FLAC to MP3, a whole folder at a time

The actual job: a library that will not fit

Nobody converts one FLAC. The reason to be here is a folder — a ripped collection, a Bandcamp download, a set of live recordings — that is four or five times larger than it needs to be for the way you are about to listen to it. On a phone, in a car, over Bluetooth, through earbuds on a train, FLAC's advantage over a good MP3 is not something you are going to hear.

So this is a deliberate one-way trade: perfect archival copies stay where they are, and a portable set gets made from them. Keep the FLACs. That is the whole point of having had them.

You only get to do this once, and that is fine

FLAC is lossless, so every sample in it is exactly what came off the disc. Converting to MP3 is the first lossy step that audio has ever taken, which is the best possible case — the encoder gets a clean master rather than something already thinned out by a previous pass. It is also irreversible: converting the MP3 back to FLAC later produces a large file containing the MP3's audio, not the original's.

The gap that matters is the first one. Going from FLAC to MP3 at 256 saves about two thirds. Going from 256 to 320 costs another 25% for a difference nobody has reliably demonstrated in a blind test. The bitrate comparison goes through what the listening studies actually found.

Tags and artwork survive

This is the thing that quietly ruins a library conversion, and it is worth checking whichever tool you use. Titles, artists, albums, dates and track numbers are carried into every MP3 here. Embedded cover art is carried too, because MP3 can hold an attached picture.

Two containers cannot: a raw .aac stream and a .wav. Convert to either of those and the artwork is dropped rather than written somewhere that would corrupt the file. The tags still come across.

If you are on Apple, consider ALAC instead

Apple Music does not read FLAC. That is the usual reason a lossless collection ends up being converted at all — and converting to MP3 to solve it throws away the lossless quality you were keeping.

Apple Lossless is the answer. It is a different lossless codec at roughly the same size, and it plays natively everywhere Apple software runs. FLAC to ALAC is a lossless-to-lossless conversion: every sample survives, only the encoding changes. Pick ALAC rather than MP3 in the converter above and you keep everything.

Why the file-size limit matters more here

FLAC files are big, and that is exactly where a converter with a server on the other end becomes painful: a 260 MB album has to travel the whole way there before anything starts happening, and such services generally cap free use by file size, by daily job count, or both.

Here nothing is uploaded, so the only limit is the browser tab itself — roughly two gigabytes of address space. This warns above 300 MB per file and refuses above 2 GB. Individual FLAC tracks are rarely close; a two-hour live set in one file might be.

How long a batch takes

The engine is single-threaded WebAssembly, so it is slower than a native desktop encoder, and honest numbers are better than optimistic ones: expect a few seconds per track rather than a fraction of a second. A ten-track album is a couple of minutes with the tab open. The progress bar counts through the files so you can see where it is.

The first conversion also downloads the 32 MB engine. After that your browser caches it for a year and subsequent visits start with no wait.