There is no official list of video formats Slack can play inline. People go looking for the spec their MOV failed to meet, and the spec does not exist.
Slack documents what happens when you upload a file in some detail, and almost nothing about what happens when someone presses play.
First, separate "didn't upload" from "won't play"
These look identical in a channel and have completely different causes. A failed upload is governed by rules Slack actually publishes. The size cap is stated flatly, with no plan tiers attached: "You can add files up to 1GB in size." There is no per-plan table on that page, so if someone tells you Business+ raises your per-file ceiling, ask for the link — Slack's documentation does not say it. (More in Slack file too large.)
Playback is a different story. The only preview behaviour Slack spells out is this: "Slack will include a preview of a file as part of your message." Then it gives exactly two sets of conditions, and neither is about video:
| Preview rule Slack documents | Official value |
|---|---|
| Image displays inline: longest side | smaller than 25,000 pixels |
| Image displays inline: total pixels | less than 45 million pixels total |
| Office file shows no preview above | larger than 50 MB |
The image conditions are joined with and — "smaller than 25,000 pixels on the longest side, and less than 45 million pixels total" — so a picture has to clear both, and failing either drops it out of the inline view. And these are display thresholds, not upload thresholds: a file that refuses to preview may have uploaded perfectly.
The gap matters: Slack publishes no equivalent threshold for video. No maximum dimensions for a thumbnail, no size above which a player stops rendering.
Why there is no format to "fix" your MOV to
Search this problem and you will read that Slack only embeds MP4 and MOV. That claim has no official source. What does exist is a different thing that gets mistaken for it: a filetype enumeration in Slack's developer documentation, listing the type labels the API can attach to a file object. The media entries include:
mkv Matroska video
mov QuickTime video
mp4 MPEG 4 video
webm WebM
ogv Ogg video
wmv Windows Media VideoThat is a taxonomy, not a capability list. It tells you Slack knows how to label a QuickTime file as mov; nowhere does it promise a file with that label will play in a message.
The one place Slack does name codecs is about something else entirely: "Huddles rely on AV1, VP9, and H.264 for media encoding and decoding." Read the qualifier — that sentence is about huddles, Slack's real-time calls. It says nothing about uploaded files.
You cannot make your file match Slack's video spec, because Slack has not published one.
The fix: re-encode to H.264 MP4
A .mov is a container, and the extension tells you nothing about what is inside it. When a file plays fine in QuickTime on your Mac and does nothing in a colleague's Slack window, the container is not the interesting variable. The contents are.
Slack has not asked you for H.264, and this page will not pretend it has — check the official documentation if you want a format requirement in writing. Re-encoding is just the move that removes an undocumented decoder from the chain.
Try it right here
Runs in your browser. Your file is not uploaded anywhere.
This runs inside your browser tab. The file is read from disk, re-encoded locally, and handed back — never uploaded anywhere. The MOV to MP4 converter is the same tool on its own page.
If the result is still large, run it through video compression afterwards. For the wrapper-versus-contents distinction, see codec vs container.
Other causes, roughly in order of likelihood
The upload never completed. Beyond the 1GB ceiling, Slack's developer docs list file_upload_size_restricted: "The size of provided file is too large, as the team has restricted uploads of large files." Your admins can tighten the limit below 1GB, so a file well under a gigabyte can still be refused. The API reference does not say where that setting lives — ask an admin.
The workspace hit a storage ceiling. The same error table documents storage_limit_reached: "File storage limit has been reached. This occurs when free teams have uploaded 5GB of files." Two qualifiers you should not drop: this is a workspace-wide total, not a per-file cap, and it appears in developer documentation and a 2019 changelog rather than on any current user-facing help page.
The file is old and the workspace is free. Slack's usage-limits page: "You can view and search messages and files from the last 90 days", and "Messages and files more than one year old will be permanently deleted." The 90-day rule hides; the one-year rule deletes. Hidden content comes back if you upgrade — the remaining 275 days become available on a paid plan. Deleted content does not: "It will not be possible to restore messages and files once deletion occurs."
There is a trap folded into that. Workspace Owners can set basic data retention to either keep everything for a year or delete after 90 days — and choosing deletion "will automatically delete any that become hidden by the message and file limit." Pick that setting and the 90-day hiding stops being reversible.
A virus scan blocked it. Slack is explicit: "Files you add to Slack will be scanned for viruses and other malware, and won't be uploaded if any are detected." If a file is blocked incorrectly, Slack says to contact them for help.
You are in a Slack Connect conversation. Slack restricts certain file types there — "To help keep Slack secure, certain types of files cannot be added to Slack Connect channels and direct messages (DMs)" — and publishes the blocked extension list. Note that .mp4, .mov, .mkv and .webm are not on it, while .7z, .gz and .tar are. Archiving a large video before sharing it into a Slack Connect channel turns a file that was allowed into one that is blocked outright. Send the raw video. Note the scope — that list covers Slack Connect channels and DMs, not Slack as a whole.
You are trying to save it on a phone. Slack's download instructions are explicit: "On your mobile device, you can only download image files." Pulling the MOV onto your phone to open it in another player is closed off by design.
You are comparing against a clip. Slack clips — audio and video recorded inside Slack — are capped at five minutes, and that limit belongs to clips only. Slack publishes no duration limit for ordinary video file uploads. Clips can be made from an existing video ("Upload an existing video as a clip"), but Slack does not state which containers or codecs that path accepts, so it is not a documented workaround.
How to tell it worked
Play the converted file in a browser tab before uploading — that is closer to what a teammate's Slack client will attempt than QuickTime is. Then upload and check from a second device if you can: playing on your desktop but not a colleague's points at a decoder difference.
If it still will not upload after converting, work down the list above.
FAQ
Does Slack support MOV files?
Slack's API recognises mov as a filetype label — "QuickTime video" in its own words — and MOV is not on the restricted extension list for Slack Connect. But recognising a file type is not the same as promising to play it inline, and Slack publishes no list of formats it plays. If playback matters, convert to H.264 MP4.
Why does my video upload but not play in Slack? Because upload and playback are governed by different things, and only the upload side is documented. Slack's published preview rules cover images and Microsoft Office files; there is no stated video preview threshold at all. A file can be stored fine and still show no player.
Does Slack only embed MP4 and MOV? That claim circulates widely with no official source behind it. Slack's documentation contains no playback whitelist.
Why did an old video stop playing in our Slack? On a free workspace, messages and files from the last 90 days are visible and searchable, older content gets hidden, and anything more than one year old is permanently deleted. Upgrading restores hidden content — unless your Workspace Owner chose the retention option that deletes after 90 days, which makes those files unrecoverable.
What is Slack's video file size limit?
Slack states "You can add files up to 1GB in size", with no plan-based variation on that page. Admins can restrict uploads below that, which surfaces as file_upload_size_restricted. Slack publishes no duration limit for uploaded video files; the five-minute limit applies to clips.
Should I zip the video to get it through?
Not for Slack Connect. Archive extensions including .7z, .gz and .tar are on Slack's restricted list for Slack Connect channels and DMs, while common video extensions are not — so archiving is more likely to get the file blocked than to get it through.
Everything cited here comes from Slack's help center and developer documentation. Where Slack states nothing — playback formats, video preview thresholds, upload duration limits — this page says so rather than filling the gap. Browse the other fix guides or all Clapr tools.
