Binary vs decimal: two ways to count bytes
The same file can be described with two different systems, and both are correct. In the binary system, each step multiplies by 1024 (which is 2 to the 10th power): 1 kibibyte is 1024 bytes, 1 mebibyte is 1024 kibibytes, and so on. In the decimal system, each step multiplies by 1000: 1 kilobyte is 1000 bytes. Computing was born using powers of 2 because memory is organized that way, but the metric prefix "kilo" always meant 1000 — hence the historical confusion.
Why 1 GB is not equal to 1 GiB
The difference looks small at first but it compounds. At the KB/KiB level it is just 24 bytes, but at each level the gap multiplies. By the time you reach gigabytes, 1 GiB (1,073,741,824 bytes) is about 7.4% larger than 1 GB (1,000,000,000 bytes). At terabytes the gap is around 10%. That is why it pays to know which system each tool uses before comparing figures.
The real case: disk marketing vs what the OS reports
You buy a "1 TB" drive. The maker calculates that number in decimal: 1,000,000,000,000 bytes. But when you plug it in, Windows shows about 931 GB, because internally it divides by 1024 at each level (it shows GiB but labels them GB). You did not lose 69 GB: it is exactly the same number of bytes, measured with a different ruler. macOS, on the other hand, adopted the decimal convention back in 2009, so there a 1 TB drive does show ~1 TB.
Where each system is used
- Binary (KiB, MiB, GiB): RAM, block sizes, most operating systems when reporting files, and developer tools.
- Decimal (KB, MB, GB): drive and flash-storage makers, network speeds (megabits), and cloud storage plans.
- Bandwidth: careful — it is measured in bits, not bytes. 100 Mbps is about 12.5 MB/s (divide by 8).
How to read the prefixes correctly
The IEC standardized binary prefixes in 1998 precisely to end the ambiguity: kibi (Ki), mebi (Mi), gibi (Gi), tebi (Ti), pebi (Pi). When you see the "i" in the middle (KiB, MiB), you know for sure it is base 1024. When you see plain KB or MB, it depends on context: on a drive it is almost always decimal, in your file explorer it is almost always binary. This tool lets you pick the mode and labels the units accordingly, so there is no room for doubt.
Reference: bytes per unit in each system
| Unit | Bytes (binary) | Bytes (decimal) |
|---|---|---|
| 1 KB / KiB | 1,024 | 1,000 |
| 1 MB / MiB | 1,048,576 | 1,000,000 |
| 1 GB / GiB | 1,073,741,824 | 1,000,000,000 |
| 1 TB / TiB | 1,099,511,627,776 | 1,000,000,000,000 |
| 1 PB / PiB | 1,125,899,906,842,624 | 1,000,000,000,000,000 |