A video compressor on Mac is a local tool that re-encodes one browser-decodable source into a WebM file, with input capped at 500 MiB, five minutes, and 3840 × 2160 decoded pixels. Lizely’s Video Compressor does that work in the current browser tab rather than uploading the source: the tab decodes the video, draws displayed frames into a bounded Canvas, records that stream with a WebM codec, and offers the resulting Blob for download. The process is intentionally transparent. Small, Balanced, and Quality are preset choices, not promises of a universal optimum, and compression is content dependent. A clean animation may shrink much more than noisy, grainy, shaky, or already efficient footage; in some cases, the WebM can be the same size or larger. The page calculates its percentage only from actual input and output bytes, so a larger result is reported as a larger result rather than being presented as a reduction. This makes the tool useful when you want to experiment on a Mac, compare three bounded outputs, and inspect the measured trade-off before deciding what to keep. Compression happens in real time while the source plays. Keep the tab open, wait for the source to finish, and check the downloaded file completely before deleting the original.
What stays local on your Mac
The Mac workflow runs inside the current browser tab. Video Compressor does not upload the source: source bytes, decoded frames, generated output bytes, and the completed file remain associated with that tab. Lizely receives no video, thumbnail, duration, file name, codec, or result.
A familiar Mac filename is not enough to make a file decodable. The tool can accept files named MP4, WebM, MOV, M4V, or Ogg, but the current browser must still contain codecs capable of decoding the file’s actual content. An unsupported codec produces an error and no download; the tool does not convert or truncate an unusable source silently.
| Input check | Verified limit or rule |
|---|---|
| Number of sources | One browser-decodable video |
| Duration | Five minutes, or 300 seconds |
| Byte size | No larger than 500 MiB |
| Frame sides | No more than 4096 pixels on either side |
| Decoded area | No more than 3840 × 2160 pixels |
| Extension | MP4, WebM, MOV, M4V, and Ogg names are recognized, but the installed browser codec must support the file |
These limits interact, so a source must satisfy every applicable restriction. An invalid file, unsupported decoder, excessive duration, excessive frame dimensions, missing Canvas stream, or unavailable WebM recorder ends processing with an error. Nothing is silently shortened to make a job fit.
Compress a video on Mac with Video Compressor
Use the tool in a current desktop browser on the Mac. The file must already satisfy the input rules because Video Compressor performs bounded re-encoding rather than a background upload or instant server conversion.
- Confirm the source is eligible. Check that it is no larger than 500 MiB and no longer than five minutes. Its decoded dimensions must fit within 4096 pixels on each side and 3840 × 2160 pixels of total decoded area. MP4, WebM, MOV, M4V, and Ogg are accepted file labels, but the current browser must decode their codecs. If a limit is exceeded, select a different eligible source rather than expecting automatic truncation.
- Open the tool and select the video. Launch Video Compressor in the browser tab, then choose the eligible source. File selection happens locally, and the page reports unsupported or over-limit inputs instead of creating a partial output.
- Choose a preset. Select Small for a 640-pixel maximum width and 24 frames per second, Balanced for a 1280-pixel maximum width and 30 frames per second, or Quality for a 1920-pixel maximum width and 30 frames per second. Start with the destination’s requirements because no preset guarantees a specific result size.
- Start compression and leave the tab open. The source plays in real time while Video Compressor draws its displayed frames, records the resulting stream, and tracks progress by playback time. A two-minute source therefore takes approximately two minutes rather than completing instantly. If the source contains audio, it can play during processing, so adjust the device volume if needed. Choosing a new file, changing a preset, cancelling, or leaving the page invalidates the active job.
- Review the measured result. When the source ends, inspect the reported output dimensions, input bytes, output bytes, and measured percentage change. The percentage comes only from the two actual Blob sizes. Video Compressor does not describe a larger WebM as a reduction, even when its dimensions or frame rate are lower.
- Download and play the complete WebM. Save the generated file, then play it from beginning to end. Confirm the duration, picture, sound, and playback in the browser or destination app that will use it. Keep the original until those checks are complete.
Choose a preset from the output requirements
The presets are transparent product choices rather than claims about one ideal compression level. Each preserves the source aspect ratio, never enlarges the source, and rounds output dimensions down to even pixel values. A 3840 × 2160 input can be accepted, but the output will not remain 4K because Quality has a 1920-pixel maximum width.
| Preset | Maximum output width | Target frame rate | Starting condition |
|---|---|---|---|
| Small | 640 pixels | 24 fps | The destination accepts the 640-pixel width and 24 fps target |
| Balanced | 1280 pixels | 30 fps | The destination accepts a wider output at the 30 fps target |
| Quality | 1920 pixels | 30 fps | The destination accepts the largest available output dimensions |
A bounded bits-per-pixel heuristic selects the requested recorder bitrate, with the result clamped between 180 kbps and 8 Mbps. That target does not determine the final size. Motion, noise, grain, camera shake, rapidly changing detail, the source codec, and the browser’s encoder can all affect the WebM. Run another preset when the measured output does not suit the destination, then compare actual bytes rather than assuming that a lower resolution or frame rate will always produce a smaller file.
How the browser creates the WebM
Video Compressor plays the selected source, draws each displayed frame into an opaque Canvas at the selected cadence, and records the Canvas stream. When the browser exposes an audio track through media capture, the tool adds that track to the output stream. The generated WebM uses the first codec format reported as available by the current browser: VP9, then VP8, then generic WebM.
This browser model explains an important limitation. The same source and preset can produce different output in different browsers because MediaRecorder support and codec combinations vary. The project follows the browser APIs behind that process; MDN’s HTMLCanvasElement.captureStream documentation covers canvas stream capture, while MDN’s MediaRecorder documentation covers stream recording. WebM is the output because browser-generated MP4 support remains inconsistent.
Check the downloaded WebM before replacing the original
Do not treat the reported byte change as the only acceptance test. Play the entire output and compare its actual duration with the source. Check for dropped visual detail, unexpected cropping, missing sections, and other picture changes caused by re-encoding.
Audio is included only when the browser exposes a capturable track. If the downloaded file is silent, the complete video is inaudible, or the sound differs from the source, verify the file in another player before replacing the original. Browser support and codec combinations vary, so visual and sound checks are necessary even when compression finishes successfully.
Video Compressor also does not normalize volume, mix tracks, repair corrupt media, or preserve subtitles, chapters, attachments, rotation tags, color metadata, HDR signaling, and every possible container feature. If the destination requires any of those elements, the WebM is not a transparent substitute. The finished file may also use a different codec than a WebM produced in another browser, so test the actual download where it will be used.
When to choose a desktop encoder
For sharing or a quick local WebM, the three bounded presets provide a direct workflow. For an archival master, professional delivery specification, exact codec profile, two-pass rate control, fixed keyframe interval, subtitle preservation, or deterministic result across machines, use a maintained desktop encoder such as FFmpeg with a reviewed command. Video Compressor reports the outcome of its own WebM job; it does not promise archival completeness or cross-platform bit-for-bit encoding.
For a deeper look, see Test a Video Cropper Online: A Local Browser Walkthrough.
For a deeper look, see Extract Frames from Video on Mac in Your Browser.