Why MP3 to FLAC cannot improve quality
MP3 reaches its small size by permanently discarding sound it judges inaudible, often including high frequencies. FLAC is lossless, so it stores exactly what the MP3 decodes to: the same sound as the MP3, with nothing added back. No converter can reconstruct the missing detail.
A FLAC made from an MP3 is therefore not a "lossless" copy of the music in the usual sense; it is a lossless copy of the MP3. Label such files clearly so they are not mistaken for CD-quality rips later.
How much bigger the file gets
Decoded, an MP3 becomes 10.58 MB per minute of 16-bit/44.1 kHz stereo — about 11 times a 128 kbps file and 4.4 times a 320 kbps file. FLAC typically removes 30–50% of that, so expect the FLAC to be roughly 2 to 8 times the size of the MP3.
The extra space holds no extra music. If storage matters and your device plays MP3, keeping the MP3 is the better choice.
When converting still makes sense
- A device, app or library workflow accepts only FLAC.
- You want a single lossless format for an editing project, so later exports do not stack another round of lossy compression.
- A music server or DJ tool handles FLAC better than MP3 in your setup.
In those cases the conversion is useful and harmless: FLAC reproduces the MP3's sound bit for bit, and tags and cover art are carried over.
Choose 16-bit, not 24-bit
MP3 has no bit depth of its own; 16-bit already holds more precision than the MP3 carries, so the default "Original" setting writes 16-bit FLAC at the MP3's sample rate. 24-bit only makes the file about 1.5 times larger with nothing audible gained.
Keep the sample rate at "Original" too. Resampling a 44.1 kHz MP3 to 48 kHz changes every sample without adding detail.
Spotting a FLAC that came from an MP3
Lossy encoders usually cut off high frequencies, so in a spectrum view a FLAC made from an MP3 typically shows a hard ceiling somewhere below 20 kHz, while a true CD rip extends further with natural roll-off. File size is another hint: a FLAC far smaller than 50% of WAV size for dense music is unusual.
These signs are indicators, not proof; some genuine recordings are band-limited too. The honest way is to keep the source format in the file name or tags, for example "(from MP3)".