Convert · runs on your device

Convert MP4 to WebM in your browser

MP4 to WebM is always a real re-encode — WebM only accepts VP8, VP9 or AV1 video, so H.264 has to be decoded and compressed again. Video Mender does that here in the browser and shows genuine progress while it works.

Nothing is uploaded

Drop video files here

MP4 · M4V · MOV

Choose video files to convert

Output format

This page always writes WebM.

How to size the output

A constant quality target. File sizes vary with how demanding the footage is.

Quality — 6 of 10

Resolution limit

Files smaller than the limit are left alone. Capping usually looks better than lowering the bitrate.

Frame rate

Audio

Fast rewrapCopy streams without re-encoding when the codecs already fit

Lossless and near-instant when it applies. It cannot reduce file size, so turn it off if you need a smaller file.

WebM is a deliberately narrow container. Where MP4 will carry H.264, HEVC and AAC, WebM permits VP8, VP9 or AV1 video with Opus or Vorbis audio and nothing else. There is no shortcut: converting MP4 to WebM means decoding every frame and encoding it again in a different codec.

That has a practical consequence worth knowing before you start. Re-encoding takes real time — think minutes per minute of footage, not seconds — and it always costs a little quality. It is also unlikely to save space: VP8 is roughly as efficient as H.264, so expect a WebM of similar size to the MP4. Convert because you need the format, not to shrink the file.

One honest limitation to set expectations. WebM output here is VP8, not VP9. VP9 would produce smaller files, but the VP9 encoder in this browser build crashes, and shipping a codec that fails halfway through your queue would be worse than not offering it. VP8 encodes reliably and plays anywhere WebM plays at all. Audio is Opus, which is efficient and supported everywhere WebM itself is.

What this actually does to your file

Rewrap or re-encode

There is no rewrap path for MP4 to WebM in either direction. The codecs the two containers accept do not overlap at all, so the fast-copy badge never appears on this page.

Quality

This conversion re-encodes, so it is lossy by definition. Generation loss is real but modest at the Balanced preset and above; the Smallest preset is visibly softer because it also caps resolution at 480p. VP8 is about as efficient as H.264, so a matched-quality WebM is a similar size rather than smaller.

VP8 encoding in WebAssembly is slower than H.264 and runs on one thread. A 10-minute 1080p clip is a coffee break, not a moment.
Safari support for WebM is partial and version-dependent. If Apple devices are part of your audience, keep an MP4 as well.
Most video editors will not import WebM. It is a delivery format, not an editing format.

How to use this page

Add your MP4 files. MOV and M4V work here too.
Set a quality preset. The codec is VP8 with Opus audio — the combination this build can write reliably.
Set a quality preset, or cap the resolution if the source is larger than you need.
Start the queue and download each WebM as it finishes.

Where the result plays

Checked against the containers and codecs this page produces, stated conservatively.

Target	Works	Detail
Chrome, Edge, Firefox	Yes	VP8 and Opus play natively, including in the <video> element.
Safari and iOS	Partly	Support exists in recent versions but has never been dependable across the range of devices in use.
Video editors	No	Premiere, Final Cut and Resolve generally refuse WebM. Convert back to MP4 for editing.
Smart TVs	No	Assume WebM will not play on TV hardware or consoles.

The formats involved

Format	Read	Write	What it is
MP4	Yes	Yes	The MPEG-4 Part 14 container. It is a wrapper, not a codec — an MP4 usually holds H.264 or HEVC video with AAC audio.
M4V	Yes	Yes	Apple's MP4 variant used by iTunes. Byte-for-byte it is an MP4; the extension exists so iTunes can flag optional FairPlay DRM.
MOV	Yes	Yes	Apple QuickTime. Structurally a close cousin of MP4, which is why MOV files can often be rewrapped to MP4 with no re-encode.
WebM	Yes	Yes	A deliberately restricted Matroska profile for the web: VP8/VP9/AV1 video with Opus or Vorbis audio, and nothing else.

Tested limits

Real ceilings measured on real devices, not a marketing claim of unlimited.

Limit	Desktop	iPhone / iPad	Why
File size — rewrap	1.3 GiB	384 MiB	Rewrapping never decodes a frame, so it tolerates far larger files.
File size — re-encode	1.0 GiB	256 MiB	Decoding holds uncompressed frames in memory, which is the real constraint.
Clip length — re-encode	4:00:00	30:00	Beyond this, progress reporting and memory behaviour stop being dependable.
Resolution — re-encode	4096 × 2160	1920 × 1080	A 4K frame is about 12 MB decoded, before the encoder allocates anything.

If something goes wrong

Encoding is extremely slow.

That is VP8 in WebAssembly on a single thread, not a stall — check that the progress bar and the elapsed time are still moving. Capping the resolution to 720p cuts the time substantially.

The WebM is larger than the MP4.

Expected more often than not, because VP8 is no more efficient than the H.264 in your MP4. Re-encoding cannot recover information, so ask for fewer bits: use a lower preset or switch to target-size mode in the compressor.

The WebM will not play on my iPhone.

Expected. Apple support for WebM is partial. Keep the original MP4 for Apple devices and use the WebM for browsers where it plays.

Your video never leaves this device

This is a structural property of the application, not a policy we ask you to take on trust. There is no upload endpoint, no API route and no server action anywhere in the codebase, so there is nowhere for a video to be sent even by mistake.

Conversion runs in WebAssembly inside this browser tab. The engine is served from this domain and loaded only when you start a job.
The Content-Security-Policy restricts network connections to this origin, so the page is not permitted to contact a third party at all.
Nothing is stored. Close the tab and every queued file and finished output is gone — which is also why you should download results before you leave.
There are no accounts and no AI. Anonymous page views are counted — which page, roughly what country, what kind of device — and nothing whatsoever about your files.

The repository contains a test that fails the build if any media byte is passed to fetch, XMLHttpRequest, sendBeacon or a WebSocket. See the privacy page for how that is verified.

Questions

Why can MP4 to WebM not be done without re-encoding?

Because containers can only carry codecs their specification allows. WebM permits VP8, VP9 and AV1 video; MP4 files almost always contain H.264 or HEVC. With no codec in common, the frames must be decoded and compressed again.

Why VP8 rather than VP9?

Because VP9 would fail. The VP9 encoder in this browser build crashes with a memory fault, verified on both Chrome and Safari, so it is not offered at all. VP8 is less efficient but encodes reliably and plays wherever WebM does.

Is WebM better than MP4?

Neither is better in the abstract, and with VP8 the WebM will not be smaller. WebM is royalty-free and is what some web tooling expects; MP4 with H.264 plays on far more devices. Choose by where the video needs to play.

Can I convert WebM back to MP4 here?

Yes, through the video compressor with MP4 as the output container, or on the WebM compressor page. That direction is also a full re-encode, for the same codec reason.

Does the audio quality drop too?

The audio is re-encoded to Opus, so yes, slightly. At the default 160 kbps the difference is inaudible in practice; Opus is unusually good at low bitrates.