How do you convert an AAC file to MP3, and how is .aac different from .m4a?

What a .aac file is, and how it differs from .m4a

AAC (Advanced Audio Coding) is a lossy codec, the sound format inside most .m4a files. The codec can be stored two ways:

• .m4a: AAC inside an MP4 container, which also holds the length, tags and cover art.
• .aac: a raw stream of AAC frames, each with a short ADTS header in front of it and nothing around them.

You can tell them apart in a hex viewer: the first two bytes of our .aac files were FF F1, the start of an ADTS header, while each .m4a began with 00 00 00 1C followed by the letters ftyp, the first box of an MP4 file.

Why the length differs. An AAC encoder puts a short run of extra "priming" samples in front of the music, and pads the end to a whole frame. Apple's technical note TN2258, AAC Audio - Encoder Delay and Synchronization describes both and says that only the Core Audio (.caf) and MPEG-4 audio (.m4a) containers have a described way to carry those numbers, while raw ADTS streams have no explicit signal for them (checked 2026-10-20). So a player can trim the extra samples from an .m4a but not from a .aac. Apple's note gives 2,112 priming samples for its own encoder; FFmpeg's encoder, which made our files, added 1,024.

Opening a .aac. Microsoft's page File types supported by Windows Media Player lists .aac, .adt and .adts as well as .m4a (checked 2026-10-20). If you just want to play the file, you may not need MP3 at all.

How we tested

• Music: Mozart's Symphony No. 40, first movement, played by the Musopen Symphony. It is in the public domain on Wikimedia Commons as a 48 kHz, 24-bit FLAC.
• Test files: from 1:00 to 4:00 of that file (exactly 180 s) we encoded AAC-LC with FFmpeg 4.4.2's built-in encoder: 96, 128, 192 and 256 kbps stereo at 44.1 kHz, 128 kbps stereo at 48 kHz, and 64 kbps mono. Each was written as .m4a, then copied frame for frame, without re-encoding, into a raw .aac. Each pair holds the same audio frames.
• Not tested: HE-AAC (the "AAC+" used by some streams and low-bitrate recorders). FFmpeg's built-in encoder can't make it, so we don't know whether those files open.
• Browser: Chromium 153 driven by Playwright, opening a local copy of this site's audio cutter. One fresh tab per file; the region from 0 to the end, no fades; WAV and MP3 at 128, 192, 256 and 320 kbps saved.
• Score: signal-to-error ratio in dB against the 24-bit original, converted to 44.1 kHz with FFmpeg's high-quality resampler, after lining the files up. Higher is closer. It measures how far the waveform moved, not what you hear, so compare rows with each other rather than reading the numbers as sound quality.

What each MP3 bitrate keeps from an AAC

The first score column is the .aac itself, before the cutter touched it. Every MP3 column is the same file after conversion.

No MP3 scored higher than the AAC it came from. Matching the bitrate is not enough: a 128 kbps AAC saved as 128 kbps MP3 fell from 26.7 to 15.2 dB, and a 192 kbps AAC saved as 192 kbps MP3 fell 6.0 dB. At 256 kbps the 96 and 128 kbps files stayed within 0.4 dB; the 192 and 256 kbps files needed 320 kbps to stay within 0.5 and 1.2 dB. The .m4a copy of the 128 kbps file scored the same at 192, 256 and 320 kbps, and 0.1 dB lower at 128 kbps.

File size

The raw .aac was a little larger than the .m4a holding the same frames: 2,944,225 against 2,921,805 bytes for the 128 kbps pair. The 64 kbps mono .aac (0.50 MB a minute) stayed mono in the MP3, but an MP3 at a given bitrate came out the same size as from a stereo file, so 192 kbps made it 2.88 times bigger. For mono speech, see the bitrate advice in How do you compress an MP3?

Length and start: .aac against .m4a

• .m4a: the WAV export held exactly 7,938,000 samples per side, 180 s at 44.1 kHz, and lined up with the original with 0 samples offset. The priming samples were dropped.
• .aac: the WAV export held 7,939,072 samples, 1,072 more (24.3 ms). The music started 1,024 samples (23.2 ms) late, and the other 48 samples were at the end. FFmpeg's own decoder gave the same 7,939,072 samples, so this is the file, not the browser.
• 48 kHz .aac: the music started 941 samples late at 44.1 kHz (21.3 ms, which is 1,024 samples at 48 kHz).
• MP3 start: MP3 encoding adds its own 1,105 samples (25.1 ms), explained in Can you trim an MP3 to the exact millisecond? From the .m4a the MP3 started 1,105 samples late; from the .aac, 2,129 samples (48.3 ms).
• The time label: the cutter showed the .aac files as 3:00.024 (3:00.032 at 48 kHz) and the .m4a files as 3:00.000, and named the downloads to match.

For converting a whole song, 23 ms of lead-in is a short gap before the music. It matters if you line the MP3 up against other audio or cut it to the millisecond. If you have both, convert the .m4a.

Several .aac files at once: the M4A to MP3 page

The M4A to MP3 converter takes many files in one go, but its file picker only lists .m4a files. Dragging a .aac onto its box did work in our test: the 128 kbps and 48 kHz files converted, and the 192 kbps MP3 was byte for byte the same as the one from the cutter. Two things to know:

• Before converting, the list showed the .aac files as 3:03.5 and 3:03.6 long, and filled those in as the end time. A raw .aac has no length field, so the browser guessed from the bitrate. The finished MP3s were 3:00.0, so nothing was added or cut.
• The 23.2 ms lead-in stays, as in the cutter.

The page's batch guide covers timing and memory for many files.

How long it took

Opening a three-minute .aac took 0.5 to 1.8 s. MP3 export took 17.6 to 26.6 s for stereo files at any bitrate, 9.4 s for the mono file; WAV took 0.8 to 2.3 s. That was Chromium 153 on a server, measured 2026-10-20, not a phone. For long recordings, How long a file can an online MP3 cutter open? has the limits we found.

Steps

1. Open the audio cutter in Chrome, Edge or another Chromium-based browser and choose your .aac file.
2. Type 0 as the start and any large number, such as 99999, as the end. The end stops at the last sample.
3. Leave both fades at 0.
4. Under Export format pick MP3 and 256 kbps, or 320 kbps if the AAC is 192 kbps or more.
5. Press Download. The MP3 has no tags; add them in your music player if you need them.

Questions

Can I just rename .aac to .mp3?

No. The name changes but the sound inside is still AAC, so a device that only plays MP3 still can't play it. Converting re-encodes the audio as MP3.

Can I turn the MP3 back into AAC?

Not on this site, which saves only MP3 and WAV. Each lossy step loses a little more, so keep the .aac you started with.

Is the file uploaded?

No. The cutter reads and encodes the file in your browser; see how to check whether an online tool uploads your file.