MOV, WebM, MKV & AVI Compressor

Why MOV, WebM, MKV and AVI files are so large

If you record video on an iPhone, a DSLR, a GoPro or a screen recorder, you often end up with enormous files — a few minutes of footage can easily run into the hundreds of megabytes. There are good reasons for that. Phones and cameras prioritise capture speed and quality over file size, so they record at high bitrates, high resolutions and high frame rates. Apple devices save .mov (QuickTime) files, screen recorders frequently produce .mkv or .webm, and older cameras and Windows tools still output .avi. Each of these can be many times larger than it needs to be for sharing or archiving.

Large videos are awkward to deal with: they exceed email and chat attachment limits, take ages to upload, fill up phone and cloud storage, and burn through mobile data. Re-encoding them with a modern, efficient codec can cut the size dramatically — often by half or more — while keeping the footage looking essentially the same. Because this tool runs entirely on your device, you get those savings without ever handing your personal recordings to a third-party server.

Privacy is the reason that last point matters so much. Home videos, confidential screen recordings, footage of your kids, product demos you haven’t announced yet — these are not things you want sitting on an unknown company’s servers, even temporarily. Conventional online video compressors upload your entire file, process it remotely, and trust you to believe it was deleted afterwards. Here, your video is read into your browser’s memory, transcoded with WebAssembly, and handed straight back to you. No network request carries the footage, so there is nothing to leak, log or retain.

Container vs codec: what actually determines the size

A common point of confusion is the difference between a container and a codec. The file extension — .mov, .mkv, .webm, .avi, .mp4 — names the container: a wrapper that bundles the video stream, audio stream, subtitles and metadata into one file. The codec is the algorithm that actually encodes the picture and sound inside that wrapper. The same container can hold many different codecs, which is exactly why two .mkv files can have wildly different sizes and compatibility.

That separation matters because the codec, not the container, drives most of the file size. A MOV from an iPhone might use the efficient HEVC codec, while an AVI from an old camcorder could use a dated, bloated one. Containers also differ in support: MKV and WebM are flexible and open but aren’t played natively everywhere, and AVI is largely a legacy format. You can read a clear breakdown of how the common web containers relate to their codecs in MDN’s media container formats guide.

In practical terms, this is why simply renaming a file’s extension never makes it smaller and usually breaks playback: you would be relabelling the wrapper without touching the data inside. Real compression means decoding the original video stream and re-encoding it with a more efficient codec at a chosen quality level. That is exactly what happens here — your MOV, WebM, MKV or AVI is fully decoded and then rebuilt as a fresh H.264 MP4, rather than just repackaged.

Why converting to H.264 MP4 helps

This tool transcodes whatever you give it to H.264 video inside an MP4 container. There are two big wins here. First, size: re-encoding with H.264 at a sensible quality level lets you strip out the excess bitrate that cameras and recorders bake in, and you can optionally cap the resolution to shed even more. Second, and just as important, compatibility: H.264 MP4 is the closest thing to a universal video format. It plays in every modern browser, on phones and tablets, on smart TVs, and in editing software — no codec packs or conversions required on the other end.

Quality is controlled with a CRF (Constant Rate Factor) value. Lower numbers mean higher quality and bigger files; higher numbers mean smaller files with more visible compression. For most clips a CRF around 23–28 is the sweet spot, looking nearly identical to the source at a fraction of the size. If your video is already an efficiently encoded MP4, you may not gain much by re-compressing it — in that case our MP4 compressor with a higher CRF or a lower target resolution is the better lever to pull.

Tips, limits and what to expect

Everything happens in your browser using a WebAssembly build of ffmpeg, which is genuinely private but comes with one honest caveat: it runs single-threaded. That means it cannot use all your CPU cores the way a native desktop encoder can, so long or high-resolution videos take noticeably longer and can strain low-memory devices. Keep these pointers in mind for the best results:

• Favour shorter clips. A minute or two of footage compresses quickly. For very long recordings, trim to the part you actually need before compressing.
• Drop the resolution for the biggest savings. A 4K screen recording downscaled to 1080p shrinks far more than quality tweaks alone, with no visible loss on most screens.
• Use a desktop machine for heavy jobs.Large files (over roughly 150 MB) are slower on phones; a laptop or desktop with more memory handles them more comfortably.
• Pick the right tool for the job. Need a quick view of all formats in one place? Use the video compressor. Working with audio files instead? Try the audio compressor.
• Re-check the result.If the output ever comes back the same size or larger, your source was already efficiently encoded — raise the CRF or lower the resolution and run it again rather than accepting a file that didn’t shrink.