Audacity's own manual calls this a bug. If you pick a bit rate for an M4A (AAC) export and the file comes out at some other rate, you are not misreading the dialog and you have not misconfigured anything — the behaviour is written up on Audacity's own AAC page.
The failure is not uniform, though. There is a range where the setting is ignored and a range where it mostly works — and two official pages describe the same bug in noticeably different words.
What "not honored" actually means
The AAC Export Options page states it directly:
"Please be aware that there is a bug in Audacity whereby the bit rate setting that you make in an AAC export is not always honored by Audacity. See this logged bug for more details."
The same page then splits the behaviour in two, and this is the part worth remembering:
- "For stereo all selected bit rates of 196kbps or less export at 196-198kbps,"
- "Settings in the range 200kbps to 320kbps are reasonably closely honored on export."
So the slider is not inert. Below a point it collapses everything onto roughly one value; above it, it works approximately. If you asked for 128 kbps stereo and got a much larger file than you expected, that first bullet is what happened — and note the qualifier the manual attaches to it: stereo.
Two more numbers from the same page put this in perspective. "The default setting is 256 kbps which is that quality that Apple now uses for audio files sold in iTunes/Apple Music", and "The Maximum bit rate (best quality) is 320 kbps (stereo) 160 kbps (mono) and the minimum is 98 kbps (mono)."
That default of 256 kbps sits inside the band the manual calls reasonably closely honored. In other words, this bug mostly bites people deliberately turning quality down to save space, not people who leave the dialog alone.
One more thing on that page interacts with all of the above: its Channels section offers Mono, Stereo and Custom mapping, while its Quality section says "A stereo file is always produced, but a file exported from a mono track will have about half the bit rate (and hence half the file size) of one exported from a stereo track." Both sentences sit on the same page, and the manual does not reconcile them.
Two official pages, two different descriptions
Here is the detail that explains why the advice you find on this bug contradicts itself.
The Export Formats page describes the same bug without the word "always":
"Please be aware that there is a bug in Audacity whereby the bit rate setting that you make in an AAC export is not honored by Audacity. See this logged bug. The workaround is to export as uncompressed WAV or AIFF and use iTunes to make the AAC conversion, see this page for information regarding this."
Read alone, that sentence says the bit rate control does nothing whatsoever. The AAC page says "not always honored" and then supplies the 196-198kbps detail that shows exactly where it does work. Same bug, two very different impressions — and both pages carry the same version date in their footers, so the pages themselves give you no way to tell which wording is newer.
Practical reading: treat the AAC Export Options page as the authoritative one. It is the dedicated page for that dialog, and it is the one that gives the ranges.
The fix: set the bit rate through Custom FFmpeg Export
Audacity documents a second route to the same encoder, and on this route you type the number yourself. From the Custom FFmpeg Export Options page: "Custom FFmpeg Export provides an alternative way to specify the bit rate of exported M4A (AAC files) rather than using the Quality slider for "M4A (AAC) Files" in the Export Audio dialog."
The official steps, verbatim:
- "Select mp4 in the Formats list."
- "Select libvo_aacenc in the Codecs List."
- "In Bit Rate: in the General Options pane, enter a bit rate up to 320 kbps expressed in bits per second. For example, to export at 280 kbps, enter "280000" (without quotes)."
- "Click the OK button."
- "Back in the Export Audio dialog, enter your desired file name then add "m4a" (without quotes) at the end of the name."
Three caveats the manual attaches to this route. It "is available if you have installed the optional FFmpeg library." The lists you see are not fixed: "The actual formats and codecs available in the lists depend entirely on the options with which your copy of FFmpeg has been compiled" — and if the format you want is missing, "press the Show All Formats button." Step 5 is not optional either: "you must add the extension relevant to the format at the end of your chosen file name."
Audacity also warns what a bad pairing looks like: "some exports might result in zero-byte or invalid files if FFmpeg does not support the combination chosen." A zero-byte output is a codec/container mismatch, not a bit rate problem.
Try it right here
Runs in your browser. Your file is not uploaded anywhere.
If a wrong-bit-rate M4A is all you have left and what you needed was a predictable, widely playable file, converting it to MP3 in your browser takes one step and installs nothing. It runs locally in the tab; the M4A to MP3 converter is the same thing on its own page.
Do this knowing it is a second lossy encode. Where you still have the Audacity project, re-exporting from it is better — the manual's advice is to "work with a lossless format such as WAV, AIFF or FLAC and for final use then export" to the lossy one.
The workaround the manual itself recommends
Both official pages point somewhere else entirely. The AAC page: "The strongly recommended workaround is to export as uncompressed WAV or AIFF and use iTunes/Music, where you have greater control of the settings, to make the AAC conversion."
That means writing an uncompressed file first, so keep the size ceiling in mind: "WAV and AIFF files are limited to a maximum size of 4GB (this is a general restriction and not an Audacity one)." At Audacity's default 16-bit export setting that works out to about 6 hours 45 minutes of stereo audio. If you would rather land on MP3 than AAC at the end of that round trip, WAV to MP3 takes the uncompressed file straight there.
There is a third documented route for people who prefer a command line: the (external program) format choice. Audacity's manual gives this example for the native FFmpeg AAC encoder, noting "Change the value before "k" in the following command to set a different bit rate in kbps":
ffmpeg -i - -strict experimental -c:a aac -b:a 240k "%f"How to tell it worked
Check the resulting file's bit rate in whatever application you play it in, not in Audacity. Two things to keep in mind while you read that number:
- AAC is variable, so expect wobble. The manual describes AAC as "a form of VBR or variable bit rate encoding", and notes that where VBR is used "the achieved size will vary depending on the content."
- The project sample rate moves the result. "Increasing the sample rate in Audio Settings Preferences will increase the bit rate and so the file size (and vice versa)." If your number is still off after a custom export, that is the next setting to look at.
As a rough size check, Audacity's format table lists M4A (AAC) at 1.1 MB per minute for stereo at default settings. The mono cell of that table is blank, so it gives you nothing to compare a mono export against.
FAQ
Is the Audacity AAC bitrate bug officially acknowledged? Yes. Two pages of the official manual describe it in the same words — "a bug in Audacity" — and both point to a logged bug report. Neither page states whether it has been fixed or in which version, so check the official documentation for its current status.
Which AAC bit rates are actually affected? Per the AAC Export Options page: for stereo, "all selected bit rates of 196kbps or less export at 196-198kbps", while "Settings in the range 200kbps to 320kbps are reasonably closely honored on export."
Does this affect Audacity's default AAC setting? The default is 256 kbps, which falls inside the 200–320 kbps band the manual calls reasonably closely honored. The setting most likely to surprise you is a deliberately low one.
Can I set an exact AAC bit rate in Audacity at all?
Through Custom FFmpeg Export, yes: choose mp4 as the format, libvo_aacenc as the codec, and enter the bit rate in bits per second (280000 for 280 kbps), up to 320 kbps. Remember to add the m4a extension to the file name yourself.
Do I need FFmpeg installed for AAC export? Yes — "FFmpeg is required for AAC export but it is not shipped with Audacity." Audacity's manual points to its support site for installation rather than listing the steps, so check the official documentation for the current download and version details.
All quotations are from the Audacity Manual pages linked above. For more export troubleshooting see all fix guides or every Clapr tool; if your problem is AAC on a different editor, DaVinci Resolve's macOS-only AAC encoding is a separate, unrelated restriction.
