Compress · runs on your device

Compress WebM files in your browser

Add your .webm files, choose a target size or quality, and Video Mender re-encodes them in the browser — staying in WebM with VP8, or writing MP4 if you need the file to play on Apple devices.

Nothing is uploaded

Drop video files here

WebM · MKV

Choose video files to convert

Output format

Chrome, Firefox and Edge natively. Safari support is partial, and most editors reject it.

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

Platform presets — editable

These are dated observations, not permanent platform limits — all were last checked on 2026-08-01. Every one sets an ordinary target size you can edit afterwards.

WebM files usually arrive already reasonably compressed, because the format exists for web delivery and whatever produced it was aiming for small files. That sets realistic expectations: a WebM will not collapse the way a camera MOV does. Meaningful reduction here generally means lowering the resolution or accepting a visible quality step, not reclaiming waste.

The exception is browser-recorded WebM. Screen and webcam recordings made through the MediaRecorder API are encoded live at a fixed bitrate with no analysis pass, and those genuinely do compress well — often by half — with a considered re-encode.

Two things to be straight about. WebM output here is VP8, because the VP9 encoder in this browser build crashes and offering it would mean failing mid-queue; if your source is already VP9, re-encoding to VP8 will not shrink it much and MP4 with H.264 is usually the better choice. And the container is never changed without you choosing it: renaming a WebM to .mp4 produces a file no player will accept, so the output picker performs a real conversion or nothing at all.

What this actually does to your file

Rewrap or re-encode

A WebM cannot be rewrapped into MP4 — the codecs have nothing in common — so changing the container here is always a full re-encode. Staying in WebM is also a re-encode, since copying the streams would not change the size.

Quality

Lossy, and stacked on top of whatever compression the WebM already carries. Expect a visible step at aggressive settings; reducing resolution is usually kinder to the picture than starving the bitrate.

VP8 encoding in WebAssembly is slow and single-threaded. For long files, consider MP4 output with H.264, which encodes several times faster.
Already-optimised WebM files have little to give. If the size barely moves, the file was already efficient.
Safari support for WebM has never been dependable across all devices in use. Choose MP4 output if Apple playback matters.
AV1 and VP9 inside a WebM can be decoded here but not written back: the output will be VP8 or H.264.

How to use this page

Add your .webm files. MKV files also work here, since WebM is a restricted Matroska profile.
Decide the output: WebM with VP8 to stay web-native, or MP4 with H.264 for universal playback.
Set a target size or a quality preset, and cap the resolution if the source is oversized.
Start the queue and download each result as it finishes.

Where the result plays

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

Target	Works	Detail
WebM output	Partly	VP8 and Opus. Native in Chrome, Edge and Firefox; inconsistent on Safari and iOS.
MP4 output	Yes	H.264 and AAC. The choice when the file must play everywhere.
Video editors	No	WebM is generally rejected by editors. Choose MP4 output for an editing workflow.
Web pages	Yes	WebM with VP8 plays inline in every browser that supports WebM at all.

The formats involved

Format	Read	Write	What it is
WebM	Yes	Yes	A deliberately restricted Matroska profile for the web: VP8/VP9/AV1 video with Opus or Vorbis audio, and nothing else.
MKV	Yes	Yes	Matroska. The most permissive common container: it will carry almost any codec, plus multiple audio tracks, chapters and subtitles.
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.

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

The WebM hardly shrank at all.

It was already efficiently encoded, which is common for web-delivery files. Use target-size mode to force a specific reduction, or cap the resolution, and accept the visible cost.

Encoding is taking a very long time.

VP8 in WebAssembly is genuinely slow and runs on one thread. Switch the output to MP4 with H.264, which is several times faster, or cap the resolution to 720p.

The compressed WebM will not play on my iPhone.

That is a WebM limitation, not a result of compressing it. Choose MP4 as the output container for Apple devices.

Can I just rename the WebM to .mp4?

No. The container structure and codecs are completely different, and players will reject the result. Use the MP4 output option, which performs a real conversion.

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

How do I reduce WebM file size?

Re-encode at a lower bitrate or resolution. Add the file here, pick a target size in megabytes or a quality preset, and the work happens in your browser with no upload.

Why does my WebM barely compress?

Because it was probably already compressed for web delivery, so there is little waste to reclaim. Browser-recorded WebM from MediaRecorder is the exception and does compress well.

Should I keep WebM or switch to MP4?

Keep WebM if some part of your tooling expects it. Switch to MP4 if Apple devices, editors or TVs are involved, or if you want the smallest file — H.264 beats the VP8 this build writes.

Which codec should I use for the smallest file?

H.264 in MP4. VP8 — the WebM codec this build can write — is no more efficient than H.264 and is slower to encode. VP9 would be roughly 30% better but its browser encoder is not reliable, so it is not offered.

Can I compress a batch of WebM files?

Yes. Queue them all, apply one preset or override individual files, and the memory-aware scheduler works through them while showing real per-file and overall progress.