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