Why Your 1TB Drive Shows 931GB (and How to Convert GB, MB, TB Correctly)
You bought a 1TB drive, plugged it in, and Windows tells you it holds 931GB. Nobody stole 69GB from you. The drive manufacturer counted in units of 1000 and your operating system counts in units of 1024, so the same physical bytes get reported as two different numbers depending on who’s doing the math.
The rule: two different definitions of “kilo”
Storage manufacturers, and the specs on the side of the box, use decimal math because it’s the international standard for metric prefixes: 1 KB is 1000 bytes, 1 MB is 1000 KB, 1 GB is 1000 MB, 1 TB is 1000 GB. Clean powers of ten, the same logic as a kilometer being 1000 meters.
Operating systems do something different. Windows, macOS, and Linux file managers, along with anything describing RAM, use binary math: 1 KB is 1024 bytes, 1 MB is 1024 KB, 1 GB is 1024 MB, 1 TB is 1024 GB. That 1024 comes from computers working in powers of two (2^10), not powers of ten.
There’s actually a correct name for the binary version: KiB, MiB, GiB, TiB (kibibyte, mebibyte, gibibyte, tebibyte). Almost no consumer software uses that terminology, though. When Windows’ file explorer shows you “465 GB free,” it’s really showing GiB and just calling it GB. The label is wrong, but it’s been wrong for so long that everyone’s used to it.
This isn’t a new problem either. It goes back decades, to a point when computer memory was measured in binary because that’s how memory chips are physically addressed, while storage manufacturers, marketers, and later the International Electrotechnical Commission all pushed for decimal because it matches every other metric unit on the planet. Neither side backed down, and consumers ended up stuck in the middle with two different numbers for the same drive.
Worked numeric example
Here’s what actually happens to a drive’s advertised capacity once your OS gets its hands on it. The bytes don’t change, only which unit is used to describe them.
| Advertised capacity (decimal, manufacturer) | Exact bytes | Shown by your OS (binary) |
|---|---|---|
| 500 GB | 500,000,000,000 bytes | 465.66 GB |
| 1 TB | 1,000,000,000,000 bytes | 931.32 GB |
Same drive, same physical bytes, two different numbers on the label versus the screen. A 1TB drive really does contain 1,000,000,000,000 bytes, exactly as promised. Your OS just divides that number by 1024 three times over instead of 1000, and 1,000,000,000,000 ÷ 1024 ÷ 1024 ÷ 1024 comes out to 931.32, not 1000.
The same gap shows up in ordinary unit conversion, not just drive capacity. Converting GB to MB gives a different answer depending on which mode you’re using:
| Conversion | Binary (1024) result | Decimal (1000) result |
|---|---|---|
| 1 GB → MB | 1024 MB | 1000 MB |
| 4.5 GB → MB | 4608 MB | 4500 MB |
That second row is the practical one. Say a video file’s properties say 4.5GB. Your file manager is almost certainly using binary math to get that number, which means the file is 4608 MB, not 4500 MB. If you’d used decimal math instead (the way a spec sheet might), you’d land on 4500 MB and be off by 108 MB when you go to check it against the actual file size on disk.
Convert your own values
Plug in a value and unit below, switch between binary and decimal mode, and see the conversion across bytes through petabytes instantly. Useful for checking a drive’s real capacity, converting a file size, or just settling an argument about whether 1000 GB equals 1 TB (answer: only in decimal mode).
Common mistakes and edge cases
Thinking the missing space is a defect or a scam. It isn’t. The physical bytes are all present and correctly formatted; the “missing” gigabytes are purely a difference in how two systems label the same count of bytes. Nobody shipped you a smaller drive.
Mixing up Mb and MB. ISPs advertise internet speed in megabits (lowercase b), while file sizes are measured in megabytes (uppercase B). There are 8 bits in a byte, so a “100 Mbps” internet plan tops out around 12.5 MB/s, not 100 MB/s. If your download speed looks eight times slower than what you’re paying for, this is almost certainly why.
Assuming KB always means 1024 bytes. Under the strict decimal standard, KB means exactly 1000 bytes; KiB is the term reserved for 1024. Most consumer software’s “KB” is really KiB wearing a different name, which is convenient until you need to do exact math and don’t know which one you’re looking at.
Not checking which mode a spec sheet uses before doing mental math. A cloud storage plan, a drive spec, and your file manager’s properties window can each be quoting a different convention. Mixing binary and decimal figures in the same comparison, without converting them to a common base first, produces a number that looks precise but is quietly wrong.
Frequently asked questions
Why does my hard drive show less space than advertised? Because manufacturers calculate capacity in decimal (1 GB = 1000 MB), while your operating system displays it in binary (1 GB = 1024 MB). A 1TB drive contains exactly 1,000,000,000,000 bytes as promised; your OS just divides that figure by 1024 repeatedly instead of 1000, which is why it reports 931.32 GB. No storage is missing.
What’s the difference between KB and KiB? KB, under the formal decimal standard, means 1000 bytes. KiB (kibibyte) is the term that specifically means 1024 bytes. Most everyday software labels things “KB” or “MB” even when it’s actually using the 1024-based binary math, so in practice “KB” in your file manager usually means KiB in disguise.
Which mode should I use, binary or decimal? Use binary (1024) when you want to match what your operating system’s file manager or properties window shows you, since that’s the convention Windows, macOS, and Linux use internally. Use decimal (1000) when comparing against a manufacturer’s spec sheet, a cloud storage plan, or an internet speed advertisement, since those are quoted in decimal by convention.
How many MB are in a GB? It depends on the mode. In binary, 1 GB equals 1024 MB. In decimal, 1 GB equals 1000 MB. A 4.5 GB file, for example, is 4608 MB in binary (what your file manager will show) or 4500 MB in decimal.
Why doesn’t my “100 Mbps” internet download at 100 MB/s? Because Mbps measures megabits and MB/s measures megabytes, and a byte is 8 bits. Divide the advertised megabit figure by 8 to get the megabyte-per-second speed you’ll actually see in a download manager: 100 Mbps works out to 12.5 MB/s at best.
Is 1000 GB the same as 1 TB? Only if you’re working in decimal mode, where 1 TB is defined as exactly 1000 GB. In binary mode, 1 TB equals 1024 GB, so 1000 GB would actually be slightly less than 1 TB. Which one is “correct” depends entirely on which convention the number you’re comparing against is using.