MP4 to WebM Converter

This Is the Direction Most People Do Not Need

Nearly everyone converting video is going toward MP4, because that is what phones, editors, TVs, and upload forms accept. Going the other way makes sense in one specific situation: you are serving video from your own website and want smaller files for the browsers that support WebM.

At matching quality, VP9 typically produces files 20–30% smaller than H.264. On a page with an autoplaying background video, that is a real bandwidth and load-time saving. The standard pattern is to offer both and let the browser pick:

<video autoplay muted loop playsinline><source src="clip.webm" type="video/webm"><source src="clip.mp4" type="video/mp4"></video>

Browsers try the sources in order, so Chrome and Firefox take the smaller WebM while anything that does not support it falls back to the MP4. Do not ship WebM alone unless you know your audience.

VP9 or VP8?

• VP9 — the modern choice. Meaningfully smaller files, supported by every current browser including Safari since 2021. Encoding is slow, especially in a browser tab.
• VP8 — the 2010 original. Larger files and dated quality, but it encodes much faster and plays in genuinely ancient browsers. Choose it when you are converting something short and want it now.

Both are paired with Opus audio here, which is the standard companion codec in WebM and outperforms AAC at low bitrates.

Expect This to Be Slow

VP9 encoding is computationally heavier than H.264, and FFmpeg.wasm runs single-threaded inside the tab. A one-minute 1080p clip can take several minutes. The tool uses the fastest VP9 settings that still produce reasonable output, but there is no way around the underlying cost.

For anything longer than a couple of minutes, encode on the desktop instead — the same command runs many times faster with all cores available.

Technical notes

• VP9: constant-quality mode at CRF 33, with the bitrate cap disabled so quality drives the result
• VP8: CRF 30 with a 1 Mbps ceiling
• Speed settings: realtime deadline and cpu-used 5, chosen so a browser tab finishes in a usable time
• Audio: Opus at 128 kbps
• Pixel format: yuv420p for maximum player compatibility
• Alpha channel: not supported here — transparent WebM requires yuva420p and a different pipeline

Where WebM Genuinely Wins

The case for WebM is narrow but real: video you host yourself and serve to browsers. Background loops on a landing page, product demonstrations, animated hero sections, and inline documentation clips all benefit, because a 25% smaller file over thousands of page loads is meaningful bandwidth.

It is worth being honest about where it does not help. A file you email to a colleague, upload to a client portal, import into an editor, or send through a messaging app should be MP4 — WebM support outside browsers is patchy enough that you will simply cause trouble.

The one place WebM has no competition is transparency. VP9 with an alpha channel lets you overlay video on a page background, something MP4 cannot do at all.

FAQ