Windows Server licensing
on Proxmox VE

Windows Server is licensed by host cores

The most common misunderstanding about Windows on Proxmox: if the VM only "takes" 2 vCPU, do I pay for 2 cores? No. Windows Server is licensed by the physical cores of the server (host) the VM runs on — regardless of how many vCPU you assign it. You must cover all of the host's cores, with a minimum of 16 cores per server and 8 cores per processor.

On top of the core license come CALs (Client Access Licenses) — per user or per device — required for everyone using the server. Remote Desktop Services additionally require separate RDS CALs. All of this is counted independently of the core model.

The dense-host trap

Since you pay for all of the host's physical cores, the denser the server, the higher the license cost — even if it runs only a single, small Windows VM.

Under Standard, each full set of licenses covering all host cores grants the right to run two Windows VMs. Need four? You license the host cores twice ("stacking"). With many Windows VMs on one node it quickly becomes cheaper to go Datacenter — license the physical cores once and run unlimited VMs.

HA clusters and license mobility

A cluster adds the mobility dimension. If a VM can move to another node (live migration or automatic HA failover), then under the physical model every node the VM can run on must be separately and fully licensed — not just the one it usually runs on.

The per-VM with Software Assurance model gives you license mobility — it "travels" with the VM between servers. Without Software Assurance, legally moving a core license between hosts is limited (once every 90 days), which in practice conflicts with the idea of an HA cluster.

Catalog prices for Windows Server 2025

Below are indicative Microsoft catalog prices (Open channel / ERP price list, net of VAT) for Windows Server 2025. These are base prices for perpetual licenses (without Software Assurance) — a reference point for budgeting; the real price depends on the purchase channel, partner and volume.

Perpetual license — how long is it valid

The prices above are for perpetual licenses — that is, with no expiry: you buy once and get the right to run that version indefinitely, with no renewal fees. There are, however, two important caveats:

• Tied to a specific version — a Windows Server 2025 license grants the right to run that version forever, but does not entitle you to newer ones. Moving to the next version means a new license or Software Assurance (which adds upgrade rights).
• The real "clock" is the support lifecycle (~10 years) — the license doesn't expire, but security patches do: typically 5 years mainstream + 5 years extended support. After that the system still runs legally, but without fixes (a compliance and cyber-insurance problem).

Azure Arc pay-as-you-go — costs on Proxmox

Instead of buying a license, you can bill Windows Server 2025 hourly through an Azure subscription — including for VMs running in your own data center on Proxmox VE. It requires installing the Azure Arc agent on each Windows machine and an active Azure subscription (charges land on your Azure invoice); there's a 7-day trial.

PAYG pays off for short-term, seasonal or variable use and for a few small Windows VMs on a dense node. For steady, multi-year workloads a perpetual license is usually cheaper — 16 cores on PAYG is ~$537/mo, so a perpetual Standard (16 cores, $1,176) pays back in ~2 months, and Datacenter in ~13.

Software Assurance — costs and what it gives you on Proxmox

Software Assurance (SA) is a paid add-on to a perpetual license, costing for server products about 25% of the license price per year (usually bought as an L&SA bundle for 1–3 years).

In a Proxmox environment in your own data center, SA gives three things that genuinely lower cost and risk:

• Flexible Virtualization — with per-VM licenses under Software Assurance or a subscription (minimum 8 cores per VM) you can reassign the license between hosts more often than once every 90 days. That is one way to run a legal HA cluster. The other is a full license on every node the VM can land on. License Mobility does not cover Windows Server. A physical-core license stays on the 90-day rule even with SA.
• Per-VM model — SA unlocks licensing by the VM's cores (min. 8 per VM) instead of all the host's physical cores — the cheapest way to run a few Windows VMs on a dense node.
• Rights to newer versions — upgrade Windows Server during the SA term (removes the version lock of a perpetual license).

Which model to choose

• Many Windows VMs on a host → Datacenter: license the physical cores once, unlimited VMs. At list prices (16 cores: Datacenter $6,771, Standard $1,176 per 2 VMs) it pays off from about 11 Windows VMs per node versus "stacked" Standard: 10 VMs need 5 Standard sets ($5,880, cheaper), while 11 VMs need 6 sets ($7,056, more than Datacenter).
• A few Windows VMs on a dense node → per-VM with Software Assurance: pay for 8 cores per VM instead of all the host's physical cores.
• Consolidating Windows onto a dedicated, smaller node — fewer physical cores to license; the same "fewer cores = fewer licenses" logic as with per-core databases.
• HA cluster → count licenses for every node a VM can move to, or restrict it with a strict node-affinity rule, or reach for Software Assurance for mobility.
• CALs (per user/device) always apply, regardless of the core model.

Planning an OS upgrade during the migration? See our article Windows Server 2016 vs 2025 — the differences, end of support and running WS2025 as a VM on Proxmox VE.

Do Windows licenses bought under VMware carry over to Proxmox

Yes — in most cases. A Windows Server license is independent of the hypervisor: Microsoft licenses it by the server's physical cores, not by whether VMware ESXi, Hyper-V or Proxmox VE (KVM) runs underneath. If you keep the same physical servers and only swap the virtualization layer, your existing per-core licenses remain valid — they cover exactly the same cores. You don't rebuy Windows just because you're leaving VMware.

The devil is in the details — what to keep in mind:

• Volume licenses (Open / EA / MPSA) — transferable between servers. Without Software Assurance you can reassign a license to a specific host once every 90 days; with Software Assurance in the per-VM model (Flexible Virtualization) — more often than once every 90 days. SA alone does not lift the 90-day rule for physical-core licensing, and License Mobility does not cover Windows Server. That matters for an HA cluster.
• OEM licenses — permanently tied to the original server. They stay valid if you swap VMware for Proxmox on the same hardware, but you may not move them to a new/different physical server.
• CALs (per user/device) and RDS CALs — entirely independent of the hypervisor, they carry over unchanged.
• New, denser hardware? If the migration also moves VMs onto different servers, re-count the cores — a license covers a specific number of cores, and a 128-core host needs far more than an old 16-core one (see "the dense-host trap").

Do Microsoft tickets cover Proxmox

Microsoft does not support Proxmox. The SVVP (Server Virtualization Validation Program) names the hypervisors on which Microsoft supports Windows Server as a guest. Joint support covers vendors such as VMware, Red Hat, Citrix, Nutanix, SUSE, Canonical and Virtuozzo. Proxmox VE is not on the SVVP list in the Windows Server Catalog, so Microsoft does not support Windows Server as a guest on Proxmox.

Sharing a KVM codebase with an SVVP-validated distribution does not put Proxmox on that list. Microsoft validates a specific product and version, not the KVM project itself. Windows Server runs correctly on Proxmox VE. Not being listed in SVVP is a matter of Microsoft technical support, not the validity of the license — the license depends on the host's physical cores, not on the hypervisor.

Microsoft does not take bugs in Proxmox itself, in Proxmox's QEMU build, in live migration, or in host-side storage and networking. Those tickets go to Proxmox support.