How converting in a browser tab actually works
Every other online audio converter uploads your file. This one does not, and the difference is worth understanding — including the places where it makes this tool worse, not better.
The engine is FFmpeg, compiled to WebAssembly
FFmpeg is the program almost every media tool on earth uses underneath, including the ones that charge you a subscription to run it on their servers. Version 0.12.10 of it has been compiled to WebAssembly, which is a format browsers can execute directly. When you drop a file on this page, that build starts up inside the tab and does the work there.
The engine is a 32 MB download the first time you use it. After that your browser caches it for a year, so the second visit starts with no wait at all. It is served from this site's own domain, never a CDN — which matters for the next section.
Why "nothing is uploaded" is enforced, not promised
Any site can write "your files are safe with us". This one ships a Content Security Policy that says connect-src 'self' blob:, which instructs your browser to block every outbound network request this page might try to make to anywhere but this domain. There is no fetch, no XHR, no WebSocket, no form post that could carry your audio off the machine, because the browser itself would refuse it.
The practical demonstration: load this page, turn off your Wi-Fi, and convert a file. It works. A converter that uploads cannot do that.
Stream copy: the conversions that are free
An audio file is a container wrapped around a compressed stream. An .m4a, an .m4r ringtone and the soundtrack inside an .mp4 can all hold the exact same AAC stream — the difference is the wrapper. So converting between them does not need an encoder at all: the stream is lifted out and put in the new wrapper untouched.
The audio comes out identical — we checked, on a real pair of files: the compressed AAC stream extracted from each matched byte for byte across all 244,087 bytes, and so did the decoded audio across all 2,117,710. It finishes in about a second whether the file is three minutes or three hours. This site does that automatically whenever it is possible, and tells you when it is not.
One honest caveat, because it is the kind of detail other sites round off: the file is not byte-identical. FFmpeg writes its own container around the copied stream, so the metadata block differs — in the case above, one region starting at byte 240,994. Nothing that affects what you hear changes, and nothing that affects what you hear is re-encoded.
The catch worth knowing: FFmpeg will let you copy an AAC stream into a WAV container and report success. The file it produces plays nowhere. This site keeps its own list of which pairs are genuinely legal rather than trusting FFmpeg to refuse.
What a conversion costs
There are only three cases, and the tool tells you which one you are in before you press the button:
- A copy — nothing changes. Free.
- Lossless to lossless — FLAC to WAV, WAV to ALAC. Every sample survives; only the size changes.
- An encode — anything to MP3, AAC, OGG or WMA. Detail is discarded to make the file smaller. Doing it once from a lossless master is the best case; doing it to a file that has already been through an encoder once stacks the damage.
The one thing no converter anywhere can do is put back what an earlier encode threw away. Converting a 128 kbps MP3 to a 320 kbps MP3 gives you a file two and a half times the size containing exactly the same audio, slightly degraded by the second pass. The tool will say so.
Where this is the wrong tool
Being honest about this is the point:
- Very large files. A browser tab has roughly two gigabytes of address space. Audio rarely comes close, but a multi-hour uncompressed recording can. The apps have no such ceiling.
- The first visit on a slow connection. 32 MB is a real download. The apps are smaller and already installed.
- Files that are on your phone. Getting a voice memo onto a laptop to convert it is more work than converting it where it already is.
- Opus output. The good Opus encoder crashes in this WebAssembly build. Rather than silently substituting a worse one, this site does not offer Opus as an output at all. Opus files convert in perfectly.
- Anything DRM-protected. Purchased audiobooks and Apple Music downloads are encrypted. No converter will open them, and one that claims it can is lying to you.
What it is built from
FFmpeg is licensed under the GPL and LGPL, and this site's build includes GPL components, so the licence terms travel with it. The full notice and the source links are on the open source licences page.