Client Access Licences (CALs) Explained
A complete UK-focused guide to Microsoft CALs — what they are, when you need them, the difference between User and Device CALs, and how to size your purchase. Written for IT managers, procurement teams, and business owners buying Windows Server, RDS, Exchange or SharePoint licences.
What is a CAL?
A Client Access Licence (CAL) is the legal permission for a user or device to connect to a Microsoft server product. It is not software you install — it is a licence that proves the user/device is entitled to access the server. CALs apply to every version of Windows Server, Exchange Server, SharePoint, SQL Server (under the Server+CAL model), Skype for Business and System Center.
Crucially: the server licence by itself does not give anyone the right to connect to it. Every individual person or device that authenticates to the server needs a CAL, unless that user is one of the few exempt cases (see “Who doesn’t need a CAL?” below).
User CAL vs Device CAL — which to buy?
| Scenario | Best choice | Why |
|---|---|---|
| Each member of staff has their own laptop and phone, but connects from anywhere | User CAL | A single User CAL covers all of that person’s devices — cheaper when users have multiple devices. |
| Shift workers share workstations on the shop floor or call centre | Device CAL | One Device CAL covers every user that logs on to that machine, so you license shared hardware rather than rotating staff. |
| Hot-desking + mobile workforce | User CAL | Users move between many devices — you only buy one CAL per person. |
| Manufacturing line PCs operated 24/7 by rotating staff | Device CAL | Devices stay fixed, users rotate through them — fewer CALs needed than user-based licensing. |
| Mixed environment | Both | You can mix User and Device CALs against the same server. Buy each type for the population it fits. |
The simple rule of thumb: if you have more devices than users, buy User CALs. If you have more users than devices, buy Device CALs. The CAL count must equal or exceed the larger of the two populations.
The main CAL types
Windows Server CAL
Required for every user or device that authenticates to a Windows Server. Includes basic file/print sharing, AD authentication, DNS, DHCP, and most native server roles. Available for each Windows Server release (2016, 2019, 2022, 2025).
Browse Windows Server CALs →Remote Desktop Services (RDS) CAL
Additional to the base Windows Server CAL. Required for anyone accessing the server via Remote Desktop Services (RDS), including app virtualisation, RemoteApp, and full session-based desktops. If you’re running a session host or VDI farm, every connecting user/device needs both a Windows Server CAL andan RDS CAL.
Detailed RDS CAL guide →Exchange Server CAL (Standard & Enterprise)
Required for every mailbox user. Standard CALenables core email, calendar, and contacts. Enterprise CAL(purchased in addition, not as a replacement) unlocks unified messaging, archiving, advanced compliance, and DLP. You always need the Standard CAL; Enterprise is additive.
SharePoint Server CAL (Standard & Enterprise)
Standard CAL covers core collaboration sites and search. Enterprise CAL adds Business Intelligence, advanced forms, Visio Services and Excel Services. Same additive model as Exchange — Enterprise is purchased on top of Standard.
SQL Server CAL
Only used when SQL Server is licensed under the “Server + CAL” model (typically Standard Edition for small deployments with a known user count). Enterprise SQL is almost always licensed per-core and does not use CALs. See our dedicated SQL Server licensing guide.
System Center CAL
Required to manage non-server devices (laptops, desktops, mobile devices) via System Center Configuration Manager / Endpoint Manager. Each managed client OSE needs a Client Management Licence.
CAL versioning — can I use Server 2019 CALs with Server 2022?
No. The CAL version must match or exceed the server version. If you upgrade from Windows Server 2019 to 2022, every existing CAL also needs upgrading. The good news is thatCALs are forward-compatible — a 2025 CAL can be used with a 2019 server, but a 2019 CAL cannot connect to a 2025 server.
In practice this means: if you’re buying a fresh server in 2026, buy the matching version of CAL (e.g. Windows Server 2025 CALs for a Server 2025 deployment) even if your existing infrastructure is older. Future-proofing costs nothing extra.
Who doesn’t need a CAL?
- Anonymous external users accessing your public website (covered by the External Connector Licence or by SharePoint Server licence depending on workload).
- Administrators — up to two admins are licensed by the Windows Server licence itself for management purposes only.
- Microsoft 365 / Azure AD users hitting cloud services rather than your on-premises servers. The cloud subscription includes the access right.
External Connector Licence (ECL)
If you have a large or unknown number of external users (e.g. partners or customers logging in to an extranet), it can be cheaper to buy a per-server External Connector Licenceinstead of a CAL per external user. ECLs are pricey per server but license unlimited external users.
Rule of thumb: under ~100 external users, CALs are cheaper. Over that, look at an ECL. We’re happy to quote both for you.
How we help you size the right number of CALs
- Tell us how many users authenticate (including remote workers and apprentices).
- Tell us how many devices — including kiosks, tills, and meeting-room PCs.
- List the workloads each user/device touches (file shares, Exchange, SharePoint, RDS).
- We compare User vs Device totals and recommend the cheaper mix.
- We supply a written compliance summary you can keep on file.
Email our licensing teamand we’ll send a free CAL sizing within one working day.
Need to buy now?
We stock User and Device CALs for Windows Server 2016, 2019, 2022 and 2025, plus RDS CALs in 1/5/25/50-user packs. UK billing, VAT invoices, instant digital delivery.