Use AAC by default for music and podcasts — at the same bitrate it sounds better than MP3, and the file is the same size. Switch to MP3 only when the file has to play somewhere that will not touch AAC, such as an older car stereo, a hardware player or an MP3-only upload form. Audio Converter turns AAC and M4A files into MP3 for exactly those cases, on your device.
Bitrate is what sets file size, not the codec. A 256 kbps AAC file and a 256 kbps MP3 file are the same size; the difference is how much of the original sound survives at that size. AAC is the newer codec and does more with the same bits, so it wins on quality at equal bitrate — or matches MP3 quality at a lower bitrate and therefore a smaller file.
When to use AAC
AAC is the right pick for anything you keep and listen to yourself, and for anything you publish to modern devices.
- Music libraries and phone playback. Apple devices, Android, most streaming services and modern browsers all decode AAC natively. It is the default for a reason.
- Podcasts at modest bitrates. Speech at 64–96 kbps AAC is noticeably cleaner than MP3 at the same rate, which matters when you are paying for hosting by the gigabyte.
- Smaller files at matched quality. If you are happy with how MP3 sounds at 192 kbps, AAC can typically reach comparable quality at a lower bitrate — the file gets smaller because the bitrate is lower, not because AAC is magically denser at the same number.
- Files that came from a phone or a video. Voice memos, screen recordings and most video soundtracks are already AAC or M4A. Keeping them as they are avoids a needless re-encode.
AAC's weak spot is age and licensing history. It is well supported now, but plenty of hardware and software from the 2000s and early 2010s still expects MP3 and nothing else.
When to use MP3
MP3 is the compatibility answer, not the quality answer.
- Old hardware. Car head units, dedicated MP3 players, some AV receivers and cheap radios read MP3 and may ignore or refuse an AAC file entirely.
- Forms and platforms that ask for MP3 by name. Some podcast hosts, voice-over marketplaces and sample libraries accept MP3 only.
- Sharing with people whose setup you do not know. MP3 has been decodable by essentially everything for two decades. If you cannot test the destination, MP3 is the safe bet.
- Simple, unencumbered files. MP3 has no licensing baggage left and no container quirks — it is a bare audio stream, which is why it still appears in places AAC does not.
The cost is real: at the same bitrate, MP3 keeps less of the original. If you must use MP3, encode at a higher bitrate than you would for AAC to land at similar quality, and accept the larger file that follows.
Why convert locally in the browser
Most 'free online converter' sites take your file, send it to a server, process it there and hand back a download. That means a copy of your recording — a voice memo, an unreleased track, an interview — sits on someone else's disk, under their retention policy, for however long they choose. Many of those sites also cap file size, add a watermark or queue you behind an ad wall.
Alvseike's audio tools do the work inside the browser tab. The file is opened from your disk, decoded and re-encoded by your own machine, and the result is saved straight back to your disk. Nothing is uploaded, so there is no size cap and no copy left behind. That is also why it works the same on a locked-down work laptop where you cannot install software.
The free allowance is 5 exports a day, or 15 with a free account. Pro is $12.99/month and covers all 67 Alvseike tools — images, video, audio, PDF and marketing — not just this one, and it adds offline use. See pricing if you convert often.
Two related tools are worth knowing about. Audio Trimmer cuts a clip down before you convert, so you are not encoding minutes you will throw away. Audio Normalizer measures loudness in LUFS and brings a recording to -16 or -14 LUFS with peaks under -1 dBTP, which stops a converted podcast from arriving quieter than everything else in a listener's queue. For recording a take in the first place, Voice Recorder captures straight from your mic.
Common mistakes
Assuming AAC files are smaller at the same bitrate. They are not. Bitrate sets the size. AAC's edge is quality at that size, or the same quality at a lower bitrate you choose deliberately.
Re-encoding lossy to lossy more than once. Every AAC-to-MP3 or MP3-to-AAC pass throws away more detail. Convert once from the best source you have, and keep that source.
Converting to MP3 'just in case' when nothing needs it. If every device you own plays AAC, converting only degrades the file. Check the destination first.
Encoding speech at music bitrates. A mono voice recording at 256 kbps wastes space. Speech is fine at 64–96 kbps AAC.
Trusting the extension. An .m4a file is usually AAC audio in an MP4 container; an .aac file is usually a raw stream. Both are AAC, and both convert to MP3 the same way.
Forgetting that video sound is often AAC. If you only want the audio from a video, Audio Converter can keep just the sound and write it as MP3 or WAV — no separate video editor needed.