01. Counting Physical Cores and Licence Requirements
Under the core-based model, the number of core licences required equals the number of physical cores in the server, subject to mandatory minimums. The process for determining your licence requirement for any given server is straightforward, but the details matter.
1
Identify physical cores. For each physical server, determine the number of processors (sockets) and the number of cores per processor. Multiply to get the total physical core count. For example, a server with 2 CPUs x 10 cores each = 20 physical cores.
2
Apply minimum rules. Microsoft enforces two minimum thresholds that override the actual core count when the physical count falls below them. These ensure a baseline licence requirement per server regardless of hardware size.
3
Acquire licence packs. Windows Server core licences are sold in 2-core packs (standard in volume licensing). Some channels also offer 16-core pack SKUs. Pricing is proportional. All core licences for a server must be assigned to that server; you cannot split a pack across multiple machines.
Calculation example. Server with 2 CPUs, 12 cores each = 24 physical cores. Both minimums are naturally exceeded (12 per CPU > 8, 24 total > 16). You purchase twelve 2-core licence packs to cover 24 cores. Alternatively: one 16-core pack + four 2-core packs = 24 cores.
02. The 8/16 Rule: Minimum Licence Requirements
Microsoft's minimum licence requirements are the most commonly misunderstood aspect of core-based licensing. Two rules always apply, and they are non-negotiable.
8 cores per processor. Even if a CPU has fewer than 8 physical cores, you must licence it as 8. A 4-core CPU requires 8 core licences.
16 cores per server. Every server must have at least 16 core licences, regardless of actual core count. A 12-core single-socket server still needs 16.
2-core licence packs. Licences sold in 2-core packs. Minimum purchase: eight 2-core packs (16 cores) per server.
| Server Config | Physical Cores | 8/CPU Min | 16/Server Min | Licences Required |
|---|
| 1 x 6-core CPU | 6 | 8 | 16 | 16 |
| 1 x 12-core CPU | 12 | 12 | 16 | 16 |
| 2 x 4-core CPUs | 8 | 16 | 16 | 16 |
| 2 x 10-core CPUs | 20 | 20 | 16 | 20 |
| 2 x 20-core CPUs | 40 | 40 | 16 | 40 |
| 4 x 28-core CPUs | 112 | 112 | 16 | 112 |
03. Hyper-Threading, Disabled Cores, and Edge Cases
Hyper-threading does not count. Intel Hyper-Threading or AMD SMT creates two logical threads per physical core. Microsoft does not count hyper-threads as cores. An 8-core CPU with HT enabled shows 16 logical processors in the OS, but you still need only 8 core licences. Always verify core counts against the physical CPU specification, not the OS-reported logical processor count.
Disabled cores still count. Microsoft's rule is clear: all physical cores must be licensed, even if disabled via BIOS. You cannot reduce your licence requirement by turning off cores. Licensing is based on hardware capability, not current utilisation. Microsoft auditors verify core counts against hardware specifications. A 16-core server with 4 cores disabled still needs 16 core licences.
04. Physical vs Virtual Core Licensing
Physical Core Licensing (Default Method)
In on-premises deployments without Software Assurance or subscription rights, you must licence all physical cores on the host hardware, even if you run only one or two VMs. By licensing all cores, you activate the edition's virtualisation rights (Standard: 2 VMs per licence stack; Datacenter: unlimited VMs).
Virtual Core Licensing (Requires SA or Subscription)
If you have active Software Assurance or equivalent subscription licences, you can assign licences to individual VMs instead of the entire host. You count the virtual cores (vCPUs) allocated to the VM and licence those.
Per-VM licensing rules. Minimum 8 virtual core licences per VM, even if the VM has only 4 vCPUs. Minimum 16 core licences overall. SA or subscription required. Primary use cases: Azure Hybrid Benefit, authorised cloud provider deployments, hosting scenarios where you do not control the physical host details.
When to use physical core licensing. Your own datacentre, on-premises servers you control, any environment without SA. This is the default for the vast majority of deployments.
When to use virtual core licensing. Azure VMs (via Hybrid Benefit), authorised outsourcer cloud environments, third-party hosting where physical host details are unknown, or scenarios where licensing a few VMs costs less than licensing the entire host.
The 90-day reassignment rule. Once assigned to a server (physical or virtual), a core licence cannot be reassigned to a different server for at least 90 days, except when permanently retiring the original server. This prevents "floating" licences between hosts to follow VM migrations.
Clustering impact. In Hyper-V or VMware clusters with Live Migration, you cannot "float" a single Windows Server licence to whichever host runs the VM. You must licence each host for the maximum number of VMs that could run on it during failover, or use Datacenter on all nodes. With SA and per-VM licensing, you gain more flexibility, but careful tracking is still required.
05. Core Licensing and CALs
Core licensing covers only the server installation itself. It grants the right to install and run Windows Server on the hardware. It does not grant users the right to connect to it. Separate Client Access Licences (CALs) are required for every user or device that accesses Windows Server services.
CALs are not tied to cores. The number required is based purely on users or devices, regardless of server size. A 16-core server and a 112-core server both require the same CAL count if the same users access them.
CAL types are additive. Base Windows Server CALs are required for general access. RDS CALs and RMS CALs are additional requirements on top of the base CAL when those services are used.
Azure exception. Azure-hosted Windows Server instances do not require CALs. If you migrate workloads to Azure, the CAL requirement is eliminated for those instances, a meaningful cost savings for environments with many users accessing cloud-hosted Windows Server services.
06. Practical Scenarios with Calculations
1
Small office server. Single tower, 1 CPU with 8 cores, Windows Server 2022 Standard, file sharing for 10 employees. Core calculation: 8 physical cores, but the 16-core server minimum applies. Must licence 16 cores (eight 2-core packs). Standard with 16 cores covers 1 physical instance + 2 VMs. CALs required: 10 User CALs. Result: 16 core licences (Standard) + 10 User CALs.
2
Highly virtualised host. VMware host, 2 CPUs with 20 cores each = 40 cores total, running 10 Windows Server VMs. Option A (Datacenter): licence all 40 cores once, covers unlimited VMs, twenty 2-core packs. Option B (Standard stacked): each 40-core Standard licence set covers 2 VMs, so for 10 VMs: 5 licence stacks x 40 cores = 200 core licences. Far more expensive. Result: 40 core licences (Datacenter) + appropriate CALs.
3
Cloud BYOL with per-VM licensing. Windows Server Datacenter subscription with SA, bringing own licence to an authorised outsourcer. VM configuration: 8 vCPUs. Per-VM calculation: 8 vCPUs meets the 8-core minimum exactly. Allocate 8 core licences to that VM. Datacenter SA dual-use rights preserve on-premises entitlement. Result: 8 core licences assigned to the cloud VM via per-VM licensing.
4
Server consolidation. Before: two servers, each 1 CPU x 8 cores, each requiring 16 core licences due to minimums. Total: 32 core licences. After: consolidate onto one server with 1 CPU x 16 cores, running 2 VMs within Standard's 2-VM limit. Total: 16 core licences. Result: 50% reduction in core licence count from 32 to 16.
07. Licence Optimisation Considerations
Consolidation reduces minimum-driven overhead. Because each server carries a 16-core minimum cost, consolidating workloads onto fewer larger servers can dramatically reduce total licence requirements. Instead of three 8-core servers (3 x 16 = 48 core licences due to minimums), one 24-core server handles the same workloads with just 24 core licences. A 50% reduction.
Software Assurance unlocks strategic options. SA grants version upgrade rights (Windows Server 2025 automatically), per-VM licensing for cloud BYOL scenarios, Azure Hybrid Benefit (saving 40-50% on Azure VM costs), and enhanced reassignment flexibility for server farm and cloud scenarios.
Monitor core counts through hardware changes. If you add processors, upgrade to higher-core-count CPUs, or enable previously disabled cores, your licence requirement increases immediately. Failing to update licences after hardware upgrades is one of the most common audit findings.
Plan Standard vs Datacenter transitions. Track VM density on each host. When Standard stacking approaches the Datacenter cost threshold (typically around 6-8 VMs per host), plan the transition proactively. Converting from stacked Standard to Datacenter at renewal is usually straightforward but requires planning the timing and budget.
Avoid common audit pitfalls. Microsoft audit methodology specifically verifies that each server has at least 16 core licences and at least as many licences as physical cores. Non-compliance typically occurs when organisations spin up development or test servers without full licensing, repurpose hardware without adjusting licence assignments, or enable additional cores after initial purchase.
08. SAM Recommendations
Maintain a hardware inventory with CPU and core counts for all Windows Server hosts. Update documentation whenever hardware is changed, upgraded, or decommissioned.
Use the right licensing mode. If you have SA or subscription entitlements, evaluate per-VM licensing for cloud, hosting, or mixed-environment scenarios where it is more cost-effective. Otherwise, stick to physical core licensing and ensure all cores are covered.
Plan for the Standard-to-Datacenter tipping point. Model the cost of Standard stacking vs Datacenter up front for heavily virtualised hosts. Investing in Datacenter licences early avoids complexity and potential compliance gaps of managing multiple Standard stacks.
Include CALs in your strategy. Even a perfectly core-licensed server is non-compliant if users lack CALs. For external-facing systems, evaluate whether an External Connector is more economical than individual user CALs.
Verify core counts against hardware specs, not OS reporting. Hyper-threading doubles the logical processor count shown in the OS, but only physical cores need licensing. Use BIOS, iLO, iDRAC, or CPU specification sheets to confirm physical counts.
Document all licence assignments. Maintain a centralised record mapping licence entitlements to specific servers: server name, physical core count, edition, number of licence packs assigned, SA status and expiry date, and any cloud allocations. This documentation is invaluable during audits and true-ups.
Frequently Asked Questions
Do I need to licence hyper-threads as cores?+
No. Microsoft counts only physical cores, not logical processors created by hyper-threading. An 8-core CPU with HT enabled shows 16 logical processors in the OS, but requires only 8 core licences. Always verify core counts against the physical CPU specification.
Can I save licences by disabling cores in BIOS?+
No. Microsoft requires licensing all physical cores, even if some are disabled via BIOS. Licensing is based on hardware capability, not current utilisation or configuration. Microsoft auditors verify core counts against hardware specifications, not OS-reported active cores.
What is the minimum number of licences for any server?+
Every server requires a minimum of 16 core licences, and each physical processor requires a minimum of 8 core licences. For most modern servers with 16+ physical cores, the actual count exceeds all minimums, making the calculation straightforward.
What is per-VM licensing and who can use it?+
Per-VM licensing allows you to licence individual virtual machines by their allocated virtual cores rather than the entire physical host. Each VM must have at least 8 virtual core licences. This option is only available to customers with active Software Assurance or equivalent subscription licences. If SA lapses, those VMs must revert to physical core licensing.
Can I move a Windows Server licence between servers?+
Yes, but with restrictions. Once assigned to a server, a core licence cannot be reassigned for 90 days, unless the original server is permanently retired. For Hyper-V or VMware clusters with Live Migration, you must licence each host for the maximum VM count it could handle during failover.
How do core licence packs work?+
Windows Server core licences are sold in 2-core packs as the standard unit in volume licensing. Some channels also offer 16-core pack SKUs. All core licences for a server must be assigned to that single server. You cannot split a pack across multiple machines. To meet the 16-core server minimum, you need at least eight 2-core packs per server.
Do I still need CALs in addition to core licences?+
Yes. Core licences grant the right to install and run Windows Server. They do not grant users the right to connect to it. Every user or device accessing Windows Server services requires a separate CAL. The one major exception: Azure-hosted Windows Server instances do not require CALs.
Related Resources
FF
Fredrik Filipsson
Co-Founder, Redress Compliance
Fredrik Filipsson brings over 20 years of experience in enterprise software licensing, including senior roles at IBM, SAP, and Oracle. He specialises in helping Fortune 500 organisations optimise Microsoft, Oracle, SAP, and IBM licensing, ensuring compliance, reducing costs, and securing favourable contract terms through independent, vendor-neutral advisory.
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