How lossless compression is even possible
FLAC (Free Lossless Audio Codec) compresses audio the same conceptual way a ZIP file compresses documents — by finding and removing statistical redundancy in the data, not by discarding any actual content. This is fundamentally different from MP3's approach: MP3 shrinks files by permanently removing audio detail it judges to be less perceptible, while FLAC shrinks files by encoding the exact same data more efficiently, with every original bit recoverable on playback.
This is why decompressing a FLAC file reproduces the audio absolutely identically to the original uncompressed source — there's no quality trade-off involved anywhere in the process, only a computational one (FLAC files take slightly more processing power to decode than an uncompressed format, though this is trivial for any modern device).
The real-world size difference
FLAC typically achieves roughly 40-60% the size of the equivalent uncompressed WAV file, with the exact ratio depending on how much redundancy exists in the specific audio content — a factor influenced by things like how much silence, how much predictable repetition, and how much genuine acoustic complexity the material has. It will never compress anywhere close to MP3's size, because MP3 achieves its much smaller files specifically by throwing away information that FLAC is designed never to touch.
Who actually needs FLAC over MP3
Audiophiles building a music collection meant to last and be re-purposed for any future use, musicians and audio engineers who need pristine intermediate files for further processing, and archival situations where you genuinely don't want to lock in any compromise are the main cases where FLAC's extra size over MP3 is worth it. For casual listening on a phone with limited storage, MP3 or AAC at a reasonable bitrate typically sounds indistinguishable to most listeners while taking a fraction of the space.
Converting between FLAC and other lossless formats
Converting FLAC to WAV or another lossless format loses nothing, since both preserve full audio data — the choice between them then comes down to compatibility and file size preference rather than any quality consideration. Converting FLAC (or WAV) down to a lossy format like MP3 is the point where an actual, permanent quality trade-off enters the picture, so that conversion direction deserves the bitrate consideration that a lossless-to-lossless conversion doesn't need.