Going MP3 to FLAC won't bring back what MP3 encoding threw away — the audio you hear in the FLAC will sound identical to the source MP3. What you get is a lossless container, so the next time the file gets re-encoded for any reason, you don't compound MP3 loss with another generation of compression.
Any MP3, any bitrate. The decoder doesn't care.
Up to 200 MB
FLAC compression levels 0-8 trade speed for size; level 8 squeezes the most. Decoded audio is bit-identical regardless of level — only the file size differs.
Encoder: ffmpeg flac · level 8 · ~50-60% of WAV size
The FLAC will be smaller than the equivalent WAV but bit-identical to the decoded MP3 audio. Tags carry over from the MP3.
Standard .flac, Vorbis comments preserved
Two reasons. One: archiving — keep the audio in a lossless container so re-encoding it for a different format doesn't add another generation of MP3 artifacts. Two: importing into music libraries that prefer FLAC, like audiophile players or hi-res streaming apps.
Typically 40-60% smaller. FLAC compresses by finding patterns in PCM data. Music with quiet sections compresses more; loud, full-spectrum material compresses less. Compression is reversible — decoding the FLAC gives you bit-identical PCM back.
Only if the source was high-res to begin with. A FLAC encoded from a 320 kbps MP3 is still 320 kbps quality in lossless wrapping. Hi-res FLAC means a 24-bit / 96 kHz or higher PCM source — that has to come from the master, not from converting an MP3.