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Bitrate refers to the size, over time, of a video or audio stream. Although most computer oriented applications measure bitrate in terms of binary kilobits and Megabits where 1kb (1 kilobit) = 1024 bits and 1Mb (1 Megabit) = 1024 kilobits, bitrate calculations use standard metric values for these prefixes, meaning 1kb = 1000b and 1Mb = 1000kb. Bitrate calculations are typically made in kilobits per second (kbps) or Megabits per second (Mbps). They can also be expressed in other units, including bits per second (bps) or even Megabytes per second (MBps). Notice the capital B denoting Bytes. You should always use a lower case b to denote bits and upper case for Bytes.
The size of any stream (video, audio, or otherwise) can be calculated by multiplying the bitrate its encoded at by its length. When calculating filesize based on bitrate you also have to take into account any overhead introduced by the Container being used, such as AVI or MPEG. Container overhead varies from one to another, and can even be affected by the number or type of streams being muxed.
Bitrate can refer to either a constant number of bits used to Encode each second of video, or the average number per second. While Variable Bitrate encoding is generally preferable for consumer video applications like DVD or MPEG-4, hardware video encoding or formats designed for editing often benefit from being encoded at a Constant Bitrate. In either case the calculations for stream size are the same.
Online Bitrate Calculators
A simple DVD/SVCD/DivX bitrate calculator
Another DivX bitrate calculator
3IVX Bitrate Calculator
Basic DVD Authoring Project Part 5 - Calculate Video Bitrates
Digital Video Fundamentals - Color Formats
Related glossary terms
Related software tools
Bitrate Viewer (Shareware)
A handy little tool that displays the DVD bitrate in a graphical form.
XviD Bitrate Calculator (Freeware)
Nice little bitrate and resolution calculator for XviD videos, which does everything you can ask from a bitrate calculator. Only nag is the installation of the tool, which can get painful.