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VMware Memory Overcommit Calculator

Measure ESXi memory overcommit and see which reclamation stage it triggers: page sharing, ballooning, compression or host swap.

About VMware Memory Overcommit Calculator

ESXi reclaims memory in escalating stages. Page sharing is free and always on, ballooning asks the guest to give back idle pages, compression stores reclaimed pages at roughly 2:1 in a memory cache, and host swapping writes them to disk — the only stage that genuinely hurts. Overcommit above 1.0 is normal and safe as long as the summed active working set still fits in physical RAM. Overcommit alone is not a problem; a working set that exceeds physical memory is, because host swapping turns nanosecond memory access into millisecond disk access and every VM on the host feels it at once.

Formula

VMware Memory Overcommit

overcommit = VMs × (configured RAM + VMX overhead) ÷ (installed RAM × (1 − host overhead)); the stage is chosen from that ratio and from whether the active working set still fits.

host overhead
VMkernel and agent memory, typically 5-6% of installed RAM
active working set
The share of configured RAM a guest is actually touching
overcommit
Configured guest memory divided by usable physical memory

Worked example

With the default values below, VMware Memory Overcommit Calculator returns Memory Overcommit Ratio of 1.17. Change any input above and every figure here is recalculated in your browser.

Inputs used

Hosts in clusteraccepts 1–96 hosts
4 hosts
Installed memory per hostaccepts 8–24,576 GiB
384 GiB
ESXi host memory overheadaccepts 0–20 %
6 %
Powered-on VMsaccepts 1–20,000 VMs
140 VMs
Configured memory per VMaccepts 0.25–6,144 GiB
12 GiB
VMX overhead per VMaccepts 0–2,048 MiB
90 MiB
Active working set per VMaccepts 5–100 %
55 %
Page sharing and compression savingsaccepts 0–50 %
12 %

Results

Memory Overcommit Ratio
1.17 :1
Configured Memory Allocated
1,692.3 GiB
Usable Host Memory
1,443.8 GiB
Active Working Set
924.0 GiB
Memory to Reclaim
45.4 GiB
Reclamation Stage Reached
Ballooning — the guest balloon driver reclaims idle pages

Step by step

  1. 1Values used
  2. 2VMware Memory Overcommit
  3. 3Memory Overcommit Ratio= 1.17 :1
  4. 4Configured Memory Allocated= 1,692.3 GiB
  5. 5Usable Host Memory= 1,443.8 GiB
  6. 6Active Working Set= 924.0 GiB
  7. 7Memory to Reclaim= 45.4 GiB
  8. 8Reclamation Stage Reached= Ballooning — the guest balloon driver reclaims idle pages

Assumptions and limits

  • The result is derived only from the 8 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

How is VMware Memory Overcommit calculated?

overcommit = VMs × (configured RAM + VMX overhead) ÷ (installed RAM × (1 − host overhead)); the stage is chosen from that ratio and from whether the active working set still fits. ESXi reclaims memory in escalating stages. Page sharing is free and always on, ballooning asks the guest to give back idle pages, compression stores reclaimed pages at roughly 2:1 in a memory cache, and host swapping writes them to disk — the only stage that genuinely hurts. Overcommit above 1.0 is normal and safe as long as the summed active working set still fits in physical RAM.

Why does VMware Memory Overcommit matter?

Overcommit alone is not a problem; a working set that exceeds physical memory is, because host swapping turns nanosecond memory access into millisecond disk access and every VM on the host feels it at once.

What values do I need to enter?

This calculator takes 8 inputs: Hosts in cluster, Installed memory per host, ESXi host memory overhead, Powered-on VMs, Configured memory per VM, VMX overhead per VM, Active working set per VM, Page sharing and compression savings. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

Do memory reservations change this arithmetic?

Yes. A full reservation removes that VM's memory from the reclaimable pool entirely and also eliminates its swap file, so heavy use of reservations lowers the overcommit you can safely run across the rest of the cluster.

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