1,000 or 1,024
In the metric system kilo means one thousand, and by the international standard that governs these units — IEC 80000-13 — that is what it means here too. One kilobyte is 1,000 bytes. One megabyte is a million, one gigabyte a billion, one terabyte a trillion. The correct SI symbol is lower-case kB, incidentally; KB with a capital K is the near-universal informal spelling and means the same thing.
Computers, though, address memory in powers of two, so 2^10 = 1,024 is the quantity that keeps appearing in practice. Early on it was simply called a kilobyte because it was close enough, and by the time the imprecision started to matter the habit was set. In 1998 the IEC named the binary quantities properly: kibibyte (KiB) = 1,024 bytes, mebibyte (MiB) = 1,048,576, gibibyte (GiB) = 1,073,741,824, tebibyte (TiB) = 2^40.
Adoption has been uneven, which is the practical problem. Windows Explorer divides by 1,024 and labels the result KB, MB and GB. macOS switched to decimal in Snow Leopard, so the same file reads differently on the two systems. Most Linux tooling uses KiB and MiB correctly, and ls -h and df -h default to binary. Storage manufacturers use decimal. Memory (RAM) is genuinely binary — a 16 GB stick really is 16 GiB — because memory is addressed in powers of two in a way that disk capacity is not.
| Unit | Decimal | Binary equivalent | Gap |
|---|---|---|---|
| kB / KiB | 1,000 B | 1,024 B | 2.4% |
| MB / MiB | 10^6 B | 1,048,576 B | 4.9% |
| GB / GiB | 10^9 B | 1,073,741,824 B | 7.4% |
| TB / TiB | 10^12 B | 1,099,511,627,776 B | 10.0% |
The gap compounds by roughly 2.4% per step, which is why nobody argues about kilobytes and everybody notices at the terabyte.
Why a 1 TB drive shows as 931 GB
Work it through. The manufacturer sells 1 TB, meaning exactly 1,000,000,000,000 bytes — that is the number on the box and it is honest. The operating system divides by 1,024 three times: 1,000,000,000,000 ÷ 1,073,741,824 = 931.32 GiB. Windows then labels that "931 GB".
Both figures describe the identical drive. No capacity was lost, no bytes are hiding; one number is a count of bytes in thousands and the other the same count in 1,024s, and the two labels collide on the same three letters. Class action lawsuits have been filed over this and settled on labelling rather than on capacity, because the arithmetic was never in dispute.
Formatting takes a further, real bite on top: the file system's own structures, a recovery partition, and reserved space. On a 1 TB drive that is commonly 10 to 30 GB, so somewhere around 900 to 920 GB usable is normal. That part is genuinely consumed, unlike the 931.
Mbps against MB/s
Network speed is quoted in bits per second, file sizes in bytes, and there are eight bits in a byte. A 100 Mbps connection therefore tops out at 12.5 MB/s, and a gigabit line at 125 MB/s. Lower-case b is bits and upper-case B is bytes — the entire distinction rests on one letter's case, which is why a download that seems eight times slower than advertised usually is not.
Real throughput is lower again. TCP and IP headers, TLS, retransmissions and the gap between packets consume overhead; the server on the other end has its own limits; Wi-Fi contends with distance, interference and every other device on the channel. Sixty to eighty percent of the nominal rate is the usual range, which is why the transfer table shows all three. A 5 GB download over 100 Mbps is 6.7 minutes in theory and typically eight to eleven in practice.
Sizes worth carrying in your head
| Thing | Roughly |
|---|---|
| Phone photo, JPEG | 2 to 5 MB |
| Phone photo, RAW | 20 to 30 MB |
| Song, 320 kbps MP3 | 10 MB |
| An hour of 1080p streaming | 1.5 to 3 GB |
| An hour of 4K streaming | 7 to 15 GB |
| CD | 700 MB (binary — 700 MiB) |
| Single-layer DVD | 4.7 GB (decimal) |
| Single-layer Blu-ray | 25 GB (decimal) |
Note the CD, which is one of the few consumer capacities quoted in binary — 700 MB there means 700 MiB, about 734 million bytes. DVD and Blu-ray figures are decimal. There is no rule; each format simply chose, decades apart, and both choices stuck.
Questions people ask
Is 1 GB 1,000 MB or 1,024 MB?
1,000 MB by the standard. The value that is 1,024 MiB is 1 GiB, a different unit with a different name. In practice, storage vendors and macOS mean 1,000, while Windows and most mobile carriers mean 1,024 while writing "GB". When precision matters — a contract, a data cap, a capacity spec — the byte count is the only unambiguous figure.
My 1 TB drive shows 931 GB. Am I missing 69 GB?
No. 1 TB is 1,000,000,000,000 bytes; divide by 1,024 three times and you get 931.32 GiB, which Windows labels GB. Same drive, same bytes, two naming conventions. What is genuinely consumed is the file system overhead and any recovery partition on top of that, usually 10 to 30 GB, which is why the usable figure tends to land nearer 900 GB.
Why does my 128 GB phone only have about 100 GB free?
Two effects stack. The 128 GB is decimal, which is 119 GiB by the binary count the phone reports. Then the operating system, preinstalled apps and system caches take 10 to 20 GB. Neither is a defect and neither is recoverable.
The transfer time does not match what I actually get.
The line speed is a ceiling, not a delivery. Protocol overhead, the source server, Wi-Fi conditions and other traffic on the connection all subtract from it, which is what the 80 and 60 percent rows are for. Upload is also frequently much slower than download on cable and DSL connections, so a plan advertised as one figure often means that figure in one direction only.