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MP4 to WebM

Convert an MP4 video into WebM, the open web video format that HTML5 players, Chrome, Firefox and Android play natively. Choose VP9 or VP8, set the quality with a slider, scale it down if you like, and compare the result side by side. The video never leaves your computer.

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Video codec

How it works: the video is encoded by FFmpeg (libvpx for VP8/VP9 and libopus for audio) compiled to WebAssembly, running in a background thread of this tab. It downloads once (about 31 MB, then cached). Your video is read straight from your disk and never uploaded. Encoding in a browser is slower than desktop software: a 1-minute 720p clip takes roughly 1 to 4 minutes on Fast.

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How to use the MP4 to WebM

  1. Choose an MP4 video or drop it on the box. Its details appear next to a preview player.
  2. Pick the codec: VP9 for the smallest file, VP8 if it must play on old browsers or devices.
  3. Set the quality with the slider (CRF 32 is a good default), and optionally lower the resolution or frame rate.
  4. Press Convert to WebM and keep the tab open. FFmpeg downloads once (about 31 MB) the first time.
  5. Compare and download: play the WebM next to the original, then press Download WebM.

MP4 vs WebM at a glance

MP4WebM
Typical video codecH.264 (AVC), HEVCVP9, VP8, AV1
Typical audio codecAACOpus, Vorbis
LicensingPatent-licensed codecsRoyalty-free (open source)
Browser supportAll browsersAll current browsers (Safari from 14.1 on macOS)
Best forPhones, editors, TVs, sharingHTML5 video on websites, Android, open platforms

Recommended VP9 settings by resolution

Constant quality (CRF) lets the encoder spend more bits on complex scenes and fewer on static ones. Starting points based on Google's VP9 encoding guidance for on-demand video:

Output heightVP9 CRFTypical average bitrate (30 fps)Opus audio
2160p (4K)15 to 2412 to 18 Mbps128 to 160 kbps
1080p31about 1.8 Mbps128 kbps
720p32about 1 Mbps96 to 128 kbps
480p33about 0.5 to 0.75 Mbps96 kbps
360p36about 0.3 Mbps64 kbps

Actual bitrates depend heavily on motion and detail: a screen recording can be a tenth of these figures, sports footage much more. WebM file size is roughly (video kbps + audio kbps) × seconds ÷ 8 kilobytes.

Using WebM on a website

  • Offer both formats so every browser gets the best one: <video controls><source src="clip.webm" type="video/webm"><source src="clip.mp4" type="video/mp4"></video>.
  • For autoplaying background loops, choose No audio, 720p or lower and 24 fps. Browsers only autoplay muted video, and a silent track saves bytes.
  • Animated GIFs converted to WebM are typically many times smaller with smoother colours.
  • To go back the other way, use WebM to MP4.

Frequently asked questions

How do I convert MP4 to WebM for free?

Choose your MP4, keep the default VP9 codec and quality, and press Convert to WebM. The video is encoded by FFmpeg with libvpx and libopus compiled to WebAssembly, entirely inside your browser, so it is free, has no watermark and nothing is uploaded.

Should I choose VP9 or VP8?

VP9 produces files roughly 30 to 50% smaller than VP8 at similar quality and is supported by every current browser, including Safari 14.1 and later on macOS. VP8 encodes faster and plays on older browsers, smart TVs and Android 4 devices.

What CRF value should I use?

CRF is a constant-quality setting: lower numbers mean better quality and bigger files. Google's VP9 guidance suggests about 31 for 1080p, 32 for 720p, 33 for 480p and 36 for 360p. Use 24 to 28 for visually lossless results, and 40 or more for small previews.

Why is converting slow?

Video encoding is heavy work, and in a browser it runs in WebAssembly on a single thread without GPU acceleration. Expect roughly 1 to 4 minutes per minute of 720p video on the Fast setting. Lowering the resolution or frame rate speeds it up a lot.

Will the WebM be smaller than the MP4?

Usually, with VP9 at default settings, especially for phone videos recorded at high bitrates. If the MP4 is already heavily compressed, the WebM can be similar or larger; raise the CRF value or reduce the resolution to make it smaller.

Does WebM support audio?

Yes. WebM uses Opus (or older Vorbis) audio. This converter encodes Opus, which is transparent for music at about 128 kbps and for speech at 64 kbps. You can also remove the audio to create silent background or loop videos for websites.

Is my video uploaded?

No. The file is read directly from your disk by FFmpeg running in your browser tab. Only the FFmpeg program itself is downloaded from a public CDN the first time.

Are all the tools on FreeToolCabin tested for quality and accuracy?

Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.

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