Data units, from bit to petabyte
The whole system rests on two facts: a byte is 8 bits, and every prefix has a decimal meaning (powers of 1,000) and a binary one (powers of 1,024).
Bits and bytes
A bit is a single 0 or 1. A byte is 8 bits, enough for 256 different values. File sizes and storage are counted in bytes (capital B). Network speeds are counted in bits per second (small b): 100 Mbps is 100 million bits per second, which is 12.5 million bytes per second.
Decimal and binary prefixes
The SI prefixes are exact powers of ten: a kilobyte is 1,000 bytes and a gigabyte 1,000,000,000. In 1998 the IEC added binary prefixes for the powers of two that computers use internally: a kibibyte (KiB) is 1,024 bytes and a gibibyte (GiB) 1,073,741,824. Storage, networking, macOS and iOS use decimal; RAM and Windows use binary, though Windows writes "GB".
| Decimal | Bytes | Binary | 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 % |
| petabyte (PB) | 1,000,000,000,000,000 | pebibyte (PiB) | 1,125,899,906,842,624 | 12.6 % |
Speeds: Mbps vs MB/s
Divide a speed in Mbps by 8 for MB/s. Downloads also lose about 5 % to TCP/IP packet headers, so a 100 Mbps line peaks at about 11.9 MB/s in a download window.
| Plan speed | Top speed in MB/s | After TCP/IP overhead | In MiB/s |
|---|---|---|---|
| 10 Mbps | 1.25 MB/s | 1.19 MB/s | 1.19 MiB/s |
| 25 Mbps | 3.12 MB/s | 2.97 MB/s | 2.98 MiB/s |
| 50 Mbps | 6.25 MB/s | 5.93 MB/s | 5.96 MiB/s |
| 100 Mbps | 12.5 MB/s | 11.9 MB/s | 11.9 MiB/s |
| 300 Mbps | 37.5 MB/s | 35.6 MB/s | 35.8 MiB/s |
| 500 Mbps | 62.5 MB/s | 59.3 MB/s | 59.6 MiB/s |
| 1,000 Mbps | 125 MB/s | 119 MB/s | 119 MiB/s |
| 2,000 Mbps | 250 MB/s | 237 MB/s | 238 MiB/s |