How to Convert MP4 to WebM — VP9, Licensing-Free Web Video

MP4 with H.264 is the universal video format — it plays everywhere. WebM is the format designed specifically for the open web — no codec patent licensing fees, native browser support, and better compression than H.264. Converting MP4 to WebM makes sense when you are embedding video in a web page and want the smallest possible file without paying for H.264 patent royalties.

This guide explains VP9 versus H.264 technically, walks through when WebM makes sense and when it does not, and shows every method from a free browser tool to FFmpeg command-line encoding.

Why WebM Exists: The Licensing Problem with H.264

H.264 (also called AVC — Advanced Video Coding) is the dominant video codec worldwide. It is excellent — efficient, hardware-accelerated on every device, and universally supported. But it comes with a catch: H.264 is covered by patents held by MPEG LA. Historically, device manufacturers, browser makers, and software companies had to pay licensing fees to include H.264 decoding and encoding capabilities.

Google responded by acquiring On2 Technologies in 2010 and releasing the VP8 (and later VP9) video codec as royalty-free and open-source. WebM is the container format that wraps VP9 video and Opus audio — the open-source alternative stack to H.264/AAC in MP4.

In 2013, MPEG LA made H.264 free for internet video that is free to end users, which reduced the licensing pressure. But VP9 and its successor AV1 remain technically superior in compression efficiency, and the ecosystem has continued developing around them.

VP9 vs H.264 — Technical Comparison

When WebM Makes Sense (and When It Doesn't)

Use WebM for:

• HTML <video> embeds on your website: Serve WebM to browsers that support it, MP4 as fallback. Reduces bandwidth by 20–40% for web users.
• Background video loops: The autoplay, muted, looping video that plays as a page background. Every byte matters here since it loads on every page view.
• Open-source projects and platforms where licensing matters: If you are building a platform that cannot or will not pay H.264 licensing fees, WebM is the answer.

Do not use WebM for:

• Downloads for users: Most video players on computers and phones (Windows Media Player, VLC on mobile, QuickTime) do not support WebM. Recipients would need to install VLC or another player.
• Social media uploads: YouTube accepts WebM, but Instagram, TikTok, Twitter/X, and Facebook all require or strongly prefer MP4 H.264.
• Sharing via messaging apps: WhatsApp, iMessage, and most messaging platforms expect MP4.

Method 1: Convertlo — Free Browser Conversion

Method 2: FFmpeg CLI — Fast, Precise Control

# Basic MP4 to WebM with VP9 (CRF 31 = good quality, ~60% smaller than H.264)
ffmpeg -i input.mp4 -c:v libvpx-vp9 -crf 31 -b:v 0 -c:a libopus output.webm

# Higher quality (CRF 24 = excellent quality)
ffmpeg -i input.mp4 -c:v libvpx-vp9 -crf 24 -b:v 0 -c:a libopus output.webm

# Faster encoding with lower quality (good for testing)
ffmpeg -i input.mp4 -c:v libvpx-vp9 -crf 31 -b:v 0 -speed 4 -c:a libopus output.webm

# Two-pass encoding for target bitrate (best quality at specific size)
ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 1M -pass 1 -f webm /dev/null
ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 1M -pass 2 -c:a libopus output.webm

# Fix libvpx even-dimension requirement (always include for safety)
ffmpeg -i input.mp4 -c:v libvpx-vp9 -crf 31 -b:v 0 \
  -vf "scale=trunc(iw/2)*2:trunc(ih/2)*2" -c:a libopus output.webm

The CRF (Constant Rate Factor) for VP9 ranges from 0 (lossless) to 63 (worst quality). Values 28–36 are typical for web video. Lower CRF = better quality but larger file. The -b:v 0 flag enables pure CRF mode (variable bitrate controlled by quality target). The even-dimension scale filter is required because libvpx requires even width and height.

HTML <video> Element — Serve Both WebM and MP4

The correct way to use WebM on a web page is with the <video> element's multiple source capability — the browser picks the first format it supports:

<!-- Serve WebM first (smaller), MP4 as fallback -->
<video width="800" height="450" controls preload="metadata">
  <source src="video.webm" type="video/webm">
  <source src="video.mp4" type="video/mp4">
  Your browser does not support HTML5 video.
</video>

<!-- Autoplay background loop (muted required for autoplay) -->
<video autoplay loop muted playsinline>
  <source src="background.webm" type="video/webm">
  <source src="background.mp4" type="video/mp4">
</video>

Browsers that support WebM (Chrome, Firefox, Edge, Safari 14.1+) load the .webm file and save 20–40% bandwidth. Safari older than 14.1 and Internet Explorer fall back to the MP4 source. The playsinline attribute is required on iOS to prevent the video from opening in fullscreen mode.

Frequently Asked Questions