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vSAN Storage Calculator

Convert vSAN raw disk capacity into effective usable capacity after RAID overhead, slack space and dedupe or compression.

About vSAN Storage Calculator

vSAN applies overheads in a strict order: the storage policy's redundancy comes first, then slack space is held back from what remains, and dedupe and compression multiply only the data actually written. RAID-5 and RAID-6 erasure coding are far more space-efficient than mirroring but need at least 4 and 6 hosts respectively and cost more write amplification. Sizing vSAN on raw terabytes overstates real capacity by two to four times, and a cluster that hits the slack threshold stops rebuilding objects — which is precisely when a second disk failure becomes data loss.

Formula

vSAN Storage

effective = hosts × disks × disk size × RAID factor × (1 − slack) × dedupe ratio, where the RAID factor is 0.50 for FTT=1 mirroring, 0.33 for FTT=2 mirroring, 0.75 for RAID-5 and 0.67 for RAID-6.

RAID factor
Share of raw capacity left after the policy's redundancy overhead
slack
Reserve vSAN needs for rebuilds, rebalancing and snapshots
dedupe ratio
Space saving applied after RAID, only on all-flash clusters

Worked example

With the default values below, vSAN Storage Calculator returns Effective Usable Capacity of 72.58. Change any input above and every figure here is recalculated in your browser.

Inputs used

Hosts contributing storageaccepts 2–64 hosts
6 hosts
Capacity disks per hostaccepts 1–40 disks
6 disks
Size of each capacity diskaccepts 0.1–64 TB
3.84 TB
Storage policyone of RAID-1 mirroring, FTT=1 (50% usable), RAID-1 mirroring, FTT=2 (33% usable), RAID-5 erasure coding, FTT=1 (75% usable), RAID-6 erasure coding, FTT=2 (67% usable)
RAID-1 mirroring, FTT=1 (50% usable)
Slack space reservedaccepts 0–50 %
30 %
Dedupe and compression ratioaccepts 1–8 ratio
1.5 ratio
Storage per VMaccepts 0.01–64 TB
0.5 TB

Results

Effective Usable Capacity
72.58 TB
Raw Capacity
138.24 TB
Capacity After RAID Overhead
69.12 TB
Capacity After Slack Reserve
48.38 TB
VMs Supported
145 VMs
Host Count Compliance
Compliant with a spare host for maintenance rebuilds

Step by step

  1. 1Values used
  2. 2vSAN Storage
  3. 3Effective Usable Capacity= 72.58 TB
  4. 4Raw Capacity= 138.24 TB
  5. 5Capacity After RAID Overhead= 69.12 TB
  6. 6Capacity After Slack Reserve= 48.38 TB
  7. 7VMs Supported= 145 VMs
  8. 8Host Count Compliance= Compliant with a spare host for maintenance rebuilds

Assumptions and limits

  • The result is derived only from the 7 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax 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 vSAN Storage calculated?

effective = hosts × disks × disk size × RAID factor × (1 − slack) × dedupe ratio, where the RAID factor is 0.50 for FTT=1 mirroring, 0.33 for FTT=2 mirroring, 0.75 for RAID-5 and 0.67 for RAID-6. vSAN applies overheads in a strict order: the storage policy's redundancy comes first, then slack space is held back from what remains, and dedupe and compression multiply only the data actually written. RAID-5 and RAID-6 erasure coding are far more space-efficient than mirroring but need at least 4 and 6 hosts respectively and cost more write amplification.

Why does vSAN Storage matter?

Sizing vSAN on raw terabytes overstates real capacity by two to four times, and a cluster that hits the slack threshold stops rebuilding objects — which is precisely when a second disk failure becomes data loss.

What values do I need to enter?

This calculator takes 7 inputs: Hosts contributing storage, Capacity disks per host, Size of each capacity disk, Storage policy, Slack space reserved, Dedupe and compression ratio, Storage 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.

Why does FTT=1 mirroring need three hosts and not two?

vSAN stores two data copies plus a witness component, and all three must live on separate hosts so no single failure can take out a majority. FTT=2 mirroring needs five hosts for the same quorum reason with three copies and two witnesses.

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