AAC vs FLAC: Lossy vs Lossless, Which to Use?

AAC and FLAC are not really rivals. They solve different problems. FLAC keeps an exact copy of the recording in less space than WAV. AAC throws away detail you are unlikely to hear so that the file is small enough for phones, streaming and Bluetooth. This guide compares them on sound, size and device support, explains where the difference is audible and where it is not, and shows when to use each.

The short answer

• FLAC is lossless. Decoding it gives back exactly the audio that went in.
• AAC is lossy. At 256 kbps it is roughly a third of the size of FLAC, or less.
• At 256 kbps, the gap is small for most listeners on most equipment.
• Keep FLAC as the master; make AAC copies for phones, Apple devices and wireless listening.

How each format works

FLAC (Free Lossless Audio Codec) compresses audio without losing anything. It predicts each sample from the ones before it and stores only the small error, then rebuilds every sample exactly when the file plays. It was published as an internet standard, RFC 9639, in December 2024.

AAC (Advanced Audio Coding) is a lossy codec from the same MPEG family as MP3, designed to do the same job more efficiently. It uses a model of human hearing to decide which sounds are masked by louder ones, spends few or no bits on them, and fits the rest into the bitrate you choose. What it removes cannot be recovered.

File size compared

Sizes are stereo, in decimal megabytes. FLAC varies with the music; AAC size depends only on bitrate and length.

Can you hear the difference?

It depends on the bitrate, the music, the equipment and your ears.

At 256 kbps, many listeners cannot reliably tell AAC from the FLAC it came from in a blind test, especially on phone speakers, earbuds, in a car or anywhere with background noise. Some trained listeners can hear differences on certain tracks with good wired headphones.

At 128 kbps and below, the loss is easier to hear on music: cymbals lose their shimmer, reverb tails get grainy and busy passages can sound smeared.

The fair way to decide is a blind test. Make a 256 kbps AAC from one of your FLAC files, match the volume, and use an ABX tool that plays them in random order and scores your guesses. If you cannot beat chance on your own gear, AAC is good enough for that listening.

Bluetooth changes the picture

Over standard Bluetooth the comparison mostly disappears. Apple's support page says AirPods use the Apple AAC Bluetooth Codec and that Bluetooth connections are not lossless. Most other wireless headphones also receive a lossy stream. So when you listen to a FLAC on wireless earbuds, it is compressed again on the way, and a good AAC file will sound essentially the same. Our guide on whether lossless audio works over Bluetooth explains the codecs.

Device and app support

• Apple devices and apps: AAC plays everywhere in Apple's world. The Music app and iTunes do not add FLAC files to a library; for lossless there, use Apple Lossless (ALAC). See FLAC vs ALAC.
• iPhone and iPad: iOS 11 and later can play FLAC files, for example from the Files app, but not inside the Music app.
• Android: plays both; FLAC has been supported since Android 3.1.
• Windows and Mac: both play FLAC natively (Windows 10 and later, macOS 10.13 and later) and handle AAC.
• Car stereos and older players: check the manual. Many read MP3 and AAC; fewer read FLAC.
• Streaming: services make their own copies. Spotify's support page, for example, lists AAC for its web player and a lossless FLAC option for Premium.

AAC bitrates by use

AAC quality is set by its bitrate. As a practical guide:

• 256 kbps: a good choice for music you care about. Many listeners cannot tell it from the lossless original.
• 192 kbps: fine for everyday listening and saves a quarter of the space of 256.
• 128 kbps: acceptable for casual listening and speech; the loss is easier to hear on music.
• 96 kbps and below: best kept for voice, such as podcasts and audiobooks.

Our MP3 vs AAC guide compares AAC with MP3 at the same bitrates.

AAC or Apple Lossless? Check the codec, not the extension

AAC files usually end in .m4a, but so do Apple Lossless (ALAC) files. The extension only tells you the container, not what is inside. To be sure which one you have, open the file in a tool that shows the codec. MediaInfo, a free app for Windows, Mac and Linux, lists it as AAC or ALAC, along with the bit depth and sample rate. This matters before you delete a FLAC: an ALAC .m4a is a lossless copy, an AAC .m4a is not.

Avoid converting lossy to lossy

Every lossy encode removes detail, and the losses stack. Converting an AAC file to MP3, or an AAC to a smaller AAC, makes the result worse than either file on its own. Always make your AAC copies from the FLAC, and keep the FLAC so you can make new copies later at a different bitrate or in a newer format.

Common myths

• "AAC is Apple's own format." AAC is an international standard from the MPEG group, the same group behind MP3. Apple uses it heavily, but so do many other companies and devices.
• "An .m4a file is always lossy." Not if it holds Apple Lossless; check the codec.
• "Converting AAC to FLAC improves it." It makes a bigger file that sounds exactly like the AAC.

Which should you choose?

Converting between AAC and FLAC

FLAC to AAC: pick 256 kbps for music, or 192 kbps to save more space, and always convert from the FLAC rather than from another lossy file. Our FLAC to AAC converter runs in your browser with FFmpeg and never uploads your files. If you want to stay lossless in Apple's apps, use FLAC to Apple Lossless instead.

AAC to FLAC: possible with our AAC to FLAC converter, but it cannot restore what AAC removed. The FLAC sounds exactly like the AAC and takes more space, so only do it when an app insists on FLAC. Free users get 5 conversions per month. The output is limited to the first 10 seconds as a preview, with a 20MB input file size limit. Upgrade to Pro for unlimited, full-length conversions.

The bottom line

FLAC is the format to keep: exact, open and self-checking. AAC is the format to carry: small, efficient and native to Apple's devices and Bluetooth headphones. Keep your music as FLAC, make AAC copies at 256 kbps for the places where size and compatibility matter, and run a blind test if you want to know whether the difference matters to your ears.