A WebAssembly build of FFmpeg loads in this page, decodes your video, and re-encodes it with H.264 at a chosen Constant Rate Factor (CRF). CRF lets the encoder spend bits where they matter and skip them in flat areas — entirely on your device.

Server tools use multi-threaded native FFmpeg, often with hardware H.264 encoders. WebAssembly is single-threaded inside the browser and uses the software libx264, typically 4–8× slower than a native run. The trade-off is total privacy: your video never leaves your device.

Balanced (CRF 23) is the recommended sweet spot — visually transparent for most content. High (CRF 20) is for color-critical work. Small (CRF 28) is great for sharing on chat. Tiny (CRF 32) trades quality for the smallest possible file.

Almost always. Already-compressed sources (e.g. YouTube downloads at 720p) might end up similar or even slightly larger at the Lossless or High preset — pick Balanced or Small for guaranteed shrinking.

Both produce H.264 MP4 output. Video Compressor is a fast preset-based tool; the Compress Video tool offers more granular controls (target file size, exact bitrate, two-pass, audio bitrate).

Up to 500MB. Compressing 4K sources is memory-hungry — for very long 4K clips, downscale to 1080p first using Video Resolution Reducer.

Yes — audio is re-encoded to AAC at 128 kbps, which is transparent to the human ear for music and voice. The total file size benefit comes from the video reduction.

No. The compression runs entirely in your browser tab. Nothing is transmitted, stored, or logged. Closing the tab erases the file from memory immediately.

It depends entirely on the source. A typical phone-camera 4K video at the default bitrate (often 50–100 Mbps from the camera) can be re-encoded to 1080p at 4 Mbps with no perceptible visual change for normal viewing — that is roughly a 90% size reduction. Screen recordings compress even harder because most of the frame stays static between frames. Heavily-motion content like sports or games compresses less because there is more new visual information per frame.

Yes — video compression is fundamentally lossy at any reasonable bitrate. The codec discards detail that human eyes are bad at noticing in motion. At sensible bitrates the loss is invisible to a viewer; at very low bitrates you start to see blocky artefacts (especially in dark scenes or fast motion). The "web" and "archive" presets land in the invisible-loss zone for typical content.

Roughly 1–3× real time on a modern laptop. A 10-minute video takes 10–30 minutes to compress; a 1-hour video can take 1–3 hours. Older or low-spec hardware is slower. The tool shows real-time progress and you can keep the tab open in the background — compression continues even when the tab is not focused.

Yes by default — the audio track is re-encoded to AAC at 128 kbps which is essentially transparent for music and speech, and adds only a few MB to the output regardless of video length. If you want maximum audio fidelity (mastering work, music videos), pick a higher audio bitrate from the advanced settings.

Most likely the source video had its audio in a codec the WebAssembly build does not include — some specialised audio codecs (AC-3, certain DTS variants) are excluded from the bundled FFmpeg to keep the WASM binary size manageable. To check, run the source through Video Inspector first; if the audio codec shows up as something exotic, you can convert the source through a different tool first.

Subtitle tracks and chapter markers are preserved through the compression unless you explicitly remove them. The tool re-encodes the video and audio streams but copies metadata streams as-is.

H.265 produces files roughly 30–50% smaller than H.264 at the same visual quality, but it is more CPU-intensive to encode and decode, and not every viewer/device supports it. The tool defaults to H.264 because it works literally everywhere. If you know your destination supports HEVC (modern smart TVs, recent iPhones, most laptops from 2018+), switch to H.265 in the advanced settings for an additional 30%+ size reduction.

Not in the compressor itself — compression always re-encodes the entire selected range. If you only need part of the video, run Video Trimmer first to isolate the section you want, then compress the trimmed clip. That is also faster than compressing the full file and then trimming the output.