Windows Server licensing: cores, virtual machines and access licences

A Windows Server licence is really three things: core licences for every physical core in the server (at least 16 per server), enough of them to cover the virtual machines you run, and an access licence for every user or device that connects. Count all three before you buy.

Windows Server has been licensed per core since 2016, and the 2025 release kept the same model. The rules are not complicated once they are laid out, but they are easy to get wrong when a quotation only says "Windows Server Standard, 1 unit". This article walks through what you actually need to count, where businesses tend to overpay or underpay, and how to plan the purchase.

The short answer

For each physical server you license every physical core, with a minimum of 8 core licences per processor and 16 per server. Standard edition then lets you run two virtual machines on that server; Datacenter lets you run as many as the hardware will take. On top of the server licences, every person or device that uses the server needs a Windows Server client access licence (CAL), and Remote Desktop users need a separate Remote Desktop Services CAL as well.

Step one: count the cores

Microsoft licenses Windows Server Standard and Datacenter by physical core, not by server or by processor. Three rules apply to every physical server:

• Every physical core is licensed. A server with two 12-core processors needs 24 core licences.
• At least 8 core licences per processor. A processor with fewer cores still counts as 8.
• At least 16 core licences per server. A small server with a single 8-core processor still needs 16.

Core licences are sold in packs of 2 and packs of 16, so a 16-core pack covers a typical small server and 2-core packs top up larger ones. The core count comes from the hardware, so the first document we ask for is the server's specification sheet, not the number of staff.

Step two: count the virtual machines

This is where the two editions differ.

• Standard covers the physical server plus two virtual machines running Windows Server, once all physical cores are licensed. For a third and fourth virtual machine you license all the physical cores again. This is often called stacking.
• Datacenter covers an unlimited number of Windows Server virtual machines on the licensed server.

A small office server running a domain controller and a file server sits comfortably on Standard. A host running a dozen or more virtual machines usually costs less on Datacenter than on several stacked Standard licences. The crossover depends on the core count and the pricing on the day, so it is worth working out per host rather than by rule of thumb.

Step three: count the users and devices

Core licences cover the server. Access is licensed separately with client access licences, which come in two types:

• User CALs suit staff who connect from several devices, such as a laptop, a phone and a shared PC.
• Device CALs suit shared devices used by several people, such as a reception PC or a shift terminal.

You can mix the two to keep the total down. Two further points catch people out. CALs must match or be newer than the server version, so an older CAL does not cover a Windows Server 2025 server. And users who sign in through Remote Desktop Services need an RDS CAL in addition to the Windows Server CAL.

Some Microsoft 365 plans carry Windows Server access rights for their licensed users. If you already pay for one of them, check before buying CALs for the same people twice.

Where businesses overpay, and where they underpay

The common mistakes look like this:

1. Licensing the processor instead of the cores. The quotation lists one licence per server, and the server has more cores than the licence covers.
2. Running more virtual machines than Standard allows. A third virtual machine was added during a project and never licensed.
3. Buying Datacenter for a server that runs two virtual machines. The extra rights are paid for and never used.
4. Forgetting the CALs entirely. The server licences are in place, the access licences are not.
5. Old CALs on a new server. The server was upgraded and the access licences were not.
6. Missing RDS CALs. Remote Desktop was switched on for a few users and never licensed.

The first, second, fourth, fifth and sixth leave you under-licensed and exposed if Microsoft asks for a licence review. The third simply costs more than it needs to.

Perpetual licence, subscription or pay-as-you-go

Windows Server 2025 can be bought in more than one way, and the right choice depends on how long the server will live.

• Perpetual licences suit a server you expect to run for several years on your own hardware.
• Pay-as-you-go through Azure Arc bills Windows Server 2025 monthly through an Azure subscription for servers connected to Azure Arc. Microsoft states that a pay-as-you-go subscription does not require Windows Server CALs, although Remote Desktop Services CALs are still required. It suits short-lived, seasonal or test servers more than steady production ones.

If you also run workloads in Azure, ask about Azure Hybrid Benefit, which lets eligible Windows Server licences be used for Azure virtual machines.

If you still run Windows Server 2016

Extended support for Windows Server 2016 ends on 12 January 2027. After that date Microsoft stops releasing security updates unless the server is enrolled in paid Extended Security Updates through Azure Arc, which are billed monthly from 13 January 2027. A server that is being replaced anyway is a good moment to recount cores, virtual machines and users, because the new hardware rarely matches the old.

Frequently asked questions

How many core licences does a small server need?

At least 16. Even a server with a single 4-core or 8-core processor needs 16 core licences, because Microsoft sets a minimum of 8 per processor and 16 per server.

How many virtual machines can I run on Windows Server Standard?

Two Windows Server virtual machines per server once all physical cores are licensed. For more, license all the cores again for every additional two, or move to Datacenter.

Do I need CALs for Windows Server 2025?

Yes, on perpetual licences. Every user or device that accesses the server needs a Windows Server 2025 user or device CAL. Remote Desktop users also need an RDS CAL. A pay-as-you-go subscription through Azure Arc does not require Windows Server CALs, but RDS CALs are still needed.

Should I choose Standard or Datacenter?

Choose Standard for a physical server or a host with one or two virtual machines. Consider Datacenter for a host running many virtual machines, where it can cost less than stacking Standard licences. Compare both for each host before buying.

Can I buy Windows Server licences through a Microsoft CSP partner?

Yes. Windows Server core licences and CALs are available through the Microsoft Cloud Solution Provider programme, so they can be bought from the same partner that supplies your Microsoft 365 licences.

Where to start

Before you ask for a quotation, gather three numbers for each server: physical cores, Windows Server virtual machines, and the users and shared devices that connect to it. If any of them is uncertain, that is where to begin.

Onprem2Cloud IT Solutions Co. is a Microsoft CSP partner based in Muntinlupa City, Metro Manila. We count the cores, the virtual machines and the users with you, then quote Windows Server Standard or Datacenter with the right access licences, billed in pesos with official receipts. See our Microsoft CSP licensing service or send us your server details. If your Microsoft 365 renewal is also coming up, our guide to Microsoft CSP licensing for Philippine procurement covers the other half of the bill.