How do I make a WebM with a transparent background?

A WebM transparent background is a VP9 file with a real alpha channel (usually yuva420p)—what people mean by “WebM transparent alpha channel export.” Two approaches: upload here and export transparent WebM, or encode locally with FFmpeg. Common uses include fog/overlay layers and OBS-friendly clips. Your input must already have alpha, or you need background removal first; an opaque MP4 cannot invent holes.

Method 1: Using ExportAlpha

1. Upload your video (MP4, MOV, WebM, or M4V)
2. Select "Transparent" export option
3. Download the resulting .webm file
4. Verify with ffprobe to confirm yuva420p or check alpha_mode metadata

The tool processes background removal and exports with VP9 codec and yuva420p pixel format. This is the standard for web-compatible transparent WebM.

Method 2: Using FFmpeg (reference)

If you have FFmpeg installed and already have a video with alpha (e.g., ProRes 4444 MOV or image sequence), you can encode WebM locally:

ffmpeg -i input.mov -c:v libvpx-vp9 -pix_fmt yuva420p -c:a libopus output.webm

Important: This command is a reference. FFmpeg syntax and options change across versions. Check current FFmpeg and libvpx documentation for your version. This is not guaranteed to work in all environments without adjustment.

Input requirements

Your input must already have alpha or need background removal. You cannot make a video transparent by re-encoding it. Specifically:

• Good inputs: ProRes 4444 MOV, PNG sequences, green-screen footage (for keying), or any format with an existing alpha channel
• Won't work: H.264 MP4 that already has black or white filled in—transparency is already lost

If your video doesn't have alpha and needs background removal, that's a separate step. This tool handles background removal + WebM export in one workflow.

Verifying your WebM has alpha

Use ffprobe:

ffprobe -v error -select_streams v:0 -show_entries stream=pix_fmt -of default=noprint_wrappers=1:nokey=1 output.webm

Look for:

• yuva420p — standard alpha WebM
• yuva444p10le — higher quality alpha

Also test in Chrome: layer the video over a colored div using HTML/CSS. If the background color shows through, alpha is working. Don't rely on system video players—they often composite onto black, which makes correct files look broken.

Common issues

Desktop player shows black background

This doesn't mean your file is wrong. Most desktop video players composite transparent pixels onto black. Import the WebM into Chrome or an editing timeline to verify alpha.

Safari shows grey (not transparent)

Safari on macOS: docs say it can play VP9 WebM but does not composite alpha (grey background per WebKit bug #275908 — not a lab test by us). iOS WebM alpha is not verified. Your file is likely correct. See why Safari does not show WebM transparency, the full alpha compatibility matrix, and use HEVC-with-alpha for Safari instead.

FFmpeg says "Conversion failed"

Your input might not have alpha, libvpx-vp9 may not be compiled in your FFmpeg build, or the pixel format isn't compatible. Check FFmpeg's capabilities with ffmpeg -encoders | grep vp9.

Frequently Asked Questions

Can I make WebM from Premiere or After Effects?

Premiere and After Effects don't natively export WebM. You can export ProRes 4444 MOV (with alpha), then convert to WebM using FFmpeg or this tool. Don't export H.264 MP4—it won't have alpha.

What's the file size difference between ProRes and WebM?

ProRes 4444 is much larger (editorial intermediate); WebM VP9 is compressed for delivery.

Should I use VP8 or VP9?

This guide and our tests use VP9 with alpha (yuva420p). VP8 also supports alpha, but VP9 is what we encode and what the matrix covers.

Sources

• FFmpeg libvpx documentation: FFmpeg Wiki
• WebM specification: WebM Project Docs
• ExportAlpha Sources & Method