WEBM is Google's open video format, designed for web use. It uses VP8, VP9, or AV1 video codecs with Opus or Vorbis audio. Chrome, Firefox, and Edge support WEBM natively. The format is common in several contexts:
- Browser-based screen recorders (many produce WEBM by default)
- Downloaded web videos (some sites serve WEBM)
- WebRTC recordings (video calls recorded in the browser)
- Canvas/WebGL captures from web applications
The problem: WEBM does not play on many devices and applications outside the browser. iPhones cannot play WEBM files in the native Photos app. Windows Media Player does not support WEBM without codec packs. Many email clients, messaging apps, and social platforms reject WEBM uploads.
Converting to MP4 solves all of these compatibility issues.
The conversion process
WEBM to MP4 requires re-encoding because the codecs are different. VP8/VP9 (WEBM's video codecs) must be decoded and re-encoded as H.264 (MP4's standard codec). Opus/Vorbis (WEBM's audio codecs) must be decoded and re-encoded as AAC (MP4's standard audio codec).
This re-encoding takes longer than a simple remux because every frame must be processed. For a 5-minute screen recording, expect 30-90 seconds of processing time depending on resolution and your device's speed.
Quality during conversion
The re-encoding introduces one generation of quality loss. For screen recordings (sharp edges, flat colors, limited color palette), the loss is completely invisible. Screen content compresses so efficiently that the H.264 encoder at CRF 23 produces output that is indistinguishable from the WEBM original.
For photographic or cinematic content, use CRF 18-20 to preserve maximum detail during the format conversion.
File size changes
VP9 (the most common WEBM video codec) is more efficient than H.264. Converting VP9 WEBM to H.264 MP4 at the same quality level typically produces a file that is 20-40% larger. This is the trade-off for universal compatibility.
If file size is a concern, you can increase the CRF value slightly (from 23 to 26) to offset the efficiency difference. The file size will be comparable to the original WEBM at a quality level that is still excellent.
Screen recording specific tips
Screen recordings are the most common WEBM-to-MP4 conversion. Before converting, consider:
1. Trim the recording to remove the setup/teardown at the start and end 2. Crop to the region of interest if you recorded the full screen but only need a portion 3. Reduce frame rate if the recording is 60fps but the content does not need it (most UI interactions look fine at 30fps)
These optimizations reduce the file size and make the MP4 more practical for sharing. The MediaBrew format conversion tool handles the WEBM-to-MP4 re-encode in your browser using FFmpeg WebAssembly, producing a universally playable file for Slack, documentation, or issue trackers.