Convert WEBM to MP4
WEBM uses VP8 or VP9 video and Opus audio - codecs MP4 doesn't accept. So this one really is a re-encode. Wave uses only LGPL codecs and no server; the file stays on your device.
What actually changes
WEBM was designed as a royalty-free alternative to MP4, so its codecs (VP8, VP9) and audio (Opus) don't share DNA with MP4's family. Converting WEBM to MP4 requires actually decoding and re-encoding both streams - there's no fast-copy path.
Wave prefers your browser's built-in WebCodecs H.264 encoder for this - hardware-accelerated on almost every modern machine, and several times faster than a software encode. On browsers without a WebCodecs H.264 encoder, Wave falls back to the LGPL-only MPEG-4 Part 2 encoder that ships inside the wasm ffmpeg core. Either way, audio re-encodes to AAC at 192 kbps and every codec Wave ships is license-clean (never libx264, which is GPL).
Three steps.
-
01
Step 1
Drop your WEBM file onto the card.
-
02
Step 2
Wave will show "Re-encode needed" - the streams need to be rewritten to fit MP4.
-
03
Step 3
Convert. Slower than a remux, but everything happens on your device.
Answers, specifically.
Why is WEBM to MP4 slower than MKV to MP4?
Because it can't be a remux. Every video frame has to be decoded from VP8/VP9 and re-encoded, and every audio sample has to be decoded from Opus and re-encoded to AAC.
Will the MP4 look identical to the WEBM?
Very close. Wave targets high quality, but small transcoding losses are unavoidable whenever you go through decode-then-encode.
Do I get H.264 or MPEG-4 Part 2?
H.264 whenever your browser exposes a WebCodecs H.264 encoder - which is Chrome, Edge, Safari 16.4+, and most Chromium-based mobile browsers. On browsers without WebCodecs H.264, Wave falls back to MPEG-4 Part 2 because the GPL-licensed libx264 can't be bundled without pulling the whole site under GPL. The player-compatibility difference is small; the file-size difference is a few percent in H.264's favour.
Does the audio come out as AAC or Opus?
AAC. Opus in an MP4 file is legal but plays inconsistently in non-web players, so we default to AAC.
Do YouTube downloads work here?
Wave doesn't download from YouTube - that's a separate legal and technical animal. If you already have the .webm file, this converts it.