Written by the founder of BreezeConvert — a creative director working across set design, game 3D, Unreal/VFX and AI content pipelines. More about the author.
You export a video and get asked to choose between MP4 and WebM, or you're handed a WebM that won't play in the app you need. Both are perfectly good formats, but they were built with different priorities, and knowing which serves which purpose saves a lot of "why won't this play" frustration. Here's the practical breakdown.
First, what these formats actually are
MP4 and WebM are containers — wrappers that hold a compressed video stream, an audio stream, and metadata together in one file. The container isn't the whole story; what's inside (the codec) matters just as much. MP4 almost always carries H.264 video with AAC audio — the most universally supported combination in existence. WebM carries VP8/VP9 (or newer AV1) video with Opus/Vorbis audio, an open, royalty-free stack developed largely for the web.
MP4 — the universal safe default
If you only remember one thing: MP4 (H.264) plays essentially everywhere. Phones, TVs, editing software, social platforms, messaging apps, old and new browsers — all of it reads MP4 without complaint. That near-perfect compatibility is why MP4 is the right default whenever you're sending a video to someone, uploading it to a platform, or aren't sure what the other end supports. The trade-off is that H.264 is a slightly older codec, so at the same quality an MP4 can be a little larger than a modern WebM.
WebM — built for the web
WebM's strengths are efficiency and openness. Its VP9 and AV1 codecs compress better than H.264, so a WebM can be smaller at the same quality, and because it's royalty-free it's popular for web streaming and background videos on websites. Modern browsers play it natively. The catch is compatibility outside the browser: many desktop players, editing tools, social platforms, and older or Apple devices handle WebM poorly or not at all. So WebM is excellent on a web page you control and awkward almost everywhere else.
The decision, in one line
Sending a video to a person, uploading to a platform, or editing it? Use MP4 — it just works. Embedding video directly on a website and optimising for size? WebM is worth it (ideally with an MP4 fallback for older visitors). Handed a WebM that won't open in your player or editor? Convert it to MP4 and the problem disappears.
Converting between them
Our video converter goes both ways — WebM to MP4 for compatibility, or MP4 to WebM to shrink a web video — and it runs entirely in your browser using a WebAssembly build of FFmpeg, so the clip is never uploaded. Because browser encoding is CPU-heavy, it's best for short clips. And if your goal is simply a smaller file to email or upload rather than a format change, the video compressor re-encodes to a smaller MP4 directly. For fitting a clip under an attachment limit, see the email size guide.