Anyone know how to check file size BEFORE exporting?

Using 2.1.2 on Win 10. Just wondering if there is any way to estimate what your file size may be (after adjusting quality perimeters under Preferences) BEFORE exporting. Purpose is to get a mp3 track down below a certain size (1 MB) for sending by text message, and hate to export many, many times by trial and error, adjusting the quality settings infinite times before getting it right. Thanks

The size of an audio file depends on the number of “bits” per second, and the length of the audio:
Note that the number of bits per second is usually quoted as kbps.
Note also that the file also has “header” information and may have metadata, which will make the file a little bigger than the estimate.

10 minutes at 128 kbps
10 minutes = 600 seconds
128 kbps = 128 / 8 = 16 kB per second
Estimated file size = 600 x 16 = 9600 kB = 9.6 MB

For variable bit-rate (VBR) MP3s, the calculation will be approximate because the encoder will tweak the bit rate depending on the complexity of the audio. In Audacity, VBR settings (and the “Preset” settings) show the typical range for the desired sound quality. For example, “Standard” quality (which is pretty good quality for music) will typically have a bit rate of 170 to 210 kbps. I don’t think the actual bit-rate ever goes over the top end of the range, but in some cases, where the audio is ‘easy’ to compress, the bit-rate may be less than the quoted range.

By comparison, a standard CD quality WAV file (16-bit, 44100 Hz, stereo), is approximately 10 MB per minute.

Forgot to say:

With “constant bit rate” (CBR) encoded MP3. the bit rate is “per second”, so the number of channels (mono / stereo) is irrelevant in terms of file size. However, stereo audio is more ‘complex’ than mono. so, for example 64 kbps mono will usually sound higher quality than 64 kbps stereo.

On the other hand, “variable bit rate” (VBR) encoded MP3 aims to produce a specified sound quality. Because mono is less ‘complex’ than stereo, mono recordings will usually be smaller than stereo for the specified sound quality. For example, for “Standard (170 - 210)” VBR, a 10 minute stereo music file might be around 15 MB (600 x 210 / 8 = 15750), whereas a mono version may be around 8 MB, which works out as around 110 kbps (because there is only one channel).

In case it isn’t clear, the division by 8 is because file sizes like kB and MB are measurements in bytes, and there are 8 bits in a byte. See Bits and Bytes.