AAC to FLAC Converter

Converting AAC to FLAC: What to Expect

This tutorial walks through decoding a lossy AAC file into FLAC, the open, royalty-free lossless codec from the Xiph.Org Foundation, and explains what the Compression level slider actually does. The honest caveat comes first: AAC already discarded audio detail when it was encoded, and FLAC cannot invent it back. What you get is a lossless container around the decoded AAC — a stable master that survives future re-encodes and plays on lossless-only DACs, streamers, and music servers.

How to Convert AAC to FLAC

1. Upload Your AAC File: Drag and drop your .aac file onto the page, or click "+ Add Files" to browse. Several files can be queued and converted with the same settings.
2. Set the Compression Level: The Compression level slider runs from 1 to 12 and starts at 12, the hardest setting. FLAC is lossless at every value, so this only trades encoding time against file size — it never touches the audio.
3. Leave Audio Channel and Audio Sample Rate on ORIGINAL: Both start on ORIGINAL, which preserves exactly what the AAC decodes to. Choose MONO or a lower rate only when you deliberately want a smaller, altered file — those are the two settings here that genuinely change the audio.
4. Convert and Download: Click Convert and save your .flac file. Files upload over an encrypted connection, are processed on our servers, and are deleted automatically after a few hours. No sign-up, no watermark.

Walk-through: The Compression Level Slider

This is the one control on the page that confuses people, because "compression" here does not mean what it means in a lossy codec. FLAC works by fitting a mathematical function to each block of audio and storing the difference between the prediction and the real samples. A higher level makes the encoder search harder for a better-fitting function; as the Xiph.Org FLAC FAQ explains, decoding only has to compute the one function that was chosen, which is why the setting changes encode time but leaves decode time essentially flat.

• If you want the smallest file and do not mind waiting: leave the slider at its default of 12. This is the right choice for an archive you write once and read many times.
• If you are converting a large batch and want it done quickly: drop to a low number. The output is a bigger file that is byte-for-byte identical in audio to the level-12 version, and it decodes just as fast.
• If you are wondering whether a higher level sounds better: it cannot. Every level is lossless, so all of them reconstruct exactly the same samples — the differences are file size and CPU time only.
• If the size barely moved between levels: that is normal. The returns diminish sharply at the top of the range, and content that is already dense or noisy compresses poorly at any setting.

AAC vs FLAC at a Glance

FLAC Format Specification

Common Errors and How to Fix Them

• "My car stereo or older phone will not play the FLAC" — Plenty of head units, Bluetooth speakers, and legacy players decode only MP3 and AAC. Keep the original AAC for those devices, or convert back with FLAC to AAC.
• "Apple Music will not add my FLAC to the library" — Apple's ecosystem uses ALAC as its lossless format and does not import FLAC into the Music library. For an Apple-only workflow, AAC to AIFF gives you uncompressed PCM that Apple's apps read directly.
• "The FLAC is several times larger than the AAC" — Expected. FLAC stores the entire decoded waveform losslessly, while AAC stored a heavily compressed approximation. If the goal was a smaller file, FLAC is the wrong target — use the Audio Compressor.
• "The compression level made almost no difference" — Also expected. The slider changes how hard the encoder searches, and the last few levels typically buy very little. Nothing is wrong with the file.
• "My tags and cover art did not survive" — AAC stores metadata in MPEG-4 atoms and FLAC uses Vorbis comments; the mapping between them is not standardised, so re-tag in your library manager after converting.

When This Doesn't Work

FLAC is the wrong destination whenever the real goal is a smaller file or wider device support, because it is strictly larger than the AAC you started with and less widely decoded on consumer hardware. It is also pointless as a "quality upgrade" — if you want genuinely lossless audio, the source has to be a CD rip or a lossless download, not a re-wrapped AAC. And if the AAC came from a DRM-protected purchase, it cannot be decoded here at all; that restriction lives in the file, not in the converter. Where FLAC earns its place is as the master copy you keep so that every future MP3, Opus, or AAC you generate comes from one clean decode instead of stacking generation loss.

Frequently Asked Questions

Does converting AAC to FLAC improve the sound quality?

No. AAC is lossy, so the detail it discarded during its original encode is gone permanently, and no lossless format can reconstruct it. The FLAC is lossless relative to the AAC — it reproduces the decoded samples exactly — but it is no closer to the studio master than the AAC was. Genuine lossless audio has to come from a CD rip or a lossless download.

If there is no quality gain, why bother?

Two practical reasons. It stops a lossy-to-lossy cascade: when you later need MP3 or Opus for a phone, encoding from the FLAC uses one clean decode instead of stacking a second generation of loss on top of the AAC. And a fair number of DACs, network streamers, and library managers prefer or require a lossless container, with FLAC's tagging and embedded cover art being more robust than what raw AAC offers.

Will the FLAC be larger than my AAC?

Yes, usually several times larger, because FLAC stores the complete decoded waveform rather than a compressed approximation of it. FLAC's own documentation notes that the resulting bitrate is roughly proportional to how much information the signal actually contains, so busy, loud material compresses less than sparse material. You are trading disk space for a stable, royalty-free master.

What does the Compression level slider actually change?

How hard the encoder searches for an efficient way to model each block of audio, and nothing else about the result. Every level produces a lossless file, so all of them decode to the same samples; higher values simply spend more CPU time to shave off a bit more size. Decoding speed stays essentially constant regardless of the level you picked, which is why the default sits at the top of the range.

Does FLAC preserve the bit depth and sample rate of my source?

Comfortably. RFC 9639 defines FLAC as supporting 4 to 32 bits per sample, sample rates from 1 Hz to 1,048,575 Hz, and 1 to 8 channels — far beyond anything an AAC file will decode to. Leaving Audio Channel and Audio Sample Rate on ORIGINAL keeps an exact copy; change them only when you specifically want a mono downmix or a lower rate.

Is FLAC actually a standard, or just one project's format?

It is a published standard. The IETF released RFC 9639, "Free Lossless Audio Codec (FLAC)", in December 2024, which specifies the format formally after two decades of use. The reference implementation remains with the Xiph.Org Foundation, and the format has always been royalty-free and patent-unencumbered — which is a large part of why it is the default for archival audio.

Can I convert the FLAC back to AAC later without extra loss?

Going back to AAC is inherently lossy, so that re-encode adds its own generation of loss. What FLAC buys you is that the source feeding the encoder is a clean, exact decode rather than a second lossy stage, so the result is no worse than re-encoding the original AAC and the workflow is simpler once you have edited anything. Use FLAC to AAC for that trip.

Why is my FLAC rejected by a device that claims to support it?

Usually a sample rate, bit depth, or channel count outside what the hardware implements. FLAC's specification allows far more than any consumer player supports, so many devices decode only the common cases — stereo at 44.1 or 48 kHz, 16 or 24-bit. If your source is unusual, set Audio Sample Rate to 44100 Hz or 48000 Hz and Audio Channel to STEREO before converting.

Should I use FLAC or AIFF for a decoded AAC?

FLAC when you want a lossless archive, because it is meaningfully smaller than uncompressed PCM and carries proper tags. AIFF when the file is heading into Apple's apps, a hardware sampler, or any tool that insists on raw uncompressed PCM. Both preserve the decoded AAC identically; the difference is size and which software will open it. AAC to AIFF covers the second case.

How are my files handled during conversion?

Files upload over an encrypted connection, are processed on our servers, and are deleted automatically after a few hours — no sign-up, no watermark, never shared or made public. The practical limit on a large batch is upload size and time rather than anything in the FLAC format itself.