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The Capacity page of the vSphere Infrastructure view provides an estimate of the additional VMs that can be added to a cluster based on a typical VM configuration. Using this estimate, you can identify the clusters with residual or spare capacity. You can customize this typical VM configuration and review the changed estimate of additional VMs that can be added to the cluster.


To recalculate the spare VM basis

  1. Access the vSphere Infrastructure view and ensure that the Clusters > Capacity page is open.
  2. In the Spare VM basis column of the table, click TYPICAL or CUSTOM. The details of the spare VM basis are displayed in a new window.
  3. Click Change. The following options are displayed: TYPICAL and CUSTOM.
  4. Specify the spare VM basis:
    1. To use the calculation that is provided by default, select TYPICAL. The typical spare VM basis is computed by using the average utilization of the VMs for the time period that is selected in the time filter of the page. For more information, see Indicators.
    2. To use specific VM attributes to compute the number of spare VMs, select CUSTOM and type values for the following attributes and select their corresponding measurement units:

      1. CPU Utilization

      2. Memory Utilization

      3. Disk Utilization

  5. Click Apply.
    Depending on the option that you selected, the spare VM basis settings are saved and applied for the selected cluster. 

  6. Click x to close the window.

The table in the Capacity page displays the modified number of spare VMs based on the updated settings in the Spare VMs column.

Spare VM calculation

Spare VMs are computed by using the resource consumption of an average VM in the cluster. Average VM can be a typical VM in the cluster or a custom VM.

The following table lists formulas for spare VM calculation.

Number of VMs that will fit in the cluster on the basis of the CPU needs of an average VM

SpareVM[CPU] = ((total_cpu_mhz * threshold[cpu_util]) - cpu_utilmhz) / cpu_of_average_vm

Expression used in the above formulaDescription


Total CPU capacity of the cluster (latest sample)


CPU utilization of the cluster (95th percentile)


Total CPU consumed by the cluster in MHz


cpu_of_typical_vm or cpu_of_custom_vm


cpu_of_custom_vmUser-specified value
Number of VMs that will fit in the cluster on the basis of the memory needs of an average VM

SpareVM[MEMORY] = ((total_real_mem * threshold[mem_util]) – memory_consumed) / memory_of_average_vm

Expression used in the above formulaDescription


memory_of_typical_vm or memory_of_custom_vm


Total physical memory configured for the cluster (latest sample)
mem_utilPercentage of total memory utilized by the vSphere cluster (95th percentile)


Memory consumed by the cluster, measured in Gigabytes




User-specified value

Number of VMs that will fit in the cluster on the basis of the storage needs of an average VM 1

SpareVM[STORAGE]=((total_dstore_size * threshold[total_dstore_util]) - total_dstore_used) /storage_of_average_vm

Expression used in the above formulaDescription


Total size of the datastore (latest sample)
total_dstore_utilPercentage of total datastore space utilized by the cluster


Total datastore space used by the cluster


Spare VM

SpareVM = MIN (SpareVM[CPU], SpareVM[MEMORY], SpareVM[STORAGE])

Limiting Resource

CPU, MEMORY, or STORAGE based on:


  1. The value of Threshold[total_dstore_util] ranges from 0 to 1. If the threshold is 75%, 0.75 must be specified in the formula. This is applicable for all threshold values in all the formulas.