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Kilnfile

Compress Video

Cut a video's file size by more than half without it leaving your device.

Processed on your device, nothing uploadedLast updated

To compress a video, drop it into the Compress Video tool, pick a Compression level and a Max resolution, then click Compress video; the smaller MP4 downloads from your browser. Medium (CRF 30) at 720p typically saves 50 to 80%, which brings most phone clips under WhatsApp's 16 MB warning and Gmail's 25 MB limit.

Your file stays in this tab.On your device
Options
Add several files to get a single ZIP back.

FFmpeg (about 30 MB) is downloaded once on first use and cached by your browser. Speed depends on your device.

How do I compress a video?

  1. Drop your video onto the drop zone, or click to browse. MP4, MOV, WebM, MKV and AVI are accepted.
  2. Choose a Compression level: Light (CRF 26, near original), Medium (CRF 30, good for sharing) or Strong (CRF 34, smallest).
  3. Choose a Max resolution. 720p is the default and the biggest single saving for phone footage; Keep original leaves the frame size alone.
  4. Click Compress video and wait; progress is shown as FFmpeg works through the frames.
  5. Check the new size next to the download, then download the MP4 or Download all as ZIP for a batch.

The tool re-encodes your video with the H.264 encoder libx264 using constant rate factor (CRF) quality control, downscales it if you asked for a lower Max resolution, and encodes the audio as AAC at 96 kbps. CRF is a quality target rather than a bitrate: the encoder spends bits where the picture needs them and saves them where it does not. Every step runs in your browser through FFmpeg compiled to WebAssembly, so the video is never uploaded.

Which level to pick depends on where the file is going. Light (CRF 26) is hard to tell from the original on a phone or laptop screen and typically halves a camera file. Medium (CRF 30) is the sweet spot for WhatsApp, Messenger, email and Slack; combined with 720p it usually removes 50 to 80% of the size. Strong (CRF 34) shows some softening in fine detail and motion but gets a 1 minute clip down to a few megabytes.

Resolution matters as much as CRF. A 4K phone clip has four times the pixels of 1080p and nine times those of 720p, so dropping to 720p removes most of the data before the encoder even starts. WhatsApp recompresses anything you send to roughly 720p anyway, so sending it 720p yourself loses nothing and avoids the 16 MB warning. For email the targets are 25 MB on Gmail and 20 MB on Outlook.com.

Because the WebAssembly build is single-threaded, expect about real-time to 3x real-time for 720p output on a recent laptop: a 2 minute clip takes two to six minutes. 1080p output is slower and phones are slower again, so plug in and keep the tab in the foreground. Files up to about 1 GB are fine; the 2 GB WebAssembly memory limit is the practical cap. If a clip is long, trim it first and compress only the part you need.

How it works: this tool uses FFmpeg compiled to WebAssembly (ffmpeg.wasm), loaded into your browser. No server is involved, so Kilnfile never sees your file. Last reviewed 20 September 2026.

Frequently asked questions

How much smaller will my video be?

Typically 50 to 80% smaller at Medium with 720p, and 30 to 50% at Light with the original resolution. Phone footage shrinks the most because phones record at high bitrates. The exact figure depends on the content: static talking-head video compresses far better than sport or handheld action.

How do I compress a video for WhatsApp?

Use Medium (CRF 30) and 720p, which is roughly what WhatsApp would do to the file itself. That gets most 1 to 2 minute clips under 16 MB. If it is still too large, switch to Strong or trim the clip down first.

Does compressing reduce quality?

Yes, a little; H.264 re-encoding always discards some detail. At Light and Medium the loss is hard to see on a phone or laptop screen. Keep the original file if you may need to edit it later, since each re-encode compounds the loss.

Why is it slow?

Video encoding is heavy work and the browser version of FFmpeg runs on a single CPU thread, roughly 5 to 20 times slower than desktop FFmpeg. A 720p clip runs at about real-time to 3x real-time on a laptop. Choosing 720p or 480p speeds it up considerably.

Is anything uploaded?

No. The video is read straight from your device into the browser's memory and the compressed file is written back to your downloads folder. Nothing is sent to a server, which is also why there is no file size limit set by us.

Cite this page: “Compress Video Online Free, No Upload, No Watermark”, Kilnfile, https://kilnfile.com/media/compress-video/

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