In telecommunications, bit rate or data transfer rate is the average number of bits, characters, or blocks per unit time passing between equipment in a data transmission system. This is typically measured in multiples of the units bit per second or byte per second.

'k' vs 'Ki'

'k' and 'Ki' stand for 'kilo' and kibi respectively. They are prefixes to units where 'k' stands for 1,000 and 'Ki' stands for 1,024, because 'Ki' comes from its use in computing where 2¹⁰ = 1,024. Unfortunately, 'K' is often incorrectly used instead of 'Ki'. Furthermore, the broad public not being necessarily aware of this subtle difference, usually uses 'Kbps' and 'Kibps' indiscriminately, creating confusion. Whenever 'Kibps' is used, it is usually accurate.

'b' vs 'B'

'b' stands for 'bit' and 'B' stands for 'byte', where one byte refers to 8 bits. This can lead to confusion, as when a "1 Meg" connection is advertised, it usually means 1 Mib/s (mebibit per second) or 1.049 Mb/s (megabit per second), meaning the maximum achievable download speed is actually about 128 KiB/s (kibibyte per second) or 131 kB/s (kilobyte per second).

Example usage

If the data rate of a data-stream is 8,192 bits per second, then using the different capitalization of letters this would be as follows:

8192 / 1000 = 8.192 kb/s
8192 / 1024 = 8 Kib/s
8192 / (8 x 1024) = 1 KiB/s
8192 / (8 x 1000) = 1.024 kB/s
8192 / 8 = 1024 B/s

Bytes are typically used in modern systems, but even when 8-bit bytes are used, the number of kbyte/s is not necessarily exactly one eighth the number of kbit/s because the count of bytes might not include framing bits. For example, a 56 kb/s RS-232 serial line transfers only 5.6 kbyte/s — not 7 kbyte/s — when used in the most common configuration (asynchronous, 8 data bits, no parity, one stop bit). It is fairly common to use kbyte/s with the binary meaning (1,024 byte/s) — more so than for kb/s — perhaps because of the close relationship with the common binary usage of kilobyte for measuring file sizes.

Notes about the related unit kibibit per second:

10³ = 1,000 bit/s = 1 kb/s (one kilobit or one thousand bits per second)

2¹⁰ = 1,024 bit/s = 1 Kib/s (one kibibit per second)

kbps is also commonly used for describing bit rates for streaming data such as video.

Related: kilobit/kibibit, kilobyte/kibibyte

kilobit per second

A kilobit per second (kb/s or kbit/s or kbps) is a unit of data transfer rate equal to 1,000 bits per second. It is sometimes thought to mean 1,024 bits per second, using the binary meaning of the kilo- prefix.

Examples

• 56k modem — 56 kb/s - 56,000 b/s
• 64k ISDN — 64 kb/s - 64,000 b/s 1
• 128 kb/s MP3 — 128,000 b/s 2
• 1536k T1 — 1,536,000 b/s (1.536 Mb/s)