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.

About VMware Cluster Capacity Calculator

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

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

Worked example

With the default values below, VMware Cluster Capacity Calculator returns VMs Supported of 280. Change any input above and every figure here is recalculated in your browser.

Inputs used

Hosts in clusteraccepts 2–96 hosts: 8 hosts
Physical cores per hostaccepts 1–512 cores: 32 cores
Clock speed per coreaccepts 0.8–6 GHz: 2.6 GHz
Memory per hostaccepts 8–24,576 GiB: 512 GiB
ESXi host memory overheadaccepts 0–20 %: 6 %
Hosts reserved for failoveraccepts 0–16 hosts: 1 hosts
vCPU per logical CPUaccepts 1–12 ratio: 3 ratio
Average CPU demand per VMaccepts 50–64,000 MHz: 900 MHz
Average memory per VMaccepts 0.25–6,144 GiB: 12 GiB

Results

VMs Supported: 280 VMs
Usable CPU Capacity: 1,747.2 GHz
Usable Memory Capacity: 3,369.0 GiB
Ceiling from CPU Demand: 1,941 VMs
Ceiling from Memory: 280 VMs
Hosts Available for Workload: 7 hosts

Step by step

1Values used
2VMware Cluster Capacity
3VMs Supported= 280 VMs
4Usable CPU Capacity= 1,747.2 GHz
5Usable Memory Capacity= 3,369.0 GiB
6Ceiling from CPU Demand= 1,941 VMs
7Ceiling from Memory= 280 VMs
8Hosts Available for Workload= 7 hosts

Assumptions and limits

The result is derived only from the 9 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

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.