MP4 to MP3

How do I convert MP4 to MP3?

1. Drop one or more MP4 files onto the drop zone, or click it to browse. Other video formats such as MOV, WebM and MKV work too.
2. Choose an MP3 bitrate. 192 kbps (good) is the default and suits music and speech alike.
3. Click Convert to MP3.
4. Wait a few seconds per file while FFmpeg decodes the audio and encodes the MP3.
5. Download each MP3, or use Download all as ZIP when you converted several files.

The converter runs FFmpeg compiled to WebAssembly inside your browser tab. It reads the MP4, discards the video stream (the -vn flag in FFmpeg terms) and passes the audio through the LAME encoder at the bitrate you chose. Nothing is uploaded, so there is no queue, no file size cap set by a server and no copy of your video sitting on someone else's disk. The first time you use any Kilnfile media tool your browser fetches the FFmpeg engine (about 30 MB) and caches it; after that the tool works offline.

Bitrate is the only decision. 128 kbps gives the smallest file at roughly 1 MB per minute and is fine for podcasts, lectures and voice recordings. 192 kbps, about 1.4 MB per minute, is transparent for most listeners on most equipment and is the sensible default for music. 256 and 320 kbps add size for a difference few people can hear in a blind test, but they are there if you want headroom for later editing or you simply prefer the bigger number.

Speed depends on the length of the audio, not the size of the video, because the video frames are skipped rather than decoded. A 10 minute clip typically finishes in 10 to 20 seconds on a recent laptop and perhaps a minute on a mid-range phone. Because FFmpeg here is single-threaded WebAssembly, a two hour recording can take a few minutes; that is still faster than uploading 2 GB to a server and downloading the result.

Files up to about 1 GB work comfortably on a typical laptop. The real limit is browser memory, around 2 GB for WebAssembly, so a very large 4K file may fail even though its audio is small. If that happens, trim the video first or use the Extract Audio tool with the M4A option, which copies the AAC stream without decoding anything. Batch mode converts as many files as you drop in, one after another, and bundles them into a ZIP.

How it works: this tool uses FFmpeg compiled to WebAssembly (ffmpeg.wasm), loaded into your browser. No server is involved, so Kilnfile never sees your file. Last reviewed 20 September 2026.

Frequently asked questions