MP4 to WebM Converter

How to convert MP4 to WebM

1. Choose the video above. MP4, MOV, M4V and MKV all work as long as this browser can decode the picture; the file card shows the resolution and frame rate it found.
2. Pick a quality: High (larger file), Balanced or Small file. Balanced is selected to start with.
3. Press "Convert to WebM" and keep the tab open while the progress bar fills.
4. Download the WebM. The result card names the video codec that was used (VP9, VP8 or AV1) and the sound codec.

What happens to the picture and sound

Going from MP4 to WebM is never a quick rewrap. MP4 files almost always carry H.264 or HEVC pictures with AAC sound, while WebM only allows the open codecs VP8, VP9 and AV1 for video and Opus or Vorbis for audio. So every frame is decoded and compressed again with the browser's own WebCodecs encoder, on your computer.

Video codec

VP9 when this browser can encode it at your video's size, otherwise VP8, then AV1

Sound

re-encoded to Opus at 128 kbps; a video without sound stays silent

Resolution and frame rate

kept as they are (an odd width or height is rounded up to an even number)

Transparency

not produced: the result is an ordinary opaque video

Because it is a full encode, speed depends on your device and on whether the browser has a hardware encoder for the chosen codec. A short clip finishes quickly; a long 1080p or 4K file can keep the tab busy for a good while. If the browser can't encode Opus sound, the tool says so instead of producing a broken file; current Chrome, Edge and Firefox on a computer can.

Choosing High, Balanced or Small

The converter first works out a bitrate that would keep the picture close to the original at its size and frame rate, then scales it:

High (larger file)

the full bitrate, for hero videos and product shots where detail matters

Balanced

about two-thirds of it, a reasonable default for most page videos

Small file

about 40 percent, for background loops and previews where a slightly soft picture is fine

A WebM is not automatically smaller than the MP4 it came from. VP9 is generally more efficient than H.264, but if your MP4 was already squeezed hard, High can still come out larger. Start with Balanced, compare the two files, and step down to Small if the page weight matters more than fine detail. The video codec explainer covers why VP9, AV1 and H.264 behave differently.

Where WebM plays

Chrome, Edge, Firefox and Opera have played WebM for years. Safari is the one people ask about: Apple's WebKit release notes say WebM with VP8 and VP9 has been supported in Safari on the Mac since version 14.1, and Safari 17.4 added it on iPhone and iPad (iOS and iPadOS 17.4). Devices stuck on older versions still need an MP4. If the converter fell back to AV1, check playback on the phones and laptops you care about, because AV1 decoding still varies by hardware.

For a website, the safe pattern is to offer both files: list the WebM source first and the MP4 second inside the video element, and each browser plays the first one it understands. Keep the MP4 you started from for that reason.

Preparing a background or hero video

• Take the sound out first if the video will autoplay. Browsers generally only autoplay muted video, and a silent file is smaller. Muting the video copies the picture and takes seconds; convert the silent copy afterwards.
• Cut it to the loop you actually need with the video trimmer. Every second you remove is a second that doesn't have to be encoded or downloaded by visitors.
• A full-width background rarely needs 4K. Resize the video to 1080p or 720p before converting and the encode is quicker and the file lighter.
• If you were about to make a GIF for a docs page or a landing page, a muted WebM loop is usually a fraction of the size of a GIF made from the same clip and keeps full colour.

When to stay on MP4

• Editing: many editors and phone galleries handle H.264 MP4 more reliably than WebM, so keep the MP4 as your working copy.
• Sharing with people: messaging apps, older iPhones and upload forms that list "MP4 or MOV" are happier with MP4.
• Round trips: each conversion re-encodes and loses a little quality. If a WebM you received won't open somewhere, WebM to MP4 goes the other way, but convert from the original whenever you still have it.
• Transparent video: alpha channels (for example from ProRes 4444 MOV exports) are a desktop job with ffmpeg. Browsers usually can't decode ProRes at all, and this tool writes opaque video.

Examples

Subtitles and captions

WebM files made here carry picture and sound only. For a web player, captions normally live in a separate WebVTT file next to the video; the guide to adding subtitles to WebM explains the options. The free subtitle editor can export that VTT, or render an MP4 with the subtitles burned in that you then bring back here for WebM.

Limits to know

• Re-encoding needs a browser with WebCodecs video encoding: current Chrome or Edge, Safari 16.4 or later, or Firefox 130 or later on a computer.
• HEVC input only works where the device can decode HEVC.
• Large results are written to the browser's private storage when it's available, otherwise built in memory, so very long 4K files may be too much for a phone or a low-memory laptop.