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VMware Datastore Capacity Calculator

Size a VMFS or NFS datastore for VMDKs, swap files, snapshot growth and the free space vSphere needs to stay healthy.

About VMware Datastore Capacity Calculator

A VM costs a datastore more than its VMDK. Every powered-on VM without a full memory reservation also writes a .vswp file the size of its configured RAM, snapshots grow delta disks that can rival the base disk, and thin provisioning only defers the VMDK cost rather than removing it. Subtracting a free reserve first is what keeps the datastore operable rather than merely full. A datastore at 95% cannot consolidate a snapshot or accept a Storage vMotion, so it fails exactly when you need the tools that would rescue it — and thin provisioning means the wall arrives without warning.

Formula

VMware Datastore Capacity

space per VM = consumed VMDK + (configured RAM × (1 − reservation)) for the swap file + snapshot allowance; VMs supported = raw size × (1 − free reserve) ÷ space per VM.

.vswp
Swap file created at power-on, sized as configured RAM minus the reservation
snapshot allowance
Delta-disk growth budgeted per VM
free reserve
Headroom left unallocated so consolidation and vMotion can work

Worked example

With the default values below, VMware Datastore Capacity Calculator returns VMs the Datastore Supports of 137. Change any input above and every figure here is recalculated in your browser.

Inputs used

Datastore raw sizeaccepts 0.1–512 TiB
20 TiB
VMs on this datastoreaccepts 1–5,000 VMs
40 VMs
Provisioned disk per VMaccepts 1–65,536 GiB
200 GiB
Actual disk consumedaccepts 1–100 %
45 %
Thin provisioning enabled
Yes
Configured memory per VMaccepts 0.25–6,144 GiB
16 GiB
Memory reserved per VMaccepts 0–100 %
0 %
Snapshot delta allowanceaccepts 0–100 %
15 %
Free space kept in reserveaccepts 0–50 %
20 %

Results

VMs the Datastore Supports
137 VMs
Space Consumed per VM
119.5 GiB
Usable Capacity After Reserve
16,384.0 GiB
Space Consumed Today
4,780.0 GiB
Datastore Utilisation
23.3%
Capacity Verdict
Plenty of headroom — this datastore is under-subscribed

Step by step

  1. 1Values used
  2. 2VMware Datastore Capacity
  3. 3VMs the Datastore Supports= 137 VMs
  4. 4Space Consumed per VM= 119.5 GiB
  5. 5Usable Capacity After Reserve= 16,384.0 GiB
  6. 6Space Consumed Today= 4,780.0 GiB
  7. 7Datastore Utilisation= 23.3
  8. 8Capacity Verdict= Plenty of headroom — this datastore is under-subscribed

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, 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 Datastore Capacity calculated?

space per VM = consumed VMDK + (configured RAM × (1 − reservation)) for the swap file + snapshot allowance; VMs supported = raw size × (1 − free reserve) ÷ space per VM. A VM costs a datastore more than its VMDK. Every powered-on VM without a full memory reservation also writes a .vswp file the size of its configured RAM, snapshots grow delta disks that can rival the base disk, and thin provisioning only defers the VMDK cost rather than removing it. Subtracting a free reserve first is what keeps the datastore operable rather than merely full.

Why does VMware Datastore Capacity matter?

A datastore at 95% cannot consolidate a snapshot or accept a Storage vMotion, so it fails exactly when you need the tools that would rescue it — and thin provisioning means the wall arrives without warning.

What values do I need to enter?

This calculator takes 9 inputs: Datastore raw size, VMs on this datastore, Provisioned disk per VM, Actual disk consumed, Thin provisioning enabled, Configured memory per VM, Memory reserved per VM, Snapshot delta allowance, Free space kept in reserve. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

Does a full memory reservation remove the swap file?

Yes. The .vswp file is sized as configured RAM minus the reservation, so a 100% reservation creates no swap file at all. That trades datastore space for a permanent claim on host memory, which lowers the memory overcommit you can run.

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