Why WebM? Open format, smaller files.
WebM is an open, royalty-free video format developed by Google for the web. The VP9 codec inside WebM delivers 30–50% better compression than H.264 at the same visual quality — meaning smaller files for the same picture quality.
WebM is natively supported in Chrome, Firefox, and Edge, making it ideal for embedding video on websites without a plugin. Audio uses Opus, also open and royalty-free, which sounds better than MP3 at the same bitrate.
Supported input formats
- MP4 (H.264 or H.265) — the most common source format.
- MOV — iPhone and Mac recordings.
- MKV — multi-track containers.
- AVI, WMV, FLV, M4V — legacy formats.
- Any format your browser can decode.
Quality presets
- High — minimal compression, closest to the original. Best for archiving or further editing.
- Balanced — the recommended setting. Cuts file size by 30–50% with quality loss that is barely perceptible.
- Small — maximum compression for sharing over slow connections or email.
How it works
Conversion runs locally in your browser. For common sources — MP4, MOV, MKV — it can use your browser's built-in encoder, which is usually fast; rarer formats fall back to ffmpeg compiled to WebAssembly with libvpx-vp9, which takes longer. No file is ever sent to a server — the entire process happens on your device, not ours, so there is no file size limit and no upload wait time either way. Everything stays 100% private, exactly as it should.
Going the other way? You probably want the reverse tool
This page is for turning something else into WebM — on purpose, for the smaller file. If instead you already have a WebM file (often a browser screen recording or a download) that won't open on a phone, in an editor, or in a messaging app, you want the opposite conversion: WebM to MP4, which fixes playback rather than shrinking size.
Where WebM actually plays
WebM has real, if uneven, support outside the browser too. It's supported natively on most Android phones, and Safari added WebM support starting in version 14.1. VLC plays WebM on every desktop platform without issue. What's genuinely inconsistent is iOS in general (outside VLC), older Smart TVs, and most desktop video editors — none of which reliably import VP9-in-WebM, which is the main reason to keep an MP4 copy around for anything you need to hand off rather than just publish to the web.