WebM to MP4 Converter

How to use it#

Drop a WebM file 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 MP4 in the preview, then download it.

Why a WebM will not open everywhere#

WebM is the web's own video format. Browsers record screen captures in it, and many sites serve their clips in it, with the picture in VP9, AV1, or VP8 and the sound in Opus. Every current browser plays it, but plenty of other software does not: QuickTime, the Photos app, iMovie, older Windows players, many editing apps, and upload forms that ask for an MP4.

An MP4 holding H.264 video and AAC sound is the one combination that plays everywhere, so that is what this makes. Both tracks have to be converted, because an MP4 player cannot be counted on to play VP9 or Opus even when the file claims to be an MP4.

Time, quality, and size#

Converting the picture means decoding every frame and encoding it again, which takes roughly as long as the video plays, often less. Each re-encode loses a little detail, so Better is the default, and Balanced or Smaller trade some of that for a smaller file.

H.264 needs more bits than VP9 or AV1 for the same picture, so the MP4 can come out a little larger than the WebM. The size is kept to at most half as large again as the original, and the page shows the real figure before you download it. To make the MP4 smaller than the WebM, use compress video, which also lowers the resolution.

The sound, and what depends on your browser#

The Opus sound is converted to AAC in Chrome, Edge, and current Safari. Firefox on a Mac cannot make AAC, so there the Opus sound is copied into the MP4 as it is. That MP4 plays with sound in browsers and on Android, but an iPhone and QuickTime play it silently, and the page says so before you download. For an iPhone, convert the file in one of the other browsers.

Screen recordings made in a browser often lack the index a player uses to jump to a point in the video, which is why some of them cannot be scrubbed. The MP4 is written with a full index, so it can.

Frequently asked questions#

Does converting WebM to MP4 lose quality?

A little, because the picture is decoded and encoded again as H.264. At Better the difference is hard to see. Converting once from the original WebM is always better than converting a copy that has already been converted.

Which quality should I pick?

Better, unless the file size matters. Balanced gives a noticeably smaller file that still looks clean, and Smaller suits a clip being sent in a message. The result is never more than half as large again as the WebM whichever you choose.

Why does my MP4 have no sound on an iPhone?

It was most likely made in Firefox on a Mac, which cannot create AAC sound, so the WebM's Opus sound was copied into the MP4 as it was. iPhones and QuickTime do not play Opus in an MP4. Convert the WebM in Chrome, Edge, or Safari, which turn the sound into AAC.

Why is the MP4 bigger than the WebM?

VP9 and AV1 squeeze a picture into fewer bits than H.264 can, so the same picture takes more room as H.264. Pick Balanced or Smaller, or use compress video to lower the resolution as well.

Why is it slow?

The picture in a WebM is VP9 or VP8, so every frame is encoded again as H.264. 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 WebM is read and converted inside this browser tab, and the MP4 is handed straight to your downloads.