Convert AAC to FLAC Online
Convert AAC audio to FLAC container. Free, in your browser, no file uploads.
.aac, .m4a · up to 100 MB
What you can do
AAC to FLAC: compatibility with Hi-Fi players that require FLAC
Naim and Linn players
Convert iTunes purchases to FLAC for Hi-Fi network players that prefer FLAC over AAC.
Honest about quality
FLAC preserves the AAC's decoded PCM without additional loss, but does not restore what AAC already discarded.
100% private
Your music never leaves your device. FFmpeg.wasm processes everything locally in WebAssembly.
Metadata preserved
iTunes tags (artist, album, cover art) migrate to FLAC's Vorbis Comment block.
How it works
Three steps, no hassle
Upload your AAC file
Drag or select your .aac, .m4a, or .mp4 file with AAC audio — iTunes purchase, podcast, iPhone audio. Up to 500 MB, no signup.
Decode to PCM and repackage as FLAC
FFmpeg.wasm decodes the AAC to uncompressed PCM and re-encodes it as lossless FLAC. The result is a lossless copy of the already-decoded AAC — quality cannot be restored.
Download your FLAC file
FLAC ready for Hi-Fi players like Naim, Linn, Bluesound, or music libraries that require FLAC container.
FAQ
Got questions?
No. This is the truth most converters don't tell you: AAC is a lossy codec. When an AAC file was created, audio information was permanently discarded by the codec's perceptual model. Converting to FLAC preserves the decoded PCM from the AAC losslessly — meaning a lossless representation of already-degraded audio. It's exactly like scanning a photocopy in high resolution: the scanner captures every pixel perfectly, but the quality doesn't exceed the original photocopy. FLAC guarantees that what goes in comes out identically on every playback, but cannot restore frequencies the AAC encoder discarded — typically above 16-20 kHz and high-frequency transient details sacrificed by the psychoacoustic model.
Hardware compatibility, not quality improvement. There are legitimate practical use cases: (1) Hi-Fi network players like Naim Uniti Atom, Linn Klimax DSM, Bluesound Node, and Cocktail Audio X45 that play FLAC but not AAC natively. (2) Music library software like JRiver Media Center and MediaMonkey, which many audiophiles configure to accept only FLAC as the ingestion format. (3) NAS systems with DLNA/UPnP servers like Synology AudioStation with FLAC-only playlists. (4) Pre-2009 iTunes purchases (when Apple sold DRM-protected AAC) that have been unlocked and need to be in FLAC for a specific player. In all these cases, the goal is container compatibility, not quality improvement.
Significantly larger. AAC at 256 kbps (iTunes Plus quality) compresses 10 minutes of audio to about 19 MB. The same audio in FLAC takes 50-80 MB depending on musical content (FLAC compresses complex signals less than simple ones). The typical ratio is 3:1 to 5:1 in size, meaning a library of 1000 songs in 256 kbps AAC (~4 GB) would become 12-20 GB in equivalent FLAC. This size increase without real quality gain leads many users to reconsider whether they actually need FLAC, or just need a player that accepts M4A/AAC directly.
FLAC (Free Lossless Audio Codec) is a lossless audio compression codec developed by Josh Coalson, first published in July 2001 under a BSD license. 'Lossless' means the decoded file is bit-for-bit identical to the original PCM: if you record audio, compress it to FLAC, and decode it, you get exactly the same bits you recorded. FLAC achieves compression (typically 40-60% of PCM size) through linear prediction of samples and Rice-Golomb entropy coding of prediction residuals. Unlike MP3, AAC, or Vorbis, there is no perceptual model or information discard. FLAC is the de facto standard in audiophile communities like Hydrogenaudio, What.CD (now RED), and platforms like Bandcamp and Qobuz.
Yes, with important nuances. Since April 2009, Apple has sold music on iTunes Store as 256 kbps AAC without DRM (iTunes Plus). These M4A files can be freely converted to FLAC. Pre-2009 purchases had FairPlay DRM, but Apple allowed upgrading to iTunes Plus at $0.30 per song at the time; in 2025, if you still have DRM files, you need to remove it via iTunes/Music on Mac before converting. Converting iTunes purchases to FLAC is completely legal for personal use under the format-shifting doctrine recognized in many jurisdictions, though legal conditions vary by country.
High-end network players with historically limited or no AAC support include: Naim NDX2, ND5 XS2, Uniti Atom, Uniti Nova (Naim only partially supported AAC until firmware 3.x in 2020, still with limitations in M4A containers); Linn Klimax DSM, Akurate DSM, and Selekt DSM (Linn historically chose FLAC and WAV as primary formats); Cambridge Audio CXN and Evo series (variable AAC support by firmware); Cocktail Audio X50Pro. In 2025, most modern players have improved AAC support, but if you have equipment from before 2018 or from European Hi-Fi specialist brands, converting to FLAC may be the simplest solution for compatibility.
Convert AAC to FLAC: what actually happens, when it makes sense, and how to do it
Converting AAC to FLAC is an operation that generates more confusion than any other in audio conversion, because it defies the intuition that 'bigger is better'. The technical truth is this: AAC (Advanced Audio Coding) is a lossy compression codec standardized by ISO/IEC in 1997 as part of MPEG-4 Audio. When an AAC file was created — whether from an original recording, from a CD, or from an iTunes download — the AAC encoder applied a psychoacoustic model based on frequency masking to permanently discard audio information that human hearing theoretically cannot perceive: high-resolution frequencies above 16-20 kHz, high-frequency transients, and signal components below the simultaneous masking threshold. This information does not exist in the AAC file and cannot be recovered. FLAC (Free Lossless Audio Codec), developed by Josh Coalson and published in July 2001, is a lossless codec: it guarantees that every bit of the PCM it encodes is reproduced identically when decoded. When an AAC is converted to FLAC, what FLAC preserves losslessly is the decoded PCM from the AAC — which is already a degraded representation of the original audio. The resulting FLAC is bit-for-bit identical on every playback, but the information discarded by the original AAC encoder is still absent.
Despite not improving quality, converting AAC to FLAC has practical and legitimate use cases in 2025, centered primarily on compatibility with high-end audiophile hardware. European Hi-Fi network players — categorized as 'network streamers' in the industry — have historically had inconsistent AAC support in M4A containers. Naim Audio, the British manufacturer founded in 1969 and a reference in high-fidelity equipment, supported AAC in limited fashion until firmware 3.x for the Uniti platform in 2020, and some users with NDX2 or ND5 XS2 equipment report incompatibilities with M4A files containing multiple tracks or high-resolution embedded cover art. Linn Products, the Scottish manufacturer of players like the Klimax DSM (retail price: ~€20,000 in 2024), historically recommended FLAC or WAV as primary ingestion formats for their Kazoo/Linn App software, though AAC support improved in 2022-2023 updates. Bluesound Node ($400), the entry-level player in the BluOS ecosystem, accepts FLAC without restriction but has documented limitations with AAC in generic MP4 containers. For users on these platforms who have collections of iTunes purchases — distributed since 2009 as 256 kbps AAC M4A without DRM — converting to FLAC is the most direct solution for feeding their playback system without reconfiguring the player software.
Convertir.ai runs AAC to FLAC conversion entirely in the browser using FFmpeg.wasm. The technical process starts with detecting the input container: AAC files can arrive in several containers — ADTS (.aac, no container, with 7-9 byte sync headers per frame), MP4/M4A (MPEG-4 Part 14, the iTunes and Apple Music format), or ADIF (Audio Data Interchange Format, now obsolete). For M4A files, FFmpeg analyzes the ISO Base Media File Format atom tree: reads the ftyp atom (confirming M4A or mp42 brand), locates the moov atom to extract audio track parameters (sample rate, channels, duration), and reads audio data from the mdat atom. The libfaad2 decoder or libavcodec's native AAC decoder converts AAC samples to 32-bit float PCM. iTunes metadata stored in the ilst atom (©nam for title, ©ART for artist, ©alb for album, ©day for year, covr for cover art) is extracted and rewritten as Vorbis Comment in the output FLAC file: TITLE, ARTIST, ALBUM, DATE, and the cover art is stored as a PICTURE block type 3 (Front cover art) with the embedded PNG or JPEG. The FLAC encoder applies variable-order linear prediction (1-12) with Rice parameters optimized by the selected compression mode. FLAC compression level 5 (default) offers the best balance between encoding speed and compression ratio for general use. All processing occurs in WebAssembly without sending data to any server.