MP4 to WebM Converter

What is inside an MP4 file

MP4 is the most widely supported video format there is, normally carrying H.264 or HEVC video and AAC audio. WebM is its open counterpart, built on Matroska and limited to royalty-free codecs. Converting MP4 to WebM therefore always means re-encoding: H.264 is not a WebM codec, and neither is AAC.

This converter writes WebM with VP8 video and Opus audio. VP8 is the original WebM codec and plays in every browser that supports WebM. VP9 would give smaller files, but the VP9 encoder in this browser build of ffmpeg crashes on the first frame, so it is not offered rather than offered and broken.

Where MP4 files come from

The MP4 files people bring here are mostly a phone or camera video destined for a web page that should serve an open format, a clip for a platform, app or game engine that asks for WebM specifically, or a short loop or background video to embed with the HTML video element. In practice none of these can be copied across unchanged, so every conversion is a re-encode.

The converter does not guess. When you drop the file it reads the container's own description of its streams — codec, resolution, frame rate, audio tracks — and shows you, before you press anything, whether the WebM will be a copy or a re-encode.

What happens to your MP4

The H.264 or HEVC video is decoded and encoded with libvpx (VP8) in constrained-quality mode: a quality target with a bitrate ceiling scaled to the resolution — about 2.5 Mbps at 720p and 5 Mbps at 1080p by default. Audio is encoded as Opus at 128 kbps; surround tracks are downmixed to stereo.

If the MP4 is HEVC from an iPhone, the rotation flag is applied during the re-encode, so the WebM is stored the right way up rather than relying on a player to rotate it.

How long MP4 to WebM takes

VP8 in its realtime mode is the faster of the two encoders here — measured at roughly twice the speed of H.264's default setting on the same clip — so a minute of 30 fps 720p video takes about two minutes on a recent computer, and 1080p about four.

Larger frames cost proportionally more: 1080p has 2.25 times the pixels of 720p, and 4K nine times. For long 4K footage, scale it to 1080p or 720p with the Resolution box, which also makes the WebM far more practical to stream.

Problems you may run into

The WebM is larger than the MP4 — VP8 is an older, less efficient codec than H.264 at the same quality. Choose a lower resolution or the 'Fastest' option, which uses a lower bitrate cap, if size matters most.

Safari on an older iPhone will not play it — Safari's WebM support arrived late and varies by version. For the widest playback, serve the MP4 alongside the WebM and let the browser choose.

Transparency is missing — MP4 sources have no alpha channel to begin with, so the WebM has none either.

When not to convert MP4 to WebM

If you just need a video to play everywhere, keep the MP4 — it is the more compatible format of the two.

If WebM is needed only to shrink a web video, consider that H.264 MP4 at a lower quality setting in the Video Compressor may well come out smaller than VP8 WebM.

Your MP4 is never uploaded

Video is where uploading hurts most. A typical online converter makes you send the whole MP4 — often hundreds of megabytes of family footage or work recordings — to its server before anything happens.

Here the work happens where the file already is. A WebAssembly build of ffmpeg in the page reads the MP4 locally — it is mounted rather than copied, so even large files do not have to be duplicated in memory — and writes the WebM on your device.

The single download involved is the 32 MB engine on first use. Your video never appears in any network request, and nothing is kept once you close the tab.