.
- The correct name for the binary unit is the gibibyte (GiB); the label “GB” is used for both.
- Divide internet speeds in Mbps by 8 to get megabytes per second.
- RAM is the one place where “GB” consistently means the binary unit.
To convert between any of these units — decimal, binary, bits and bytes — use the data storage converter or the data transfer rate converter.