MB vs MiB: Why Your 1 TB Drive Shows 931 GB
Nothing Is Missing From Your Drive
You buy a 1 TB drive, plug it into a Windows PC, and it reports 931 GB. No space has gone missing. The manufacturer and the operating system are counting with two different definitions of "giga" and "tera." Once you know which is which, the numbers line up exactly.
Decimal (SI) vs Binary (IEC) Prefixes
In the International System of Units, kilo means 1,000, mega 1,000,000 and so on. Computers work in powers of two, and early on "kilobyte" was borrowed for 1,024 bytes (2^10) because it was close. As sizes grew, the gap grew too. In 1998 the International Electrotechnical Commission introduced separate binary prefixes (kibi, mebi, gibi, tebi) to end the ambiguity, a system NIST also documents.
| Decimal (SI) | Bytes | Binary (IEC) | Bytes | Binary is bigger by |
|---|---|---|---|---|
| kilobyte (kB) | 1,000 | kibibyte (KiB) | 1,024 | 2.4% |
| megabyte (MB) | 1,000,000 | mebibyte (MiB) | 1,048,576 | 4.9% |
| gigabyte (GB) | 1,000,000,000 | gibibyte (GiB) | 1,073,741,824 | 7.4% |
| terabyte (TB) | 1,000,000,000,000 | tebibyte (TiB) | 1,099,511,627,776 | 10.0% |
The gap grows with each step, which is why it is barely noticeable on a file and very noticeable on a drive.
Worked Example: 1 TB = 931.32 GiB
- The box says 1 TB, which in decimal is 1,000,000,000,000 bytes.
- One GiB is 1,073,741,824 bytes.
- 1,000,000,000,000 ÷ 1,073,741,824 = 931.32 GiB.
- In TiB: 1,000,000,000,000 ÷ 1,099,511,627,776 = 0.909 TiB.
The same math for common sizes:
| Advertised | Bytes | Shown in binary units |
|---|---|---|
| 128 GB | 128,000,000,000 | 119.2 GiB |
| 256 GB | 256,000,000,000 | 238.4 GiB |
| 512 GB | 512,000,000,000 | 476.8 GiB |
| 2 TB | 2,000,000,000,000 | 1,862.6 GiB (1.82 TiB) |
| 4 TB | 4,000,000,000,000 | 3,725.3 GiB (3.64 TiB) |
A freshly formatted drive will show slightly less again, because the file system reserves some space for its own structures, and a computer's main drive also holds the operating system and recovery partitions.
Which Systems Use Which
| Where | Convention | What it shows for a 1 TB drive |
|---|---|---|
| Drive and SSD manufacturers, USB sticks, memory cards | Decimal | 1 TB |
| Windows File Explorer | Binary, but labeled KB, MB, GB, TB | 931 GB |
| macOS (since Mac OS X 10.6 Snow Leopard) and iOS | Decimal, per Apple | 1 TB |
| Linux desktop file managers such as GNOME Files | Usually decimal | 1.0 TB |
Linux command line: df -h, du -h, ls -lh | Binary, with short suffixes K, M, G, T | about 931G |
Linux command line: df -H, du --si | Decimal | 1.0T |
| RAM | Binary in practice | A "16 GB" memory module holds 16 GiB |
So the same drive reads 1 TB on a Mac and 931 GB on a Windows PC. Both are correct; only the unit differs.
Bits vs Bytes: Internet Speeds and Download Time
Internet plans are sold in megabits per second (Mbps), with a lowercase b. Files are measured in bytes, uppercase B. One byte is 8 bits, and network speeds use decimal prefixes. So:
- 100 Mbps ÷ 8 = 12.5 MB per second
- 1 Gbps (1,000 Mbps) ÷ 8 = 125 MB per second
Download time (seconds) = file size in MB × 8 ÷ speed in Mbps
| File | At 50 Mbps | At 100 Mbps | At 500 Mbps |
|---|---|---|---|
| Photo, 5 MB | 0.8 s | 0.4 s | 0.08 s |
| Movie, 4 GB (4,000 MB) | 10 min 40 s | 5 min 20 s | 64 s |
| Game, 80 GB (80,000 MB) | 3 h 33 min | 1 h 47 min | 21 min 20 s |
These are best cases. Protocol overhead, Wi-Fi conditions, and the server's own speed usually make real downloads somewhat slower. If a download tool reports speed in MiB/s, multiply by 8.39 (8 × 1.048576) to compare with your plan's Mbps.
Common Mistakes
- Mixing Mbps and MB/s. "My 300 Mbps connection only downloads at 35 MB/s" is not a problem: 300 ÷ 8 = 37.5 MB/s is the theoretical maximum.
- Assuming a backup will fit exactly. 900 GB of files as reported by Windows is about 966 GB in decimal units. A "1 TB" backup drive leaves less headroom than it seems.
- Cloud and data plans. Providers differ on whether 1 GB means 1,000 or 1,024 MB. On a 100 GB allowance the difference is about 7 GB, so check the provider's help page if you plan close to the limit.
- Phone storage. On an iPhone, which uses decimal units, the gap between "128 GB" and available space is mostly the operating system and system data, not the binary conversion.
- Writing "KB" for 1,024 bytes in specifications. If precision matters, write KiB/MiB/GiB or state the convention once at the top of the document.
Doing It in QuickToolApp
QuickToolApp's Digital Data category uses binary (1,024-based) factors, the same convention as Windows, even though it labels the units KB, MB, GB and TB. That means:
- To see what Windows will show for an advertised size, enter the size in bytes. Open the converter with 1,000,000,000,000 bytes to GB and it returns about 931.32.
- For several drives at once, keep Bytes → GB selected and paste one byte count per line into
Batch Convert, for example
128000000000,512000000000,2000000000000. - Converting "2.5 GB to MB" gives 2,560, the binary answer. In decimal it would be 2,500.
Whichever convention you use, say so next to the numbers, as recommended in the unit conversion best practices guide.
FAQ
Is my drive's capacity a scam?
No. Manufacturers state decimal capacity and usually say so in the fine print (for example "1 GB = 1 billion bytes"). The bytes are all there.
Why does Windows still label binary values as GB?
Historical convention. The labels predate the IEC prefixes and have never been changed in File Explorer.
Which should I use in my own work?
Use decimal units (kB, MB, GB) when communicating with general audiences and for network speeds, and IEC units (KiB, MiB, GiB) when you specifically mean powers of two, such as memory sizes or low-level storage work.