What Is Bit Rate?

Bit rate, sometimes written as bitrate, is how much data a file spends on each second of sound or picture. It is the single number that best explains why one file sounds better than another, and why it is bigger.

Where you find it and where it comes from

Every audio file has one: MP3, AAC, M4A, WAV, FLAC, OGG. So does every video file, which usually has a separate bit rate for the picture and for the sound. The value is set by the encoder at the moment the file is made, which in practice means the export preset in your editing program, the recording app on your phone, the camera, or the service the file was downloaded from. It is not something a person types in.

Reports often show more than one bit rate: one per stream, and one for the file as a whole. The overall figure is the streams added together plus the small overhead the container itself needs.

Units and typical values

Data is measured in bits and time in seconds, so the unit is bits per second, written bit/s or bps. Larger numbers get a prefix: a kilobit per second (kbit/s or kbps) is a thousand bits, a megabit per second (Mbit/s) a million. Watch the case of the letter b, because a lowercase b means bits and an uppercase B means bytes, and there are eight bits in a byte. A 1 Mbit/s stream moves 125 kilobytes of data every second.

• 128 kbit/s and 192 kbit/s - everyday MP3 and AAC music files
• 1411 kbit/s - uncompressed stereo audio at CD resolution
• 5000 kbit/s - a typical figure for the picture stream of a high definition video

A file can hold its rate steady (constant bit rate) or let it rise and fall with the difficulty of the material (variable bit rate). For a variable file the report gives an average, and some variable files declare no bit rate at all, which is normal rather than a fault.

Quality, file size and playback

With a lossy codec such as MP3 or AAC, the bit rate is the budget the encoder had to work with. The more data it can spend per second, the less it has to throw away, so as a rule of thumb the higher the bit rate the better the result. An MP3 at 192 kbit/s sounds considerably better than the same recording at 32 kbit/s. The same holds for the picture in a video: too low a rate and detailed or fast-moving scenes turn blocky and soft. The gains do flatten out at the top end, and for lossless formats like FLAC or the plain PCM audio in a WAV file the rate simply reflects the content, so it is not a quality dial at all.

Bit rate also decides file size, almost arithmetically: rate multiplied by duration. Audio at 192 kbit/s comes to 1.44 megabytes per minute, so an hour of it is around 86 megabytes whatever it contains. That is the trade-off to weigh when a file has to fit an upload limit or stream over a slow connection. The field carries nothing personal about you.