Video Converter — to MP4
Convert MOV, MKV, WEBM and AVI into a widely-playable MP4. Fast lossless remux when possible, full re-encode when needed. No upload, works offline — your video stays on your device.
Runs entirely in your browser — your video is never uploaded.
- No upload Your video never leaves your device — it is processed locally.
- Works offline After the first load it runs with no internet connection.
- No size or daily limits No file caps, no paywalls — only your device memory.
- Private & safe Skip sketchy upload sites; nothing is sent to a server.
How it works
Conversion is handled by ffmpeg compiled to WebAssembly and run inside your
browser. The tool first attempts a remux with -c copy and
-movflags +faststart, which simply repackages the existing video and audio
streams into an MP4 container without re-encoding them — instant and lossless when the
streams are already MP4-compatible (typically H.264 video with AAC audio). If that fails
because the codecs are not compatible, it automatically falls back to a full transcode to
H.264 video and AAC audio. The +faststart flag moves the MP4 index to the front
so the file begins playing before it has fully downloaded. Every step runs on your device —
your video is never uploaded.
How to convert a video to MP4
- Choose a video with the drop zone above, or drag a file onto it.
- Leave the box unticked for the fastest result, or tick Always re-encode to force standard H.264/AAC.
- Press Convert to MP4; the tool tries a fast remux and re-encodes only if needed.
- Click Download to save your MP4.
Processing happens on your device. Remuxing is quick; a full re-encode of a long or high-resolution video can take several minutes, especially on phones.
Why convert locally instead of on an upload site
Online video converters upload your file to a server, transcode it there, and return a link — slow over a normal connection, a privacy risk for personal footage, and frequently capped or watermarked. Converting in your browser keeps the file private, skips the upload, and adds no watermark. It also avoids the safety problem flagged by the FBI in March 2025, when the bureau warned that some free online file-converter sites were being used to spread malware. On-device conversion sidesteps that entirely.
| Local (this site) | Typical cloud tool | |
|---|---|---|
| Upload required | No | Usually |
| File-size caps | None (device memory) | Common |
| Works offline | Yes, after first load | No |
| Files stay private | Yes — never sent | Sent to a server |
Common uses
Making a screen recording or phone clip play on a device that only supports MP4, converting a MKV download into a more compatible format, turning a WEBM saved from the web into an MP4 for editing, or normalising a mixed folder of formats to one standard container. For files that are already H.264/AAC, the fast remux gets you there in seconds with no quality loss.
More video & audio tools
Frequently asked questions
Is my video uploaded to convert it?
No. Conversion runs locally with ffmpeg compiled to WebAssembly. Your file is read into the browser, converted on your device, and the MP4 is written there for download. Nothing is sent to a server and no copy is stored online.
What is the difference between fast remux and re-encode?
Many files (for example a MOV or MKV that already contains H.264 video and AAC audio) only need to be repackaged into an MP4 container. That is a remux: it copies the existing streams without decoding them, so it is nearly instant and completely lossless. When the streams are not MP4-compatible (such as VP9 in a WEBM, or an unusual codec), the tool re-encodes the video to H.264 and audio to AAC instead, which is slower but produces a widely playable MP4.
When should I tick "Always re-encode"?
Leave it off for the fastest result — the tool tries the lossless remux first and only re-encodes if that is not possible. Tick it if a remuxed file will not play in your target app, or if you specifically want the output normalised to standard H.264/AAC regardless of the input.
Why is re-encoding slow?
Re-encoding decodes every frame and compresses it again, which is heavy work — and because ffmpeg runs here as single-threaded WebAssembly rather than native multi-core code, a full transcode is noticeably slower than a desktop app. The remux path avoids this entirely, so most everyday container changes are quick. The payoff for the slower path is privacy: nothing uploads.
Is there a file-size limit?
No limit is built into the tool; your device memory is the practical ceiling because the whole file is processed in the browser. Remuxing is light even for large files; re-encoding a long, high-resolution video will use more time and memory, especially on phones.
Does it work offline?
Yes. After the first visit the app is cached and works offline, and the ffmpeg engine is cached after its one-time download, so conversions keep working with no connection.
All processing happens on Video Tools entirely within your browser. Videos are never uploaded to a server.