How video compression works
A video is a fast series of still pictures, usually 24 to 60 every second. Stored raw, one minute of 1080p video would take several gigabytes. Codecs such as H.264, VP9 and AV1 shrink that by keeping one full picture every few seconds and, for the frames in between, storing only what moved or changed.
The number that decides the file size is the bitrate: how many bits the encoder may spend on each second of video. Fewer bits means a smaller file, but also less detail, especially in fast motion, water, leaves and dark scenes.
This tool decodes your video and encodes it again with your browser’s built-in video engine (WebCodecs). It all happens on your device. The file is read in small pieces straight from your disk and is never uploaded.
Two ways to compress
- Quality: pick a preset and let the encoder spend what each scene needs. Simple scenes get small, busy scenes get more bits. Best when you just want a smaller file that still looks good.
- Target size: type the size you need, for example 24 MB for an email or 9 MB for a 10 MB chat limit. The tool turns that into a bitrate using the length of the video:
The 0.94 keeps 6% in reserve for the file’s index and for encoder overshoot. If the first pass still comes out too big, the tool measures how far off it was and runs one corrected pass.
Worked example: a 2-minute clip under 25 MB
You have a 2-minute (120-second) 1080p phone video of 180 MB and need it under 25 MB.
- Budget: 25 MB = 26,214,400 bytes = 209,715,200 bits. Keep 94%: 197,132,288 bits.
- Per second: 197,132,288 ÷ 120 ≈ 1,643,000 bits per second, about 1.6 Mbps.
- Audio gets about 10% of that, capped at 128 kbps, so video gets about 1.5 Mbps.
At 1.5 Mbps, 1080p looks soft in busy scenes. If you also set 720p, the same bits are spread over fewer than half as many pixels, so each frame looks sharper. That is why lowering resolution is usually better than squeezing the bitrate.
If the clip were 10 minutes long, 25 MB would leave only about 0.3 Mbps. The tool tells you when a target is too small and suggests the smallest size that still gives a watchable picture.
How much does each setting save?
| Change | Pixels or frames kept | Typical effect on size |
|---|---|---|
| 4K → 1080p | 25% of the pixels | Large saving |
| 1080p → 720p | 44% of the pixels | Large saving |
| 720p → 480p | 44% of the pixels | Large saving, visibly softer |
| 60 fps → 30 fps | 50% of the frames | Noticeable saving |
| Remove audio | No sound | Saves about 1 MB per minute at 128 kbps |
| H.264 → VP9 or AV1 | Same | Smaller at the same quality, slower to make |
Savings depend on the content. A talking head against a plain wall compresses far more than a football match.
Choosing a format
- MP4 (H.264) plays everywhere: iPhone, Android, WhatsApp, email, PowerPoint and video editors. Choose it when in doubt.
- WebM (VP9) gives smaller files at the same quality and is ideal for websites, but some apps and older iPhones cannot play it.
- MP4 (AV1) is the most efficient, but encoding is slow in most browsers and older devices cannot play it.
The tool checks which encoders your browser has and marks the ones that are not available. If you only need a different format, not a smaller file, use the Video Converter, which can often switch containers without re-encoding at all.
Tips and edge cases
- Trim first. Cutting the parts you do not need is the cheapest saving of all. Use Trim Video, then compress.
- Re-compressing loses quality. Every lossy encode throws away some detail, so start from the best copy you have, not one downloaded from a chat app.
- Already small files may not shrink. Videos from messaging apps are often compressed hard already. The tool says so when the result is not smaller.
- Large files need memory. The finished video is kept in memory until you download it. Over about 2 GB, phones and older laptops may run out. Close other tabs or compress in parts.
- Batch jobs run one at a time so your device stays responsive. You can cancel the current file at any point.
- Sound only? If you just need the audio track, MP4 to MP3 extracts it in seconds. For a short looping clip, try Video to GIF.
Which browsers work?
The tool needs WebCodecs, which is in Chrome, Edge and Opera 94 or newer, Firefox 130 or newer on a computer, and Safari 16.4 or newer. Encoding speed depends on your device: computers and recent phones with hardware video encoders are fastest. If your browser lacks the engine, you will see a clear message instead of a broken page.