Audio Bitrate and File Size: How Big Will It Be?

The size of a sound file is not a mystery. For the common lossy formats it is the bitrate times the length, and for uncompressed audio it follows from three numbers. Knowing the rule lets you choose a setting before you convert.

The rule

A bitrate is the number of bits the file uses for each second of sound. Multiply it by the seconds and divide by 8 to get bytes:

size in bytes = bitrate in bits per second × seconds ÷ 8

A 3-minute song at 192 kbps is 192,000 × 180 ÷ 8 = 4,320,000 bytes, about 4.3 MB.

Size per minute

Bitrate	About one minute	Typical use
64 kbps	0.48 MB	Speech, voice notes
128 kbps	0.96 MB	Everyday listening
192 kbps	1.44 MB	Most music
320 kbps	2.4 MB	The highest setting of MP3

MDN says that the higher the bitrate, the higher the quality can be and the larger the file is likely to be. For MP3 downloads it advises a bitrate of at least 192 kbps if possible, and it gives 32 to 320 kbps as the range of the MP3 standard’s MPEG-1 mode.

Uncompressed audio

WAV in CD quality stores 44,100 samples a second, 16 bits each, for 2 channels. That is 44,100 × 16 × 2 = 1,411,200 bits a second, about 10.6 MB a minute, whatever the music. Because WAV is not compressed, a mono recording takes half as much.

Lossy and lossless

MP3, AAC (the M4A format) and Opus (in OGG files) are lossy: they throw away sound to make the file small, and the higher the compression, the more the loss. MDN says lossy codecs can compress by 80 to 95 percent. FLAC and WAV lose nothing. MDN says lossless codecs typically compress less, by up to 40 to 50 percent in one of its figures, and that decoded lossless audio is identical to the original down to the bit.

Two consequences follow. Converting an MP3 to WAV or FLAC does not bring back what the MP3 lost; it only stores the lossy sound without further loss. And converting a lossy file to another lossy format discards a little more, so keep the original if you can.

Does mono make the file smaller?

It depends on the format. At a set MP3 bitrate the size follows the bitrate and the length, so making the sound mono does not change the size: the low bitrate does. In an uncompressed WAV, mono really does halve the file.

A checklist

1. Speech: 64 to 96 kbps is usually enough.
2. Music for everyday listening: 128 to 192 kbps.
3. Anything you will edit later: keep a WAV or FLAC.
4. Cut the part you need before converting; the size follows the length that is kept.
5. Compare the size shown for the result with the estimate above.

The audio converter shows the length and the format of your file, lets you choose the bitrate, and converts in your browser without uploading the file.