MyMediaTools

Convert MP4 to WebM — Free Online Tool

Convert MP4 files to WebM using VP9 video and Opus audio encoding — the open, royalty-free format natively supported by Chrome, Firefox, and Edge for HTML5 video. Ideal for web developers embedding video without codec licensing concerns or plugin dependencies.

FFmpeg Command

Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg

Free — no uploads, no signups. Your files never leave your browser.

Estimated output:

Conversion Complete!

Download

How It Works

MP4 files most commonly contain H.264 or H.265 video with AAC audio, neither of which is compatible with the WebM container. This means full re-encoding is required: the video stream is transcoded from H.264/H.265 to VP9 using libvpx-vp9, and the audio is transcoded from AAC to Opus using libopus. VP9 typically achieves 30–50% better compression than H.264 at equivalent visual quality, so your output WebM file will often be noticeably smaller than the source MP4. The re-encoding process is computationally intensive compared to a simple remux, so processing time scales with video length and resolution. Subtitle tracks and chapter metadata are preserved in the WebM container, though some advanced MP4 metadata fields have no WebM equivalent and will be dropped.

What Each Flag Does

Flag What it does
ffmpeg Invokes the FFmpeg command-line tool. In this browser-based tool, the equivalent FFmpeg.wasm binary runs locally in your browser via WebAssembly — no data leaves your machine.
-i input.mp4 Specifies the source MP4 file as the input. FFmpeg reads the container and identifies the internal streams — typically H.264 or H.265 video and AAC audio — which will each need to be re-encoded to be compatible with the WebM container.
-c:v libvpx-vp9 Sets the video encoder to libvpx-vp9, the open-source VP9 encoder developed by Google. VP9 is the standard video codec for WebM and offers significantly better compression efficiency than the H.264 codec typically found in MP4 files.
-c:a libopus Sets the audio encoder to libopus, which transcodes the AAC audio from the source MP4 into the Opus codec. Opus is the preferred audio format for WebM and generally outperforms AAC in audio quality per kilobit, especially at bitrates below 128k.
-crf 33 Sets the Constant Rate Factor for VP9 video quality. A value of 33 is the default for this tool and targets a good balance of visual quality and file size. VP9's CRF scale runs from 0 (best quality, largest file) to 63 (lowest quality, smallest file), unlike H.264's 0–51 range.
-b:a 128k Sets the Opus audio bitrate to 128 kilobits per second. Opus at 128k delivers excellent stereo audio quality — it is widely considered to be perceptually transparent for most content at this rate, making it a practical default for web video delivery.
-b:v 0 Forces libvpx-vp9 to operate in pure constant-quality CRF mode by disabling the default target bitrate constraint. Without this flag, FFmpeg's VP9 encoder ignores the -crf setting entirely and applies its own bitrate-based mode, so this flag is required for CRF to function as intended.
output.webm Specifies the output filename with the .webm extension. FFmpeg uses this extension to automatically select the WebM container format (based on Matroska), which is the correct container for VP9 video and Opus audio streams.

Common Use Cases

  • Embedding video on a website or web app without H.264 licensing obligations — WebM with VP9 is royalty-free and plays natively in all major modern browsers
  • Reducing file size of MP4 recordings for web delivery, taking advantage of VP9's superior compression efficiency over H.264
  • Preparing video assets for a Progressive Web App or Electron app that targets Chromium-based runtimes where WebM/VP9 is the preferred format
  • Converting MP4 screencasts or tutorial videos for hosting on platforms like itch.io or open-source project pages that favor open formats
  • Producing WebM video to use with the HTML5 <video> tag alongside an MP4 fallback, following the best-practice dual-source pattern for maximum browser compatibility
  • Archiving video content in an open, patent-unencumbered format to avoid long-term dependency on proprietary codec licensing

Frequently Asked Questions

Yes, some generation loss is inevitable because this conversion requires re-encoding from H.264 or H.265 to VP9 — two different compression algorithms. The default CRF value of 33 for VP9 targets a good balance of quality and file size, but it is not mathematically lossless. For most web video content the quality difference is visually indistinguishable, but if you need the highest possible fidelity you can lower the CRF value (e.g., to 20 or 16) in the FFmpeg command at the cost of a larger output file.
VP9 is a more efficient codec than the H.264 that most MP4 files contain. Google's own benchmarks show VP9 compressing video to roughly half the file size of H.264 at comparable visual quality. So even after a full transcode, the resulting WebM can be 20–50% smaller than the source MP4, making it attractive for web delivery where bandwidth and load times matter.
Yes. The WebM container, based on Matroska, supports subtitle tracks (using WebVTT format) and multiple audio tracks, so these will be carried over from your MP4 during conversion. However, MP4 subtitle formats such as MOV text or CEA-608 captions embedded in the video stream may need to be converted to WebVTT to be stored correctly in the WebM container. Chapter markers are also preserved.
Chrome, Firefox, Edge, and Opera all support WebM with VP9 natively without any plugins. Safari added VP9 support in version 14 (macOS Big Sur and iOS 14), so it is now broadly supported across all major browsers. Android has supported VP9 hardware decoding on most devices since Android 5.0. Apple TV and older Safari versions remain the most notable exception areas, which is why serving both a WebM and MP4 source in your HTML5 video tag is still considered best practice for maximum compatibility.
To change video quality, modify the -crf value: lower numbers (e.g., -crf 20) mean higher quality and larger files, while higher numbers (e.g., -crf 40) produce smaller files with more compression artifacts. VP9 CRF ranges from 0 to 63. To change audio bitrate, replace the -b:a 128k value with your desired rate, for example -b:a 192k for higher-quality Opus audio or -b:a 96k to reduce file size. The -b:v 0 flag must stay in the command — it tells VP9 to use pure CRF mode rather than a target bitrate, which is required for CRF to work correctly with libvpx-vp9.
Yes. On Linux or macOS you can run a one-line shell loop: for f in *.mp4; do ffmpeg -i "$f" -c:v libvpx-vp9 -c:a libopus -crf 33 -b:a 128k -b:v 0 "${f%.mp4}.webm"; done. On Windows Command Prompt use: for %f in (*.mp4) do ffmpeg -i "%f" -c:v libvpx-vp9 -c:a libopus -crf 33 -b:a 128k -b:v 0 "%~nf.webm". The browser-based tool processes one file at a time, so for large batch jobs running the command locally is the more practical approach.

Technical Notes

VP9 encoding via libvpx-vp9 is significantly more CPU-intensive than H.264 encoding, so conversion times for long or high-resolution MP4 files will be longer than you might expect — a 1080p60 video can take several times its playback duration to encode, even on modern hardware. The -b:v 0 flag is not optional boilerplate: without it, libvpx-vp9 in FFmpeg ignores the -crf setting and falls back to a default target bitrate mode. Opus audio is generally superior to AAC at equivalent bitrates, so the transcoded audio in your WebM will often sound as good or better than the source despite being re-encoded. WebM supports alpha channel transparency in VP9 video streams, which MP4/H.264 does not — if you have a source with transparency encoded in a format like VP9 inside an MKV, WebM is the appropriate delivery container. One metadata limitation to be aware of: MP4 iTunes-style tags (artist, album, comment fields) are not part of the WebM specification and will not be transferred to the output file. Standard creation-date and title tags may be preserved depending on how they were authored in the source MP4.

Related Tools

Convert MP4 to WMA Convert MP4 to TS Convert MP4 to M4V Convert 3GP to WebM Convert SWF to WebM Convert AVI to WebM Compress MP4 Extract Audio MP4 to MP3