MP3 to M4A
Nothing you choose here leaves your browser. There is no server to send it to — the conversion happens on your own device, and nothing is uploaded, stored or seen by anyone.
What this conversion costs you
Two lossy formats of the same era; you gain compatibility, not quality. AAC is somewhat more efficient than MP3, but re-encoding loses a little more on the way, so the honest summary is that this makes a file Apple software prefers rather than a better-sounding one. Do it because something will only take an M4A.
Your sample rate, kept
This is the part other browser-based converters get wrong without telling you. The web audio decoder resamples everything to your device's own rate — usually 48,000 Hz — so a 44,100 Hz file, which is every CD rip and most MP3s, comes back with its 44,100 samples a second interpolated into 48,000. The audio is subtly changed and nothing says so.
This page reads the sample rate out of the file's own header first, then decodes at that rate, so the samples are the ones you started with. The table above says which rate it found and confirms it was kept. Where a file's header cannot be read, it says that instead of pretending.
How it works, and why nothing is uploaded
Your browser decodes the MP3 itself, and then encodes the M4A itself as well — through WebCodecs, which hands a page the encoders your operating system already carries. Nothing is downloaded at all, not even the once-off encoder the MP3, Ogg and FLAC pages offer. This site ships no M4A encoder and never will; it borrows the one already on your machine. What is written here is the container around it — the MP4 file structure — which is plain format code with no codec in it.
Where to go next
- M4A to MP3 — the same conversion the other way
- MP3 to WAV — uncompressed, enormous, and readable by anything
- MP3 to OGG — Vorbis, smaller than MP3 at the same quality
What this does not do
It converts whole files and does not trim, join, change the volume or alter the tags — the artist and title in your original do not survive, because the audio is decoded to raw samples and encoded again. It does not write AAC or M4A: those patents have not expired, and this site will not open that question for a format people already have.
Nothing is uploaded, so it cannot work on a file that is not on the device you are using, and a long recording will use a lot of memory while it is being converted.