Free Video Compressor

How this tool computes its result

Runs entirely in-browser via MediaRecorder plus HTMLVideoElement.captureStream() -- there is no server-side transcoding. After a file under 300MB is selected, duration and resolution are read from a hidden <video> element's loadedmetadata event. Choosing one of three presets (Safe: 4500kbps video, Balanced: 2800kbps, Aggressive: 1800kbps, all with 128kbps audio) shows an estimated output size computed purely from duration times target bitrate, before any actual encoding runs. Pressing Compress plays the source video in a hidden element while capturing its rendered frames as a live MediaStream into MediaRecorder at the chosen bitrate; the output MIME type is picked from a list of MP4/WebM codec candidates tested against MediaRecorder.isTypeSupported(), falling back from MP4 to WebM (or the reverse) if the requested format isn't supported, with a visible message when that fallback happens.

Worked example

Uploading a 45-second video and selecting the Balanced preset (2800kbps video + 128kbps audio = 2928kbps total) shows an estimated output size of (45 x 2928 x 1000) / 8 bytes, roughly 16.5MB, before compression starts. Pressing Compress then plays the full 45 seconds in real time, since the progress bar tracks video.currentTime against video.duration -- encoding happens via live playback capture, not a fast batch transcode.

When not to use this tool

Because compression works by capturing a live playback of the source rather than decoding and re-encoding frames directly, compressing a video takes roughly as long as its own runtime -- a 10-minute video takes about 10 minutes to compress -- and needs the browser tab to stay open and unthrottled the whole time, with no way to speed it up or run it in the background.

Common mistakes

• - Assuming the pre-compression size estimate shown per preset is the guaranteed output size -- it is a straight bitrate-times-duration calculation with no accounting for scene complexity, so the browser's actual encoder output can differ noticeably from the estimate.
• - Selecting MP4 output on a browser that doesn't support MediaRecorder MP4 encoding -- the tool silently falls back to WebM and shows a one-line notice about it, easy to miss before downloading.
• - Uploading a file near the 300MB cap and expecting fast processing -- since compression happens via real-time playback capture, longer source files take proportionally longer with no shortcut, and very long videos risk the tab being backgrounded or throttled mid-compression.

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