An AUP3 file is Audacity's project format, not an audio file. The manual states the consequence in one sentence: "A saved Audacity Project can be opened and used only by Audacity. It is not readable or playable by any other application."
So when your media player, your DAW, or the person you emailed it to cannot open the file, nothing is broken and no codec is missing. You saved a project where you needed an audio file. This guide covers what each one holds, what the project quietly leaves behind, and the exact route out.
What is actually inside an AUP3
The manual's Audacity Projects page defines it plainly: "All editing in Audacity is done in a project. A project is stored in a file with an aup3 extension, for example my-project.aup3."
A project is the editing session — tracks, clips, and the state of your work — rather than a rendered result. The FAQ page describes it by purpose: "An AUP3 file can only be saved and used in Audacity. It lets you resume working on that audio in Audacity at a later time if you need to."
Two properties of that file matter more than the rest.
The audio inside stays lossless. Verbatim: "Audio data is always preserved in lossless quality. This is useful if you have already exported to a lossy audio format like MP3 but decide to edit the project further." That is the real argument for keeping the project around after you have shipped the MP3.
The Undo history is not part of it. This one surprises people, and the manual flags it deliberately: "Note carefully that the Undo history is not saved with the project and so the project history starts afresh when you re-open the project later." Save, close, reopen tomorrow, and you cannot walk back yesterday's edits. Anything you might want to revert has to be decided before you close the project, not after.
The route out is Export, not Save
Saving produces the project. Getting a file other software can read is a different command, and the manual names it: "To make an audio file (such as WAV or MP3) for playing on your music player, or for use in other applications, use File > Export Audio."
Which format depends on what the file is for:
- A safety copy or a master. The manual is emphatic here: "If you have just made a recording it is strongly recommended to File > Export Audio it immediately to WAV or AIFF (ideally to an external drive) as a safety copy before you start editing the project." At Audacity's default settings the manual's own comparison table puts 16-bit PCM WAV at 10.0 MB per minute in stereo, 5.0 mono — and note that the table's unit is megabytes per minute.
- A smaller lossless file. FLAC, per the manual, produces files "about half as large as the uncompressed WAV or AIFF formats, but larger than compressed, lossy formats" — the same table lists 5.0 MB per minute stereo.
- Something to hand around. MP3 is described as "a popular compressed, lossy format producing much smaller files than WAV or AIFF, at the expense of some loss of quality."
If you have already exported a WAV master and only need a delivery copy, converting it directly with WAV to MP3 avoids reopening the project at all. The manual's rule for production order is worth quoting in full: "never do production work in MP3, instead work with a lossless format such as WAV, AIFF or FLAC and for final use then export to MP3."
If the material you want in the project lives inside a video file, that import needs the optional FFmpeg library — the manual lists MP4 and M4A among the formats it unlocks. Pulling the audio out beforehand sidesteps that dependency entirely:
Try it right here
Runs in your browser. Your file is not uploaded anywhere.
This runs in your browser tab; the file is decoded locally and is not uploaded anywhere. It gives back a lossless track that Audacity's built-in importer reads without any extra library.
What your export will not carry over from the project
An export is a render of what is audible, not a copy of the project. Four documented differences account for most "my export does not match my project" reports:
- Muted tracks are dropped. "When using Export Audacity will only export tracks that are not grayed-out by use of the Mute or Solo buttons in the tracks' Track Control Panels." Choosing Current Selection instead behaves differently: it "will Export the selection from all selected tracks even if some if those tracks are grayed-out".
- Hidden clip audio is dropped. "Also note that any hidden parts of audio clips are not exported."
- Mono projects still come out stereo. The Stereo channel setting is "the default setting even if you only have mono tracks for exporting" — which doubles the file size for nothing.
- Lyrics and album art are discarded. "For all formats, Audacity currently discards any existing lyrics or album art when exporting." Other tags survive; those two do not. More on that in why Audacity metadata goes missing.
There is also leading silence. With "Trim blank space before first clip" unchecked, which is the default, "any leading blank space offset in the tracks or project to be exported is treated as silence and appropriate leading silence will pad the exported audio."
When Audacity itself refuses to open your AUP3
Same symptom, different cause — and here the fixes are specific.
A newer project on an older Audacity. Compatibility runs one way only: "While projects created and Saved with Audacity 2.x.x or 1.3.x can be opened in Audacity 3.x.x the same is not true the other way around." Once saved as AUP3, the project "can no longer be opened in earlier versions of Audacity", and the Projects page puts a version on it: "A project in the new AUP3 format cannot be opened in versions of Audacity prior to 3.0.0."
The manual's workaround is to move the audio, not the project: "Export as WAV files, Exporting individual tracks, using File > Export Audio choosing Split in parts based on Tracks", then import those into the older version. If a project came from a newer build than yours, the error page says: "You will need to upgrade to the latest version of Audacity in order to open this project file."
The project is on a FAT or FAT32 drive. "Projects will not open from a FAT/FAT32 formatted dive, instead you must copy your project (AUP and its associated _Data folder) to an NTFS formatted drive (APFS on macOS), normally your system drive, and open the project from there." The background on that limit is in Audacity's unsuitable drive error.
You dragged it in. "Note that you cannot use drag and drop an AUP3 project into an open Audacity Project window. To open Audacity projects - use File > Open to open projects." Dragging works for audio files; projects need the menu.
Before any risky edit, the manual's copy method is File > Save Project > Backup Project saved to a new name — described as "the best way to make a copy of an already saved project".
FAQ
Can VLC or Windows Media Player play an .aup3 file? No. The manual says a saved project "is not readable or playable by any other application". Export to WAV, FLAC, or MP3 and play that instead.
How do I convert an AUP3 to MP3?
You do not convert it — you export it. Open the project in Audacity, then File > Export Audio and choose MP3 from the Format dropdown. The AUP3 stays as your editable copy.
Why can't my older Audacity open a project I just saved? Because AUP3 compatibility is one-directional. Projects from 2.x.x and 1.3.x open in 3.x.x, but not the reverse, and AUP3 "cannot be opened in versions of Audacity prior to 3.0.0". Export the tracks as WAV and import those instead.
Where did my undo history go after reopening the project? It was not saved. Per the manual, "the Undo history is not saved with the project and so the project history starts afresh when you re-open the project later." Make revert decisions before you close.
Can I send an AUP3 to someone who doesn't have Audacity? They will not be able to open it — only Audacity can. Send an exported audio file, and keep the project for yourself. For browser-based conversions that do not require installing anything, see all Clapr tools.
Is an exported WAV as good as the project audio? The project preserves audio "in lossless quality", and WAV is uncompressed, so a WAV export carries the audio without generational loss. The project still holds your track structure, which no audio file can.
All quotes taken from the Audacity Manual's Audacity Projects, FAQ, Export Audio Dialog and error pages, checked 16 September 2026. The manual does not state a size ceiling for AUP3 files themselves — check the official documentation if you need one. More walkthroughs in our guides hub, and for the related export ceiling see the 4GB WAV limit.
