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Hypervisor Overcommit Ratio Calculator

Work out hypervisor overcommit ratio instantly with clear inputs, formula shown and shareable results.

About Hypervisor Overcommit Ratio Calculator

CPU overcommit is generally safe because vCPUs are time-shared and most VMs are idle, with 4:1 to 6:1 common. Memory overcommit is different: exceeding physical RAM forces ballooning, page sharing or host swapping, and host swapping is catastrophic for guest performance because the guest has no visibility into it.

Formula

Overcommit ratios

CPU ratio = allocated vCPUs / physical cores; memory ratio = allocated VM memory / physical memory

Worked example

With the default values below, Hypervisor Overcommit Ratio Calculator returns vCPU to core ratio of 4.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Total vCPUs allocatedminimum 1
256
Physical coresminimum 1
64
Total memory allocated to VMsminimum 0.1 GB
600 GB
Physical memoryminimum 0.1 GB
512 GB

Results

vCPU to core ratio
4.00 :1
Memory overcommit ratio
1.17 :1
Allocated memory above physical
88.0 GB
Risk assessment
Moderate - memory overcommitted, monitor ballooning

Step by step

  1. 1Values used
  2. 2Overcommit ratios
  3. 3vCPU to core ratio= 4.00 :1
  4. 4Memory overcommit ratio= 1.17 :1
  5. 5Allocated memory above physical= 88.0 GB
  6. 6Risk assessment= Moderate - memory overcommitted, monitor ballooning

Assumptions and limits

  • The result is derived only from the 4 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 2 decimal places, 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

Which metric shows CPU overcommit is hurting?

CPU ready time in VMware or steal time inside the guest. Above 5 percent indicates VMs are waiting for physical cores.

Is memory overcommit ever acceptable?

In development and VDI where many guests are idle, transparent page sharing and ballooning reclaim genuine slack. For production databases, never.

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