Slack defined modern team chat, and most teams that use it like it. But chat is the one tool literally everyone in the company uses, which makes it the most expensive possible place for per-user pricing, and your entire institutional memory sits in a vendor's cloud, with the free tier hiding your own messages after 90 days. Mattermost is the open source alternative: the same channels-and-threads model, self-hosted, which we host, configure and support on UK infrastructure for a flat monthly fee. Each has a real case. Here is the honest comparison.
The same model, different economics
Slack is the polished original: channels, threads, huddles, workflow automation, Slack Connect for cross-company channels, and an app directory that integrates with practically everything. At the time of writing, Slack Pro is £5.75 per user per month billed annually (£7 billed monthly) and Business+ is £12 per user per month billed annually (£14.40 monthly). The free tier is genuinely useful for small teams, with the significant catch that messages older than 90 days are hidden until you pay.
Mattermost is an open source collaboration platform built deliberately on the model Slack proved: channels, threads, direct messages, file sharing, search, built-in calls and screen sharing, playbooks for repeatable processes, plus webhooks, slash commands and a full API. It was designed to be self-hosted and is used in government, defence and financial services for exactly that reason. Your messages sit in your own database, not a vendor's cloud.
Mattermost with Node is that software run as a managed service: we deploy it in a production configuration, connect it to your own Keycloak realm for single sign-on, and handle upgrades, backups, monitoring and support on UK infrastructure, flat-priced at any team size.
The per-seat maths
Our Mattermost tiers are flat: Small at £23 a month, Medium at £39, Large at £71, priced by the resources the instance needs rather than by headcount.
| Team size | Slack Pro | Slack Business+ | Mattermost on Node |
|---|---|---|---|
| 25 users | £1,725 a year | £3,600 a year | £468 a year (Medium, £39 a month) |
| 50 users | £3,450 a year | £7,200 a year | £852 a year (Large, £71 a month) |
| Every new hire | Another per-user licence | Another per-user licence | £0 |
These are list prices, not a quote. The honest headline is the shape of the curve, not the day-one saving: every hire raises the Slack line and leaves the Mattermost line where it is. A lighter-use 50-person team may well fit our Medium tier; a heavier one needs Large. Current figures are on the pricing page.
Feature comparison
| Slack | Mattermost with Node | |
|---|---|---|
| Pricing | Per user per month; free tier with 90-day visible history | Flat tier from £23 a month, see pricing |
| Message history | Full on paid plans, in Slack's cloud | Full, in your own PostgreSQL database |
| Apps and integrations | App directory integrating with practically everything | Webhooks, slash commands, bots, API; smaller ecosystem |
| Calls | Huddles, polished | Built-in calls and screen sharing |
| Cross-company chat | Slack Connect | Nothing equivalent |
| Workflow tooling | Workflow Builder | Playbooks for incident response and runbooks |
| Data location | Slack's cloud; paid-plan residency for certain data at rest | UK infrastructure in your own tenant, Article 28 DPA |
| Identity | Vendor-managed, SSO on higher plans | Your own Keycloak realm, SSO included |
| Exit | Exports; scope depends on plan | Your database, in open form, already yours |
When Slack is the right choice
Fair is fair. Small teams should use Slack's free tier: it costs nothing, and if losing sight of 90-day-old messages does not bother you, nothing we host will beat free. Integration-heavy teams are the strongest Slack case: its app directory connects to practically every SaaS product, and if your daily workflow runs on those integrations, Mattermost's smaller ecosystem will feel like a step down, with more of the connections needing webhooks or custom work. Organisations that collaborate across companies get Slack Connect, shared channels with clients and partners, which has no real Mattermost equivalent. And Slack's polish, in the mobile apps, huddles and a thousand small interactions, remains the benchmark; Mattermost is close, not identical. If those outweigh cost and data control for you, stay on Slack.
When Mattermost wins
Cost at any real headcount: chat per-seat fees touch every employee. At 50 people, Slack Pro is about £3,450 a year at the time of writing and Business+ about £7,200, against a flat tier under £900. The gap only widens as you grow.
Your history is not held to ransom: the free-tier 90-day window is Slack's upgrade lever, and it is your own institutional memory. With Mattermost the complete archive lives in your database from day one.
Data sovereignty you can point to: internal chat is where the unguarded conversations happen: customers, staff, finances, incidents. On Node, all of it stays in your own tenant on UK hardware under UK jurisdiction, with an Article 28 DPA from a named UK processor.
Control of the platform: retention, access and integration policy are decisions you make on a system you control, not features distributed across a vendor's plan matrix.
Exit that is not a project: your messages are already in an open database you can query. Leaving a decade of conversation in a SaaS workspace is somewhere between painful and impossible; leaving a managed Mattermost instance is taking your database with you.
Which edition we deploy, honestly
We run the free, open source Mattermost Team Edition, and you should know exactly what that means. It includes the things this page praises: channels, threads, DMs, search, file sharing, calls, playbooks, integrations, unlimited users and unlimited history in your own database, and we wire in single sign-on through your Keycloak realm.
Mattermost sells Professional and Enterprise editions with features we do not ship: SAML and AD/LDAP group sync, automated data retention policies, legal hold, and one-click compliance export for eDiscovery platforms. If your compliance regime requires those specific mechanisms, the honest answer is to license them from Mattermost. What self-hosting gives you without them is direct access: the full history is in a PostgreSQL database you control, so answering a subject access request or a disclosure exercise is a database query run by your engineers (or ours), not a request to a vendor, and what you retain is your policy rather than a plan feature.
Message history and UK sovereignty
Slack does offer data residency on paid plans, and it is worth stating precisely what that is: you choose the region where certain types of data at rest are stored, while your workspace still runs on Slack's global infrastructure under Slack's terms. That is a real control, and for many teams it is enough.
The Mattermost-on-Node position is simpler to explain to a regulator or a client: every message, file and channel lives in your own isolated tenant on hardware we own in a UK datacentre, under UK jurisdiction and UK GDPR, covered by an Article 28 data processing agreement. There is no third-party chat cloud in the picture at all. For regulated organisations, that difference, one sentence instead of a diagram, is frequently the reason they move.
Migrating from Slack: what carries over
Mattermost has first-party Slack migration tooling, and it is genuinely good: from a Slack export it imports users, channels, complete message history with original timestamps and file attachments, and mostly preserves thread structure (some threading relationships can be lost, which is the honest caveat).
The real constraint is on Slack's side. The standard export available on all plans covers public channels only, and on the free plan, only the visible 90 days. Private channels, direct messages and group messages are only included in the full "all channels" export, which Slack provides on Business+ and Enterprise Grid plans. Slack apps and integrations do not migrate anywhere and need recreating against Mattermost's webhooks and API. We handle the export, transformation, import, user mapping and cutover, and tell you before we start exactly what your Slack plan's export will and will not contain.
The bottom line
Slack is the right tool for small teams on the free tier, integration-heavy workflows and cross-company collaboration. Mattermost is the right tool when the per-seat bill has become a headcount tax, or when "where is our message history and who can reach it?" needs a one-sentence answer. See Mattermost for what the managed service includes, the pricing page for current figures, and open source alternatives for the wider estate this thinking applies to. If Slack's free tier genuinely serves you better, we will tell you so.
Frequently asked questions
Which Mattermost edition does Node deploy?
The free, open source Team Edition, and we are explicit about that. It covers channels, threads, direct messages, file sharing, search, calls, playbooks and integrations, with unlimited message history in your own database. Mattermost's paid Professional and Enterprise editions add features we do not ship, including SAML and AD/LDAP sync, automated data retention policies, legal hold and one-click compliance export. If you need those specifically, buy them from Mattermost; we will say the same on a call.
Can we migrate from Slack with our message history?
Yes, with one caveat that is Slack's, not ours. Mattermost's import tooling brings across users, channels, full message history with original timestamps and file attachments, and mostly preserves threads. But Slack's standard export covers public channels only: private channels and direct messages are only included in the full export available on Slack's Business+ and Enterprise plans. We handle the export, transform, import and cutover either way.
What does Mattermost on Node include?
Deployment in a production configuration with a managed database and file storage, upgrades and security patching, backups, monitoring, and single sign-on through your own Keycloak realm, all on UK infrastructure under an Article 28 data processing agreement, with support answered by the engineers who run the platform.
What does it honestly cost compared with Slack?
A flat monthly fee by resource tier: £23, £39 or £71 a month at the time of writing, at any team size. Slack Pro is £5.75 per user per month billed annually at the time of writing, so a 50-person team pays about £3,450 a year against a flat tier under £900. For a team of five, Slack's free tier costs nothing and the maths favours Slack, if you accept losing sight of messages older than 90 days.
Where do our messages and files live?
In a database and file store on UK infrastructure Node owns and operates, in your own isolated tenant, under UK jurisdiction and a UK GDPR Article 28 data processing agreement. Slack offers data residency on paid plans, but it covers certain data at rest in a chosen region while the service itself runs on Slack's global infrastructure.
Do we keep full message history?
Yes. Your complete archive lives in your own PostgreSQL database with no time-based paywall: what you keep and for how long is your decision, not a plan feature. On Slack's free tier, messages older than 90 days are hidden until you upgrade.
Will the team actually get on with it?
Mattermost deliberately follows the model Slack made universal: channels, threads, emoji, search, desktop and mobile apps. Anyone who has used Slack is productive in minutes. What they will notice is fewer third-party app integrations and less polish in places; what they will not notice is any difference in the daily rhythm of channels and threads.