Why video files get so big
A phone records video for safety, not for sharing. A 1080p clip from a modern phone is typically written at 15 to 20 Mbit/s, which works out to around 120 to 150 MB per minute; 4K at 60 fps can pass 400 MB per minute. Those bitrates leave room for fast motion and later editing, but most of that data is invisible on a phone screen or in a chat window. Compression throws away detail the eye is unlikely to notice and stores the rest more efficiently, so the same minute can fit in a fraction of the space.
Common reasons to compress:
- Fitting under an attachment limit for email or a messenger.
- Uploading over a slow or metered connection.
- Freeing space on a phone or cloud storage without deleting memories.
- Embedding a background or product video on a website, where every megabyte delays the page.
What the presets actually do
All three presets re-encode the picture with H.264 and the sound as AAC at 128 kbit/s, and save the result as MP4. The resolution and frame rate stay the same as the source; what changes is the CRF value, a constant-quality setting where a higher number means fewer bits and more visible compression.
In the browser, the presets map to CRF 28 for Light, 32 for Medium and 38 for Heavy, encoded with the fastest H.264 preset so the job finishes in reasonable time on a single core. As rough guidance:
- Light suits footage you still care about, such as family videos or clips you may watch on a TV. Typical phone footage drops to a third or less of its size.
- Medium is the sensible choice for messengers, email and social posts. Expect fine textures like grass or hair to soften slightly.
- Heavy is for when size is the only goal: screen recordings, reference clips, previews. Blocky areas become noticeable in dark or fast-moving scenes.
The actual saving depends heavily on content. Static scenes and screen recordings compress far more than handheld footage of water, confetti or crowds.
Browser or server processing
By default the file never leaves your device: FFmpeg runs as WebAssembly in the page, which handles files up to 2 GB but is limited by your device's speed and memory. A ten-minute 1080p clip can take many minutes on a laptop and longer on a phone.
The page also offers a server option. There, native FFmpeg uses the slower, more efficient medium encoder preset with CRF 20, 25 and 30 for Light, Medium and Heavy, and writes the MP4 with its index at the start so it begins playing before fully downloading. Because the encoder is more thorough, the server can keep more quality at a similar file size. Without an account you get one server run a day for files up to 500 MB; Premium removes the daily limit and accepts files up to 4 GB. Server jobs and their files are deleted automatically about an hour after the job is created.
When the file barely shrinks or gets bigger
The tool shows the before and after sizes for every file. If the saving is close to zero, one of these is usually the reason:
- The video was already compressed. Files downloaded from messengers or streaming sites have been squeezed once; encoding them again mostly adds generational loss.
- The source is HEVC. Recent iPhones and many Android phones record in HEVC (H.265), which is roughly a third more efficient than H.264. Converting HEVC to H.264 at high quality can produce a larger file. Use Medium or Heavy for these sources.
- The audio dominates. A long, mostly static recording such as a lecture slide deck may spend a large share of its size on sound, and the audio is always re-encoded at 128 kbit/s.
If the result would not be smaller than the original, the tool hands back the original file unchanged and marks it as already optimal; nothing would have been gained.
Getting smaller files without losing quality
Compression is not the only lever, and it works best combined with the others:
- Cut what you do not need first with trim video. Removing half the duration halves the size with zero quality loss.
- Drop the sound track with mute video for clips where audio is just wind or background noise.
- Then compress, starting with Medium. Check the result on the screen where it will be watched, and only step up to Heavy if it is still too big.
Batch jobs are supported: drop several files and they are processed one after another with the same preset, then downloaded individually or as a ZIP.