Decision kit · reviewed September 2, 2026
Slack vs. Mattermost: count the operator, not just the license.
For a 25-person team, Slack Pro is cheaper in this fixed first-year scenario. Mattermost becomes the better decision only when control, deployment, or customization is valuable enough to justify operating it.
Fixed scenario
Vendor prices are current public inputs. Infrastructure, labor, and migration are editable assumptions—not vendor quotes or observed customer costs.
| Team | 25 active people |
|---|---|
| Term | 12 months |
| Slack | Public annual list prices; promotion excluded |
| Mattermost | Entry license assumed $0; eligibility and feature limits must be checked |
| Infrastructure | $40/month hosting + $15/month backups (planning assumptions) |
| Operations | 3 hours/month at $75/hour (planning assumption) |
| Migration | 24 one-time hours at $75/hour (planning assumption) |
Calculated cost
| Path | Year one | Following year | Calculation |
|---|---|---|---|
| Slack Pro | $2,175 | $2,175 | 25 × $7.25 × 12 |
| Slack Business+ | $4,500 | $4,500 | 25 × $15 × 12 |
| Mattermost Entry, self-managed | $5,160 | $3,360 | $660 infrastructure + $2,700 operations + $1,800 first-year migration |
| Mattermost Professional or managed cloud | Quote required | Quote required | Current public pricing sends buyers to sales |
What the price does not settle
Migration reality
- Confirm which Slack plan and export scope can supply public channels, private channels, direct messages, and file links.
- Inventory apps, bots, workflows, calls, identity, retention, and external collaboration separately; they do not migrate as chat history.
- Provision a fresh Mattermost staging environment with enough working storage for the export and transformed files.
- Validate and transform the Slack export with
mmetl, then validate the Mattermost import before processing it. - Reconcile people, channels, messages, files, threads, permissions, integrations, and search before scheduling cutover.
- Keep a rollback path and named owners for backups, upgrades, monitoring, security response, and user support.
Mattermost says migrations can take several days to multiple weeks and recommends staging, repeated validation, backups, and substantial spare storage. Slack limits full private-channel and direct-message exports by plan and approval.
Evidence and unknowns
- Slack pricing: Pro $7.25 and Business+ $15 per user/month when billed annually; temporary promotions excluded.
- Slack export documentation: export coverage differs by plan and some full exports require approval.
- Mattermost pricing: Entry can be started free; Professional and managed enterprise pricing require a quote.
- Mattermost migration documentation: transformation, staging, storage, validation, and feature limitations.
Unknown until discovery: export size, required history, legal authority, integrations, call load, identity controls, availability target, support coverage, and the value of data sovereignty. No vendor paid for inclusion or placement.
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