How do you convert FLAC to MP3, and which bitrate should you pick?
Short answer: open the FLAC in the audio cutter, leave the region over the whole file (type a large end time and it stops at the end), pick MP3 and a bitrate, and press Download. In Chromium, all 11 FLAC files we tried opened, 16-bit and 24-bit, from 44.1 kHz up to 192 kHz, mono, and FLAC inside an Ogg file (.oga).
- 320 kbps came out at 48.8% of the size of a 16-bit, 44.1 kHz FLAC (2.40 MB a minute) and stayed closest to the original: 40.2 dB signal-to-error.
- 192 kbps was 29.3% of the size (1.44 MB a minute) at 27.6 dB. 128 kbps was 19.5% (0.96 MB a minute) at 15.2 dB.
- A 24-bit or 96 kHz FLAC did not make a better MP3. At each bitrate the score was the same within 0.2 dB as from a 16-bit, 44.1 kHz copy of the same recording.
- Tags are not copied. Title, artist, album and the other 5 fields in our test file were all missing from the MP3.
For a music library you will keep, pick 256 or 320 kbps. Keep the FLAC as well: an MP3 cannot be turned back into it.
What happens when FLAC becomes MP3
FLAC is lossless: decoding it gives back exactly the samples that went in. MP3 is lossy: the encoder throws away detail to make the file smaller, and the bitrate sets how much it may keep. So converting FLAC to MP3 is a one-way step from an exact copy to a smaller, approximate one. The useful questions are how much smaller and how approximate each bitrate is, and whether anything else changes on the way.
The audio cutter does the job in two steps inside your browser. The browser's own decoder turns the FLAC into samples, and the page encodes the region you marked as MP3 with lamejs. Whether a browser can decode FLAC is up to the browser, which is why the tool page says FLAC works "if your browser can decode" it.
How we tested
- Music: Mozart's Symphony No. 40, first movement, played by the Musopen Symphony. It is in the public domain on Wikimedia Commons as a 48 kHz, 24-bit FLAC.
- Test files: from 1:00 to 4:00 of that file we made 10 FLACs with FFmpeg 4.4.2: stereo 16-bit at 44.1 and 48 kHz, stereo 24-bit at 44.1, 48, 88.2, 96 and 192 kHz, mono 16-bit at 44.1 kHz, the 16-bit 44.1 kHz file inside an Ogg container (.oga), and the same file again with the original 8 tag fields. The 11th file was the whole untouched movement (5:58.8, 65.3 MB).
- Browser: Chromium 153 driven by Playwright, opening a local copy of this site's audio cutter. One fresh tab per file; the region set from 0 to the end, no fades.
- Score: signal-to-error ratio in dB against the 24-bit original, converted to 44.1 kHz with FFmpeg's high-quality resampler, after lining the files up. Higher is closer. It measures how far the waveform moved, not what you hear.
Size and score for each bitrate
Each row is the same three minutes of music. MP3 size depends only on the bitrate and the length, so the MB per minute is the same for every source.
| Export | MB per minute | Share of 16-bit 44.1 kHz FLAC | Share of 24-bit 96 kHz FLAC | Score from 16-bit 44.1 kHz | Score from 24-bit 96 kHz |
|---|---|---|---|---|---|
| MP3 128 kbps | 0.96 | 19.5% | 6.6% | 15.2 dB | 15.2 dB |
| MP3 192 kbps | 1.44 | 29.3% | 9.9% | 27.6 dB | 27.7 dB |
| MP3 256 kbps | 1.92 | 39.0% | 13.1% | 35.9 dB | 36.1 dB |
| MP3 320 kbps | 2.40 | 48.8% | 16.4% | 40.2 dB | 40.4 dB |
| WAV 16-bit | 10.58 | 215.1% | 72.4% | 72.6 dB | 62.6 dB |
The 24-bit, 44.1 kHz and 48 kHz files gave the same scores as the 16-bit file at every bitrate (15.2, 27.6, 35.9 and 40.2 dB), and the 88.2 and 192 kHz files scored 27.7 and 27.9 dB at 192 kbps. The FLAC's bit depth and sample rate made no difference worth reporting: the bitrate you pick decides how close the MP3 stays.
The mono FLAC gave MP3s of exactly the same size as the stereo ones at the same bitrate (2,880,574 bytes at 128 kbps for three minutes), so a mono file does not get smaller as MP3. As WAV it was half the size.
What changes besides the sound
- Sample rate. The cutter writes every file at 44.1 kHz. The 48, 88.2, 96 and 192 kHz files were converted by the browser and kept their length: 7,937,999 or 7,938,000 samples for 180 s, so speed and pitch are unchanged. Against FFmpeg's resampler, the browser's 44.1 kHz WAV scored 64.2 dB from the 24-bit 48 kHz file and 58.1 dB from the 192 kHz file. Both are far above what any MP3 bitrate reached, so the MP3 scores did not move.
- Bit depth. The cutter writes 16-bit samples. From the 24-bit, 44.1 kHz FLAC, the WAV export differed from the 24-bit samples by -94.7 dBFS (RMS), never more than 1.66 steps of 16-bit.
- Tags and cover art. Our tagged FLAC carried 8 fields (title, artist, album, album artist, composer, date, genre, website). The cutter's MP3 and WAV had none, and were byte for byte the same as those made from the untagged FLAC. The site's converters that keep tags (M4A to MP3 and WAV to MP3) only accept M4A and WAV files.
- Ogg FLAC. The .oga file opened and gave byte-identical MP3 and WAV files to the plain .flac.
- The start. Every MP3 started 1,105 samples (25.1 ms) later than the FLAC, the encoder delay explained in Can you trim an MP3 to the exact millisecond?
One detail if you export WAV from a 16-bit FLAC: it is not an exact copy. 5,792,883 of the 15,876,000 samples came out one step lower, all of them positive samples, and none differed by more than one step. The cutter turns the decoded sound into 16-bit samples by cutting off the fraction. One step is 1/32,768 of full scale.
How long it took
| File | FLAC size | Open | Export MP3 | Export WAV |
|---|---|---|---|---|
| 3 min, 16-bit 44.1 kHz | 14.8 MB | 0.9 s | 18.8 to 24.2 s | 2.2 s |
| 3 min, 24-bit 96 kHz | 43.9 MB | 1.9 s | 18.6 to 26.1 s | 1.0 s |
| 3 min, 24-bit 192 kHz | 55.3 MB | 3.0 s | 18.2 s (192 kbps) | 2.2 s |
| 5:58.8, 24-bit 48 kHz | 65.3 MB | 3.1 s | 37.4 s (192 kbps) | 1.8 s |
The FLAC's resolution changed how long it took to open, not how long the MP3 took: MP3 encoding works on the 44.1 kHz audio, so it ran at about 6 to 9 seconds per minute of music whatever the source. Longer files need more memory; How long a file can an online MP3 cutter open? has the limits we found.
Steps
- Open the audio cutter and choose your .flac file.
- Type 0 as the start and any large number, such as 99999, as the end. The end stops at the last sample.
- Leave both fades at 0.
- Under Export format pick MP3 and a bitrate: 256 or 320 kbps for music you will keep, 128 kbps when the file only needs to be small.
- Press Download, then add the tags again in your music player or tag editor if you need them.
The cutter converts one file at a time. To turn several FLAC tracks into a single MP3, the audio joiner also opens FLAC.
Questions
Does Safari or Firefox open FLAC here?
We only tested Chromium, so we don't know. If a browser can't decode the file, the page says "Your browser couldn't decode this file."
Can I convert MP3 back to FLAC?
Not on this site, which saves only MP3 and WAV. Even with a tool that does, the result would only hold what the MP3 kept; what the MP3 encoder threw away does not come back.
Is the FLAC uploaded?
No. The cutter reads and encodes the file in your browser; see how to check whether an online tool uploads your file.