What does Video Compressor do?
Compress video in your browser to H.264 MP4. Choose a quality preset or a target size in MB, downscale to 1080p, 720p or 480p, and compare sizes.
- Price
- Free
- Account
- Not required
- Processing
- Entirely in your browser; your data and files are not uploaded
- Works on
- Any modern browser on desktop, tablet or phone
- Category
- Video and Audio Tools
How to use the video compressor
- Drop a video (MP4, MOV, WebM, MKV or AVI, up to 300 MB).
- Choose Quality preset for a constant visual quality, or Target file size and enter the size you need in MB.
- Optionally lower the resolution to 1080p, 720p or 480p, pick the audio bitrate, or remove the audio.
- Click Compress video. When it finishes, check the preview, compare the sizes and download the MP4.
Worked example
Fitting a 60 second clip into 25 MB
25 MB is 25 x 1,048,576 x 8 = 209,715,200 bits. Over 60 seconds that is 3,495 kbps in total. The tool keeps 3% for container overhead and subtracts 128 kbps of audio, so it asks the encoder for 3,262 kbps of video. The page shows the estimated size before encoding and the real size afterwards.
How it works
Video is re-encoded with libx264 (H.264) into an MP4 with AAC audio. Quality presets use constant rate factor: -crf 30 (small), 26 (balanced) or 21 (high quality). Target size mode computes video kbps = target MB x 1,048,576 x 8 / 1000 / duration x 0.97 - audio kbps and encodes with -b:v, a 1.5x -maxrate and a 2x -bufsize, optionally in two passes. Downscaling sets the shorter side to 1080, 720 or 480 pixels, keeping the aspect ratio with even dimensions. -movflags +faststart lets the MP4 start playing before it fully downloads.
Assumptions
- The target size is an average-bitrate goal. Single-pass results can land a few percent above or below it; two-pass is usually closer.
- 1 MB here means 1,048,576 bytes, as shown by Windows and this site.
- Audio is always re-encoded to AAC at the bitrate you choose, so the size maths holds.
- The duration comes from the file; if it has none (some screen recordings), the tool reads the whole file once to measure it.
Frequently asked questions
Are my files uploaded?
No. The file is processed by FFmpeg compiled to WebAssembly, running inside your browser. Only the engine itself is downloaded (once, from cdn.jsdelivr.net); your video, audio or images never leave your device.
Why is my compressed video bigger than the original?
If the original is already efficiently compressed at a low bitrate, a quality preset can produce a larger file. Try the Small file preset, a lower resolution, or target size mode with a number below the original size.
What is CRF?
Constant rate factor is the x264 quality setting. Lower numbers mean higher quality and bigger files; each step of about 6 roughly halves or doubles the bitrate. The final size depends on how complex the video is, so it cannot be predicted in this mode.
Faster or slower encoding speed?
The slower x264 preset (medium) finds better compression and gives a smaller file at the same quality, but takes noticeably longer in the browser. Faster (veryfast) is the default because in-browser encoding is already slow.
Why does the first run take longer?
The first time you use any of the video and audio tools, your browser downloads the FFmpeg engine, about 31 MB. It is stored in your browser, so later runs on this device start without downloading it again unless you clear site data.
Limitations
- FFmpeg runs as single-threaded WebAssembly in the browser, which is much slower than desktop FFmpeg. Processing time depends on the file length, resolution and your device; long HD videos can take many minutes.
- Browsers limit how much memory a page can use. Very long or high resolution files can fail with an out of memory message, especially on phones and tablets.
- Files up to 300 MB are accepted.
- Output is always MP4 (H.264 video, AAC audio). Subtitles and extra audio tracks are not kept.
- Very small targets for long videos are rejected because the picture would be unwatchable; the tool shows the minimum size for your clip.