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VMware Cluster Capacity Calculator

Total up a vSphere cluster's usable CPU and memory after failover reservation and see how many average VMs it carries.

Inputs

hosts
cores
GHz
GiB
%
hosts

N+1 reserves one host, N+2 reserves two.

ratio
MHz
GiB

VMs Supported

280VMs

Usable CPU Capacity

1,747.2GHz

Usable Memory Capacity

3,369.0GiB

Ceiling from CPU Demand

1,941VMs

Ceiling from Memory

280VMs

Hosts Available for Workload

7hosts

Step by step

  1. Values used

    Hosts in cluster = 8 hosts; Physical cores per host = 32 cores; Clock speed per core = 2.60 GHz; Memory per host = 512 GiB; ESXi host memory overhead = 6 %; Hosts reserved for failover = 1 hosts; vCPU per logical CPU = 3 ratio; Average CPU demand per VM = 900 MHz; Average memory per VM = 12 GiB

  2. VMware Cluster Capacity

    usable GHz = (hosts − failover hosts) × cores × GHz × overcommit; usable RAM = (hosts − failover hosts) × RAM × (1 − host overhead); supported VMs is the lower of GHz ÷ VM demand and RAM ÷ VM memory.

  3. VMs Supported

    = 280 VMs

  4. Usable CPU Capacity

    = 1,747.2 GHz

  5. Usable Memory Capacity

    = 3,369.0 GiB

  6. Ceiling from CPU Demand

    = 1,941 VMs

  7. Ceiling from Memory

    = 280 VMs

  8. Hosts Available for Workload

    = 7 hosts

How it works

Cluster capacity is aggregated in absolute units — GHz for CPU and GiB for memory — because vSphere schedules against consumed resources, not configured ones. Reserving whole hosts for failover before the division is what makes the result survivable, and sizing CPU against measured MHz rather than vCPU counts stops the answer being dominated by oversized VMs. This is the number that decides your next hardware purchase order, and getting the failover reservation wrong means either buying a host you did not need or discovering during an outage that HA cannot restart everything.

Formula

VMware Cluster Capacity

usable GHz = (hosts − failover hosts) × cores × GHz × overcommit; usable RAM = (hosts − failover hosts) × RAM × (1 − host overhead); supported VMs is the lower of GHz ÷ VM demand and RAM ÷ VM memory.

failover hosts
Capacity held empty so HA can restart VMs after a host loss
effective hosts
Hosts left to actually run workload
VM demand
Average consumed MHz per VM, not its configured vCPU count

Frequently Asked Questions

How is VMware Cluster Capacity calculated?

usable GHz = (hosts − failover hosts) × cores × GHz × overcommit; usable RAM = (hosts − failover hosts) × RAM × (1 − host overhead); supported VMs is the lower of GHz ÷ VM demand and RAM ÷ VM memory. Cluster capacity is aggregated in absolute units — GHz for CPU and GiB for memory — because vSphere schedules against consumed resources, not configured ones. Reserving whole hosts for failover before the division is what makes the result survivable, and sizing CPU against measured MHz rather than vCPU counts stops the answer being dominated by oversized VMs.

Why does VMware Cluster Capacity matter?

This is the number that decides your next hardware purchase order, and getting the failover reservation wrong means either buying a host you did not need or discovering during an outage that HA cannot restart everything.

What values do I need to enter?

This calculator takes 9 inputs: Hosts in cluster, Physical cores per host, Clock speed per core, Memory per host, ESXi host memory overhead, Hosts reserved for failover, vCPU per logical CPU, Average CPU demand per VM, Average memory per VM. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

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