Convert MP4 to WebM
Drop an MP4 and get a WebM back, with the picture in VP9 and the sound in Opus. WebM is the web's own format, and it usually carries the same picture in a smaller file than H.264.
The result appears here once a video is open.
How to use it#
- Drop an MP4 onto the box above, or click it to choose one.
- Pick a quality. Better keeps the most detail, and Smaller makes a smaller file.
- The page says what happens to the picture and the sound. Play the WebM in the preview, then download it.
Why use WebM#
WebM was made for the web. Its codecs, VP9 for the picture and Opus for the sound, are free for anyone to use, every current browser plays them, and VP9 usually needs fewer bits than H.264 for the same picture. A video on a web page served as WebM can load faster for the same quality, and some sites and apps ask for WebM specifically.
Outside a browser it is less at home: QuickTime, the Photos app, and many editing apps will not open it. Keep the MP4 for sending to people and for editing, and use the WebM where a browser plays it. WebM to MP4 goes the other way.
What the conversion does#
The picture is decoded frame by frame and encoded again as VP9, at the same resolution and frame rate, and the sound is converted from AAC to Opus. VP9 takes longer to encode than H.264, so a long video can take longer than it plays. The progress bar shows how far along it is.
Older versions of Safari cannot make sound in any format, so there the WebM comes out without sound and the page says so before you download. Chrome, Edge, Firefox, and current Safari convert the sound as well.
Putting it on a web page#
A video element can list more than one source, and the browser plays the first one it can. List the WebM first and the MP4 after it, and every browser gets a file it plays, with the smaller WebM used wherever it is supported. Converting once from the original MP4 keeps the most detail, rather than converting a copy of a copy.
Frequently asked questions#
Does converting MP4 to WebM lose quality?
#
A little, because the picture is decoded and encoded again as VP9. At Better the difference is hard to see. The resolution and frame rate stay the same.
Which quality should I pick?
#
Better for a video people will watch full screen, Balanced for most web pages, and Smaller where loading fast matters more than fine detail. The result is never more than half as large again as the original whichever you choose. Quality greys out for a video already in VP9, AV1, or VP8, which goes into the WebM as it is.
Will the WebM be smaller than the MP4?
#
Usually at Balanced or Smaller, since VP9 needs fewer bits than H.264 for the same picture. A phone video recorded at a high bitrate shrinks a lot. One already compressed hard may come out about the same size, and the page shows the real figure before you download.
Why does my WebM have no sound?
#
Most likely it was made in an older version of Safari, which cannot encode sound. Convert it in Chrome, Edge, Firefox, or a current Safari, which turn the AAC sound into Opus.
Why is it slow?
#
Every frame is encoded again as VP9, which takes more work than H.264, and many devices have no chip for it. Your file never leaves your device, so the work is done by your own computer or phone, inside this browser tab, not on a server built for it. Every frame is decoded and compressed again: a minute of 1080p at 30 frames a second is 1,800 pictures of about two million pixels each, and 4K has four times as many pixels. The browser hands the compression to your device's video chip where it has one, but each frame still passes through the tab, which shares the processor with everything else that is open. A phone also slows itself down as it warms up, and some browsers slow a tab you switch away from, so keeping this tab in front, closing other tabs, and plugging a phone in all help.
Is my video uploaded?
#
No. The MP4 is read and converted inside this browser tab, and the WebM is handed straight to your downloads.