Lossy vs Lossless Audio: Formats, Quality and File Size

Lossy audio compression reduces file size by discarding some audio information; lossless compression reduces size while allowing the encoded audio samples to be recovered. MP3 and AAC are common lossy codecs. FLAC and Apple Lossless (ALAC) are lossless codecs. Uncompressed PCM, often stored in WAV, is another way to retain samples without lossy encoding.

Lossless describes a particular encoding step, not the whole history of a recording. A FLAC made from an MP3 can preserve the decoded MP3 samples, but it cannot restore what the earlier MP3 encode discarded. For editing, keep the best available original and create a separate listening or delivery copy when needed.

Lossy vs. lossless audio at a glance

Question	Lossy	Lossless
What happens to the samples?	Encoding discards or alters some information to reduce the data required.	The codec can reproduce the PCM samples supplied to it when decoded correctly.
Examples	MP3, AAC, Opus and Vorbis.	FLAC and ALAC. Uncompressed PCM can also retain samples but is not compressed losslessly.
Typical tradeoff	Often smaller for streaming or sharing; quality depends on the source, encoder and settings.	Usually larger than a comparable lossy copy; useful when retaining the input samples matters.
Can it undo an earlier MP3 encode?	No.	No. It preserves its input, including any earlier loss or distortion.

MDN's audio codec guide describes FLAC as lossless and lists common codec and container combinations. The Library of Congress FLAC description identifies its input as PCM audio.

Which audio formats are lossy or lossless?

MP3 and AAC are lossy codecs, commonly used for listening copies. FLAC and ALAC compress compatible audio losslessly. PCM WAV commonly stores uncompressed audio. A WAV or M4A extension describes file packaging, however, not a guarantee about every possible encoding inside it: an M4A can contain AAC or ALAC, and WAV can contain non-PCM audio.

For a guide to playback and containers, read what an audio codec is. For specific file choices, compare FLAC vs. WAV, read what an MP3 file is, or learn what an M4A file can contain. Use a compatible media inspector to check the actual codec and properties when the extension is ambiguous.

How different are file sizes?

For one minute of 48 kHz, 16-bit stereo PCM, the audio payload is 48,000 frames × 16 bits × 2 channels × 60 seconds ÷ 8 = 11,520,000 bytes, or 11.52 MB in decimal units, before a WAV header or metadata. A 192 kbps constant-bitrate MP3 has an estimated payload of 192,000 bits per second × 60 ÷ 8 = 1,440,000 bytes, or 1.44 MB, before framing and tags.

Those numbers illustrate storage, not a quality score. FLAC size varies with the samples and encoder settings, so neither PCM nor MP3 bitrate determines an exact FLAC size. Variable-bitrate files, metadata and artwork also affect the final result. Use the audio file size calculator for PCM or fixed-bitrate estimates, then check the exported file.

For actual files rather than a formula, the WAV and MP3 examples provide a short original synthetic signal and measured downloads. The example shows file behavior, not an audibility test for every voice or song.

Can you hear the difference?

Sometimes. The result depends on the source, codec, settings, playback system and listener. A well-encoded lossy copy may be difficult to distinguish in a given situation, while demanding material or a lower bitrate may reveal artifacts. A larger number or a lossless label alone does not guarantee that a recording sounds better: the files might come from different masters or contain different processing.

For a useful comparison, make each copy from the same original, match playback levels and listen to the same quiet and busy passages. Avoid repeatedly converting one lossy copy into another. Preserve the original so you can make a new delivery copy without adding another lossy generation.

Which should you choose?

Task	Practical starting point	Check
Keep a working or archival source	Retain the original project and a suitable lossless or uncompressed export.	Sample rate, bit depth, channels, metadata, backups and the recipient's requirements.
Send a compact listening copy	Create MP3 or another accepted lossy format from the best original.	Listen for artifacts, check playback and verify the actual file size.
Edit an MP3 you already have	Keep the MP3 untouched and work from a suitable decoded copy if the editor requires it.	A WAV does not restore the original recording; sample-rate or bit-depth changes can alter samples.
Upload for source isolation	Use a supported original or compatible WAV/MP3 excerpt.	Input limits and whether conversion changes the source; avoid assuming a larger file improves separation.

The Library of Congress recommends lossless compression over lossy compression for certain preservation deliveries in its audio format recommendations. A particular collaborator or platform may specify a different format, so follow that requirement and keep your source.

Converting between lossless and lossy

1. Start from the best source. Identify the codec and any known prior conversions; a filename alone cannot establish provenance.
2. Choose the destination's format. A FLAC-to-MP3 conversion creates a new lossy encode. FLAC-to-PCM can preserve the encoded samples only if the decoding and export do not change relevant sample properties.
3. Check the converter's actual behavior. The local FLAC-to-MP3 tool accepts a compatible short mono or stereo FLAC up to five minutes and 50 MiB, with output also limited to 50 MiB. Browser codec support varies. It decodes at 48 kHz; MP3 is newly lossy encoded, and its PCM16 WAV option can change samples through resampling or bit-depth reduction.
4. Preview and keep both files. Listen to the exported result, check duration and metadata, and retain the original. The tool does not copy tags or artwork; MP3 encoding can add delay or padding.

Common questions

Is lossless audio always uncompressed?

No. FLAC and ALAC are examples of lossless compression. PCM WAV commonly stores uncompressed samples, although WAV can also contain other encodings.

Can I turn MP3 into lossless audio?

You can encode the decoded MP3 as FLAC or PCM WAV, but the new file cannot recover information discarded during the MP3 encode. It may be larger without containing the original pre-MP3 detail.

Is lossless always high resolution?

No. Lossless describes preservation of the encoded samples, while sample rate and bit depth describe properties of those samples. Check the actual file and its source history.

Is AAC lossless?

No. AAC is lossy. An M4A file can instead contain a lossless codec such as ALAC, so inspect the codec rather than assuming from the extension.