- Does my video get uploaded to any server?
- No. This tool uses FFmpeg compiled to WebAssembly, which runs entirely inside your browser process. Your video bytes are read from your local disk into browser memory and never sent over the network. You can even go offline after the page loads and compression will still work.
- Why does it take a moment to start the first time?
- The FFmpeg WebAssembly binary (~30 MB) needs to be downloaded from a CDN and compiled by your browser's WASM engine. This happens once per session. After it is loaded, subsequent files compress without any delay. Modern browsers (Chrome, Edge, Firefox, Safari 16.4+) cache the compiled binary, so revisiting the page is fast.
- What is the difference between CRF and target file size?
- CRF (Constant Rate Factor) tells the encoder to maintain a given visual quality level — the output file size depends on how complex the video is. A fast-moving scene will produce a larger file than a still room. Target file size mode uses two-pass encoding: the first pass profiles the video, and the second pass sets the exact bitrate needed to hit your budget. Use target size when you need to fit a specific limit (e.g. an email attachment cap or a form upload limit).
- Will compressing reduce video quality noticeably?
- At CRF 18–22 the quality loss is invisible to most viewers. At CRF 23–26 (the default range) slight softness appears in fast motion, but is acceptable for sharing. CRF 28+ noticeably blurs fine detail and is best reserved for preview clips. Lowering the resolution to 720p alongside a moderate CRF typically yields a 4–8× file-size reduction with barely perceptible quality loss on phone screens.
- What format does the compressed file come out in?
- All output is MP4 with H.264 video and AAC audio. This is the most widely compatible format — it plays on every iPhone, Android device, Windows, Mac, and streaming platform without any conversion needed. MOV files from iPhone (which are H.264 or HEVC inside a QuickTime wrapper) are re-wrapped and optionally re-encoded to H.264 MP4.
- How large a video file can I compress?
- Browser memory limits apply. Most desktop browsers can handle files up to 1–2 GB. The WASM engine reads the entire source file into memory during encoding. For very large files (10-minute 4K iPhone recordings can be 5+ GB), consider splitting first or using a desktop FFmpeg installation. Files under 500 MB work reliably on most devices.