Convert MP4 to MP3

An MP4 is a container. Inside it sit two separate streams — the picture and the sound — and pulling the sound out is, in principle, a matter of ignoring one of them. In practice almost every converter on the internet does this by uploading the entire video to a server, running ffmpeg on it, and sending back a file a fraction of the size. A one-gigabyte lecture recording travels across the internet twice so that thirty megabytes of speech can come back.

That is a strange amount of work for the result, and it means handing a recording to a company you have never heard of. This page does it differently: your browser already contains the decoders needed to play an MP4, because it plays them on every website you visit. We use those, so the video stays on your machine and the only thing that moves is the finished MP3, from one part of your computer to another.

The length of the recording, almost entirely. The default band writes 160 kbps, which is 1.2 megabytes a minute, so an hour-long lecture lands near seventy megabytes regardless of whether the video it came from was two hundred megabytes or two gigabytes. The picture is what makes video files large, and the picture is exactly what this conversion discards.

The quality control under the result moves that rate to 96 kbps or to 320 — about forty per cent smaller, or twice the size. Balanced is the default and is transparent for speech and most listening. Choose the smaller setting for something you will play through a phone speaker on a train, and the higher one for music you care about.

The audio inside an MP4 is almost always AAC. Converting it to MP3 means decoding that AAC and encoding it again as MP3, and every re-encode of already-compressed audio loses a little more. For speech this is genuinely inaudible. For music it is measurable and, on good headphones, sometimes noticeable.

Converting the same MP4 to M4A does not avoid that second generation. The AAC is decoded and encoded again here as well — no conversion on this site copies an audio stream across untouched — so an M4A is a second-generation file too. What it gives you is a better codec for that second pass: AAC is more efficient than MP3, so at the same file size a second-generation AAC keeps detail that a second-generation MP3 does not. In practice one re-encode at a sensible bitrate is inaudible for speech and very close to it for music, whichever of the two you pick. The catch is compatibility: MP3 plays on everything ever made, including car stereos, gym equipment and twenty-year-old MP3 players, while M4A wants something reasonably modern. If the file is going to a phone or a computer, M4A is the better answer. If it is going anywhere else, MP3 is the safe one.

Drop up to a hundred files at once. Each converts independently with its own progress bar, and once two or more have finished a single button offers all the results as a ZIP. Nothing is queued behind anyone else, because there is no shared server — the work happens on your machine, so the only thing that limits a batch is how much your own computer feels like doing.

This is the point at which the no-upload approach stops being a privacy argument and becomes a practical one. Converting forty lecture recordings through a site that uploads them means forty uploads and a daily limit; here it means leaving a tab open.

Chapters, subtitles and any second audio track are lost, because an MP3 has nowhere to put them. Only the first soundtrack survives, which matters for films with a commentary or a dubbed language but almost never for a recording of somebody talking.

Cover art and title metadata from the video are not carried across either. If the MP3 is going into a music library, expect to set the artist and album yourself — that is a job for a tagging tool rather than a converter, and doing it badly is worse than not doing it.