FLAC or MP3: which one to keep
FLAC is lossless: decode it and you get back exactly what was on the CD or the studio master, usually 20 to 30 MB for a four-minute song. That makes it the right format for a music archive. MP3 throws some of that away to get the same song down to 7 to 10 MB, and it plays on everything: old phones, car stereos, smart speakers, DJ software, and the iPhone's Music app, which doesn't import FLAC.
So the usual answer is both. Keep the FLAC files as your masters and make MP3 copies for the places that need them. Converting an MP3 back to FLAC later doesn't bring anything back, which is why it's worth making the copies from the FLAC every time.
320 or 256 kbps
320 kbps is the highest MP3 setting and the default here, because it's what most people converting FLAC ask for. 256 kbps is 20% smaller and, for most music and most listeners, sounds the same. Our 20-second 96 kHz test file was 7.25 MB as FLAC, 805 KB as a 320 kbps MP3 and 644 KB at 256 kbps.
| MP3 bitrate | Per minute | A 60-minute album | Good for |
|---|---|---|---|
| 320 kbps | 2.4 MB | 144 MB | Copies you want as close to the FLAC as MP3 gets |
| 256 kbps | 1.92 MB | 115 MB | Music on a phone, where space matters a little |
| 192 kbps | 1.44 MB | 86 MB | Background listening, car stereos |
| 128 kbps | 0.96 MB | 58 MB | Speech, or when size matters most |
Why an MP3 can clip when the FLAC doesn't
Many modern masters are pushed right up to the loudest value a file can hold. That's fine in the FLAC. But MP3 encoding slightly changes the shape of the waveform, and some peaks come back higher than they went in. When the MP3 is played, anything above full scale is cut off, which can sound like a faint crackle on loud drums and bass.
We measured it on a test master limited to exactly full scale. Encoded at 320 kbps and decoded with an independent decoder, the MP3 peaked 3.5 dB above full scale, with 1,876 samples over the limit in 20 seconds. Lowering the volume by a fixed 1 dB, a common tip, wasn't enough: 271 samples were still over.
So this converter decodes every MP3 it makes and counts the samples that go past full scale. If there are any, it measures how far the MP3 overshoots, lowers the volume by just that much plus a small margin, encodes again and checks again. On the test master that meant 3.7 dB quieter, a peak of −0.3 dB, and nothing cut off. Tracks that don't clip are left at their original level, and each result says which case it was. If you'd rather keep the level whatever happens, choose Keep the level; the result still tells you how many samples will be cut off.
Tags and album art
FLAC files store their tags as Vorbis comments, and MP3 uses ID3, so a converter has to translate them, and many simply drop them. Here the title, artist, album, album artist, track number (with the total), disc number, genre, year, comment and cover art are written into each MP3. We read the result with an independent tag library: every field matched, and the front cover was copied byte for byte rather than recompressed.
Drop a whole album at once and the tracks are converted in order. When they all share an artist and album, Download all as ZIP names the ZIP after them, for example Artist - Album.zip, ready to drag into a music library.
Hi-res, 24-bit and 5.1 FLAC
MP3 goes no higher than 48 kHz, so hi-res files have to be resampled. Instead of always going to 44.1 kHz, the converter picks a rate that divides the original evenly: 96 and 192 kHz files become 48 kHz, 88.2 and 176.4 kHz files become 44.1 kHz. MP3 has no bit depth of its own, so 24-bit sources need no special handling. A hi-res file makes a better-sounding MP3 only to the extent that the master itself was better; the extra resolution doesn't survive the conversion.
Surround FLAC files (5.1 concert or film soundtracks, some Blu-ray rips) are mixed down to stereo: the centre and rear channels are folded into left and right, and the subwoofer channel is left out, as is usual for a stereo mix. In our test the left output held the front-left, centre and rear-left tones, with the right-hand and subwoofer tones absent.
Gapless albums
Every MP3 encoder adds a short silence at the start and pads the end, which turns a seamless live album into a series of tiny gaps. The converter marks exactly how much was added, in the header that players read, so players that support gapless playback skip it and the tracks run into each other as they did in the FLAC.
Your music stays on your device
Everything runs in your browser, including the MP3 encoder (a WebAssembly build of LAME), so nothing is uploaded and there's no file size limit beyond your device's memory. For other formats, see M4A to MP3, WAV to MP3 and OGG to MP3. To cut a track before converting, use Trim audio.