Video Compressor Guide
Reference for compressing short videos with ffmpeg.wasm in the browser, including quick presets, MP4 output, CRF, resolution, audio bitrate, mobile limits, and errors.
Presets and encoder controls
Video Compressor runs the site's ffmpeg.wasm engine after you start processing. MP4 output uses H.264 video and optional AAC audio. WebM/VP9 video encoding is temporarily unavailable because the current encoder fails on validated inputs. WebM input decoding remains available. A file extension identifies the container, not every codec a browser can decode.
The Web upload preset uses MP4, height limited to 720 pixels, 30 fps, CRF 32 and 96 kbps audio. Email attachment uses a 480-pixel height limit, 24 fps, CRF 35 and 64 kbps audio. Silent preview removes audio. A resolution limit preserves aspect ratio, rounds encoder dimensions to even numbers and does not upscale. A 1280 × 720 source at the 480p limit becomes 854 × 480.
Quality and target size are different modes
Higher CRF usually reduces bytes at the cost of detail. It does not specify a final byte count. Resolution, frame rate, source complexity and audio affect size together. Re-encoding an already compact clip can make it larger.
Target-size mode uses MiB (1 MiB = 1,048,576 bytes), calculates a bitrate budget and may retry encodes. The displayed limit permits up to 5% above the target; it does not promise a result within ±5%. A source already below the target may not need compression, and a re-encode may be larger. Container overhead and source complexity make the target best effort; read the actual deviation and downloaded file size. Do not treat the rough pre-processing range as a delivery guarantee.
Troubleshooting failed compression
The public file limit is 200 MiB; on mobile, start below 50 MiB with a short clip. These are input guidance and a file cap, not a guarantee that every device has enough memory. If processing stalls, stop it and try a shorter clip or smaller output. If the browser cannot preview a codec, use a compatible source or a desktop conversion workflow.
Inspect duration and dimensions with Video Info Inspector. Use Video Trimmer before compression when unused footage can be removed. Keep the original until the final video has been played and its audio checked.
Evidence and output review
The measured media case provides original and exported files, exact arguments and independent stream checks. Its synthetic pattern is useful for reproduction but does not predict the legibility of a real screen recording.
Verify downloaded dimensions, duration, motion, small text and audio, then choose a cover with Video Thumbnail Extractor. Media processing uses local CPU and memory. See Data handling for the site-level boundary.