FFmpeg Cookbook
JA

Slack Video Compressor

Compress a video toward a size you choose (MB) for Slack, then check the output size and picture right away.

Your file is not uploaded
Choose a video MP4 / MOV / MKV / WebM / AVI, up to 500 MB

1 MB = 1,000,000 bytes. The presets are common sizes; check the official help of each service for its limit. Entering a shorter duration does not trim the video.

Equivalent FFmpeg command

CLI command for your current settings. Updates live as you change controls.

Pick a file and adjust settings — the command appears here.

Before sharing

Slack permits regular file uploads up to 1 GB, while this browser tool accepts input files up to 500 MB. The 100 MB starting target is for lighter sharing. Slack clips are a separate feature limited to five minutes; check conversation permissions and admin settings.

Slack help: Add files to Slack / Clips (Checked 2026-09-25)

The calculation budgets 95% of the target for video and audio. An oversized output is retried once and any remaining excess is reported.

Re-encoding changes quality. Check small text, motion and audio, and keep the original. Automatic tag and chapter copying is disabled, but this does not conceal personal information in the picture or sound.

Measured comparison: file size and quality from the same clip

What changes when you compress a video?

One 10-second clip compressed with three settings. The pictures show part of the frame at 2x.

Source 23.28 MB → 0.73 MB, encoded in 1.1 s 
ffmpeg -i input.mp4 -map 0:v:0 -map 0:a:0? -c:v libx264 -crf 36 -preset fast -pix_fmt yuv420p -c:a aac -b:a 96k -movflags +faststart output.mp4
Part of the original frame (a printed T-shirt and a jacket) at 2x The same part after compression at the selected setting Original Compressed
How this was measured

2026-09-24, FFmpeg 8.1 on Windows (H.264, fast preset), one run per setting on a 10-second 1080p live-action clip. The pictures are a 480x270 native-resolution crop at 3 seconds, shown at 2x.

Measurements (JSON)Source film

Tears of Steel (CC) Blender Foundation | mango.blender.org, CC BY 3.0. Cropped from the 4K version and scaled to 1080p; part of each encoded frame at 2x

Popular uses
Smaller Slack sharing copiesActual output size versus targetClips versus standard file guidance
Next tools
Video Trim Tool Video Compressor File Size Estimator

How to use

  1. Select a videoPick a video up to 500 MB and 7200 seconds (2 hours).
  2. Check duration and targetMake sure the detected length (seconds) is right, and set a target a little below the limit where you will send it.
  3. Inspect the resultPress Compress video, then check the output size (bytes), picture and sound before saving.

Recommended settings

Over the limit
Trim unneeded footage first
A shorter clip gets more video bitrate at the same size.
Small text is unreadable
Increase the target or share a link
Lower resolution and strong compression blur small text.
Do not share the conversation
Remove audio
This removes all audio from the output. Anything visible in the picture remains.

When it goes wrong

"Duration could not be read"

Cause: The browser cannot read the length of some formats.

Fix: Enter the length shown in your player. Entering a shorter length does not trim the video; it makes the output more likely to exceed the target.

Still over target

Cause: The picture content, the audio and the file container can make the size differ from the calculation.

Fix: Trim the video, or change the target, resolution or audio settings.

Conversion failed

Cause: Possible causes include an unsupported format, a damaged file, low memory, or FFmpeg failing to load.

Fix: Try a shorter clip, or use desktop FFmpeg.

Questions

How much can I send through Slack?

Slack lets you add files up to 1 GB (Slack Help). This tool reads files up to 500 MB, and its 100 MB default target is only a starting point. Clips, which you record inside Slack, are a separate feature limited to five minutes. In Slack Connect conversations (with people outside your organization), adding files may be restricted.

Is the selected size guaranteed?

No. The tool checks the actual bytes of the finished file. If it is over, it compresses again once at most and tells you if it is still over.

Does re-encoding change quality?

Yes. Play the result and check text, motion, faces, color and sound.

Does this remove personal information?

Not completely. Tags and chapters are not copied, but names, faces and places in the picture, and voices in the audio, remain.

Were the comparison images measured on my device?

No. They show one clip compressed with desktop FFmpeg (see How this was measured). Your own result shows the output size, and you can save a JSON report.

More detail (settings, limits, FFmpeg commands)

What each setting means

Target size
Calculated with 1 MB = 1,000,000 bytes. The default is 100 MB; change it to suit where you will send the file.
Video and audio budget
95% of the target goes to video and audio. Audio gets 128 kbps up to five minutes, 64 kbps beyond that, and zero when removed. If video would get less than 150 kbps, the tool does not compress and shows the minimum size needed. When the clip is short for the target, the video bitrate is held at the cap for the output size (4000 kbps up to 480p, 7500 up to 720p, 12000 up to 1080p, and so on; YouTube's recommended upload bitrates), so the file comes out smaller than the target.
Maximum resolution
Caps the short side: the height of a landscape video, the width of a portrait one. The default, Match bitrate, picks 480, 720 or 1080 from the video bitrate. The video is never enlarged, and width and height are rounded to even numbers.

Browsers and limits

  • Input up to 500 MB and 7200 seconds. This is the tool's limit, separate from Slack's
  • Only the first video and first audio track are used; subtitles, extra audio tracks and chapters are dropped
  • Settings and Clear are locked during conversion. Nothing hides faces or personal details automatically
  • Tool pages do not load ads or analytics (GA). Loading FFmpeg itself (cdn.jsdelivr.net) needs a network connection

The same job with the FFmpeg command line

Example budget: 20 MB for a three-minute clip
ffmpeg -i input.mp4 -map 0:v:0 -map 0:a:0? -vf "scale='if(gte(iw,ih),-2,trunc(min(iw,720)/2)*2)':'if(gte(iw,ih),trunc(min(ih,720)/2)*2,-2)'" -c:v libx264 -b:v 716k -maxrate 788k -bufsize 1432k -preset fast -pix_fmt yuv420p -c:a aac -b:a 128k -map_metadata -1 -map_metadata:s -1 -map_chapters -1 -movflags +faststart output.mp4
95% of 20 MB (20,000,000 bytes) over 180 seconds, minus 128 kbps for audio, leaves 716 kbps for video. With Match bitrate, that rate caps the short side at 720. The on-page command changes with the size and resolution you choose.

Choose a sharing method for Slack

Slack lets you add files up to 1 GB, and the desktop app takes up to ten files at a time. This tool reads files up to 500 MB, and the 100 MB default target is a starting point for lighter sharing.

Checked 2026-09-25. Slack Help: Add files to Slack. Adding files may be restricted in Slack Connect conversations; if you cannot add one, ask a workspace owner or admin.

File attachments and clips

A clip is recorded inside Slack and can be up to five minutes long. You can also upload an existing video as a clip. This is separate from the 1 GB limit for file attachments.

Slack Help: Record audio and video clips

Related tools

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Video Trim Tool Video Compressor File Size Estimator