The short answer
- Moving from Slack Pro to Mattermost at 1 seat costs $1,355 more in the first year, after setup and upkeep time (Stackcut's model: modeled, not measured).
- Mattermost's official migration guide moves 4 kinds of data in full and 1 partly; 6 things stay behind. What moves
- 4 public accounts of making this switch: 3 switched, 1 went back. Read them
- Switching help: Mattermost ships free, open-source migration tooling for Slack: the mmetl CLI converts a Slack export (including Enterprise Grid) into a Mattermost bulk-import JSONL plus attachments folder, and mmctl validates, uploads and processes the import. Imports are idempotent. docs.mattermost.com
What Slack and Mattermost cost
List prices from each vendor's pricing page, as stored in the Stackcut catalog. Each price links its source.
| Plan | Billed monthly | Billed yearly (per month) | What you get |
|---|---|---|---|
| Slack Pro | $8.75/user/mo | $7.25/user/mo | Unlimited message history and apps; group huddles (up to 50 people, 25 on video); group Slack Connect |
| Slack Business+ | $18/user/mo | $15/user/mo | Everything in Pro plus advanced AI, SAML SSO, SCIM; huddles up to 50 people |
- Slack free plan: 90 days of message history (messages older than 90 days hidden until you upgrade), up to 10 apps, huddles 1:1 only (max 2 people), Slack Connect external messages 1:1 only slack.com
- Mattermost free plan: Free self-hosted edition mattermost.com
Mattermost's plans aren't in the Stackcut catalog yet; check Mattermost's own site.
Moving from Slack to Mattermost
What Mattermost's importer carries over, what it leaves behind, and any help it offers switchers.
To Mattermost
Official guide docs.mattermost.com · Tools: mmetl, mmctl
Moves: users (full); channels (full); message history (full); file attachments (full); threads (partial). docs.mattermost.com
Doesn't move: Slack apps, bots, slash commands, webhooks and Workflow Builder automations; Canvases; Starred conversations; User groups (recreatable via Mattermost Custom Groups); User presence, profile pictures and custom profile fields; Deactivated or deleted Slack users are not migrated. docs.mattermost.com
Watch out for:
- The import is idempotent: re-running the same posts creates no duplicates. docs.mattermost.com
- Missing user emails become @example.com placeholders the admin must fix before or after import. docs.mattermost.com
- mmetl runs on Linux/macOS only; WSL is not recommended for heavy migrations. docs.mattermost.com
Community guide linkedin.com · Tools: slack-advanced-exporter, slackdump (rusq), mmetl, mmctl
Moves: public channel messages (full); user accounts (full). linkedin.com
Watch out for:
- Slack's regular export omits emails and attachments; slack-advanced-exporter (or slackdump for private channels) is needed to fill the gaps. linkedin.com
- In Mattermost v10.5.2 the import fails unless every user is in at least one channel; patch the JSONL first. linkedin.com
- Default Max Users Per Team (50) will trip mid-import on larger workspaces. linkedin.com
Switching help: Mattermost ships free, open-source migration tooling for Slack: the mmetl CLI converts a Slack export (including Enterprise Grid) into a Mattermost bulk-import JSONL plus attachments folder, and mmctl validates, uploads and processes the import. Imports are idempotent. docs.mattermost.com
Switching help: Mattermost's official Slack migration docs state that support from a Mattermost expert is available for Slack migrations, and that support negotiating an export of customer IP from Slack Enterprise can be requested by contacting a Mattermost expert. docs.mattermost.com
Switching help: Nonprofit licensing program: $250 for 3 years, up to 1,000 users, for qualifying nonprofit organizations. Mattermost also says it is considering special pricing for smaller for-profits with under $10M USD annual operating budget. forum.mattermost.com
From each destination's own migration docs and switching pages, plus community write-ups where no official guide exists, each checked against its page. Confirm terms on the linked page before you rely on an offer.
People who moved from Slack to Mattermost
4 public accounts of leaving Slack: 3 switched, 1 went back. Summaries are ours; each links the original, and each was checked against its page before it appeared here.
A 5-person team that moved from Slack to self-hosted Mattermost years ago (over the 90-day free-history limit) got caught by Mattermost's v11 free-tier changes: upgraded into the limited Entry edition, lost access to chats beyond 10k messages, and documented a manual downgrade to Team Edition that preserved their data. Mattermost Discussion Forums
An office that moved from Slack to a self-hosted Mattermost hit severe delivery and notification problems; the Mattermost team replied that websockets were likely not fully installed on the instance. Much of the office reverted to email and phone. Hacker News
A counter-anecdote in the same HN thread: a ~50-user Mattermost deployment on 4.10.1 was reliable with none of the OP's delivery issues, but the UI is bare-bones and threading poorly implemented, so he treats it like IRC. Cautions that Slack migrants should not expect a 1:1 copy. Hacker News
Vendor case study: Data For Good, a 6,000-volunteer nonprofit, left Slack for self-hosted Mattermost on Docker over data sovereignty. They benchmarked Zulip and Rocket.Chat too; deployment was smooth and they plan Mattermost Enterprise as the centerpiece of a fully self-hosted tool suite. Mattermost
How to leave Slack
Export first, then cancel. Every step links the vendor's own help page, checked on the date shown.
How to leave Slack
Export first
- Public channels (messages and file links): Admin → Workspace settings → Security → Import & export data → Export (ZIP of JSON). Free and Pro export public channels only; private channels and DMs need Business+ or Enterprise. On Free, file links cover the last 90 days. slack.com
What you lose
- Canvases and Lists turn read-only
- Existing Workflow Builder workflows stop working
- Guest accounts are deactivated and Slack Connect channels disconnect
- Everyone is removed from user groups
Cancel
- Export what you need first (see export).
- Admin → Manage billing → Overview → Change plan details → Downgrade to Free.
- Workspace Owners, or the member who upgraded the workspace, can do this.
- Takes effect
- You choose: right away or on your next renewal date.
- Refunds
- Not stated for a downgrade to Free. Slack credits unused paid time only when you move from Business+ to Pro.
- After you cancel
- Free shows the most recent 90 days of messages and files, deletes data older than 1 year, allows 1:1 huddles only and up to 10 apps.
FAQ
Is Mattermost cheaper than Slack?
Modeled, not measured: moving from Slack Pro to Mattermost at 1 seat costs $1,355 more in the first year, after setup and upkeep time at $50/hour. It depends on your plan, seats and usage. the math
Can I move my Slack data to Mattermost?
Mattermost's official guide moves: users; channels; message history; file attachments; threads. It doesn't move: Slack apps, bots, slash commands, webhooks and Workflow Builder automations; Canvases; Starred conversations; User groups (recreatable via Mattermost Custom Groups). docs.mattermost.com
What do I lose when I leave Slack?
Per Slack's own pages, leaving or downgrading loses: Canvases and Lists turn read-only; Existing Workflow Builder workflows stop working; Guest accounts are deactivated and Slack Connect channels disconnect; Everyone is removed from user groups. slack.com
Does Mattermost help people switch from Slack?
Mattermost ships free, open-source migration tooling for Slack: the mmetl CLI converts a Slack export (including Enterprise Grid) into a Mattermost bulk-import JSONL plus attachments folder, and mmctl validates, uploads and processes the import. Imports are idempotent. docs.mattermost.com
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