vSAN Capacity Calculator: How to Estimate Storage Needs

The vSAN capacity calculator helps you estimate how much raw storage capacity you need for a VMware vSAN cluster based on your usable capacity requirements, replication factor, and failure tolerance. You enter your usable capacity target, choose a replication factor and failure tolerance method, and get the raw storage and minimum host count required.

Formula

The core formula for vSAN capacity is:

Raw Capacity = Usable Capacity × (Replication Factor + 1)

Where:

• Usable Capacity – the amount of data you actually need to store after all overheads (in GB or TB).
• Replication Factor – the number of copies of data vSAN keeps (1 for FTT=1 with RAID-1, 2 for FTT=2 with RAID-1, or 1 for RAID-5/6 where parity replaces full copies).
• Raw Capacity – the total physical storage you must provision across all hosts, before any filesystem overhead.

For RAID-5 and RAID-6, the formula is different because parity is used instead of full copies. For RAID-5 (FTT=1), raw capacity ≈ usable × 1.33. For RAID-6 (FTT=2), raw capacity ≈ usable × 1.5. The calculator above uses a simplified version that treats RAID-5/6 as if they used full copies, which overestimates raw capacity. For precise planning, use the exact formulas below.

Worked Example

Using the default values from the calculator: usable capacity = 10,000 GB, replication factor = 1 (FTT=1), failure tolerance method = RAID-1 (mirroring).

1. Raw capacity = 10,000 × (1 + 1) = 20,000 GB
2. Minimum hosts = 3 (for RAID-1 with FTT=1)

The calculator returns 20,000 GB raw and 3 hosts. This matches the formula.

For a real-world example: you need 50 TB usable with FTT=2 (3 copies). Raw = 50 × 3 = 150 TB. If you use RAID-6, raw ≈ 50 × 1.5 = 75 TB, but you need at least 6 hosts.

How vSAN Capacity Really Works

vSAN is VMware’s hyper-converged storage solution that pools local disks from multiple ESXi hosts into a shared datastore. Capacity planning is tricky because you must account for:

• Replication – vSAN mirrors data across hosts for redundancy. FTT=1 means one extra copy, FTT=2 means two extra copies.
• Erasure coding – RAID-5 and RAID-6 use parity to reduce overhead compared to mirroring, but require more hosts (4 for RAID-5, 6 for RAID-6).
• Filesystem overhead – vSAN reserves about 1-2% for metadata and slack space.
• Host and disk overhead – each host needs a cache tier (usually 10% of capacity) and a small amount for vSAN system usage.
• Future growth – vSAN alerts when usage exceeds 80% of capacity, so plan for headroom.

Choosing the Right Failure Tolerance

Your failure tolerance determines both resilience and capacity overhead. Here’s how to decide:

• FTT=0 – no redundancy, only for non-critical or test workloads. Not recommended for production.
• FTT=1 with RAID-1 – tolerates one host or disk failure. Requires 3 hosts, 2× raw capacity. Good for most production workloads.
• FTT=2 with RAID-1 – tolerates two failures, requires 5 hosts, 3× raw capacity. Use for critical VMs.
• FTT=1 with RAID-5 – tolerates one failure with less overhead (1.33×), but requires 4 hosts. Good for capacity-optimized clusters.
• FTT=2 with RAID-6 – tolerates two failures with 1.5× overhead, requires 6 hosts. Best for large capacity with high resilience.

Remember: the minimum host count is not just about capacity – it’s about ensuring the cluster can sustain the failure tolerance. For example, with RAID-1 FTT=1, you need at least 3 hosts so that if one fails, the remaining two can still hold a full copy.

Common Capacity Mistakes

Many vSAN planners make these errors:

• Forgetting the cache tier – vSAN requires a cache device (NVMe or SSD) that is at least 10% of the capacity tier. This doesn’t count as usable storage.
• Mixing RAID-1 and RAID-5 – vSAN policies can be per-VM, so you might have some VMs with FTT=1 mirroring and others with RAID-5. The calculator above assumes a single policy for all data.
• Ignoring host-based overhead – each host reserves about 2-4 GB for vSAN system usage, plus a small amount for metadata.
• Using raw disk size instead of usable – disk manufacturers use decimal GB (1 GB = 1,000,000,000 bytes), while vSAN uses binary GiB (1 GiB = 1,073,741,824 bytes). This can cause a 7% discrepancy.
• Not planning for rebuilds – when a disk fails, vSAN rebuilds data on other disks, which temporarily requires extra free space. Keep at least 10-20% free.

Real-World Capacity Planning Example

Suppose you have a 4-host cluster and you want to store 40 TB of VM data with FTT=1 using RAID-5. Here’s how to calculate:

1. Usable capacity needed: 40 TB
2. RAID-5 overhead factor: 1.33 (for 4 hosts, you lose one disk’s worth to parity)
3. Raw capacity = 40 × 1.33 = 53.2 TB
4. Add 10% for rebuild slack: 53.2 × 1.1 = 58.5 TB
5. Add 10% for cache tier (if using dedicated cache): 58.5 × 1.1 = 64.4 TB total raw

Each host would need about 16.1 TB of raw storage. If you use 2 TB drives, that’s 8 drives per host (plus a cache drive).

Alternatives and Tools

While this calculator gives a quick estimate, VMware provides more precise tools:

• VMware vSAN Sizer – an official online tool that takes workload specifics (IOPS, capacity, deduplication) and produces detailed hardware recommendations.
• VMware Skyline Health – checks cluster capacity and gives alerts.
• vSAN ReadyNode configurations – pre-validated hardware bundles that simplify capacity planning.
• Manual calculation – use the formulas above with your exact disk sizes and overhead percentages.

For a quick estimate, this calculator is sufficient. For production deployment, always validate with VMware’s official tools and consult your hardware vendor.

Frequently Asked Questions

What does FTT mean in vSAN?

FTT stands for Failures To Tolerate. It defines how many host or disk failures the cluster can survive without data loss. FTT=1 means one failure is tolerated, FTT=2 means two failures. Higher FTT increases capacity overhead because more copies or parity are stored.

How much overhead does vSAN add?

vSAN adds about 1-2% for metadata, plus a small amount per host (2-4 GB) for system usage. The cache tier is separate and typically 10% of capacity. For planning, add 10-20% extra for rebuild slack and future growth.

Can I use different policies for different VMs?

Yes, vSAN storage policies are applied per VM or per VMDK. You can have critical VMs with FTT=2 and less critical ones with FTT=0. This calculator assumes a uniform policy, so for mixed environments you need to calculate each group separately.

What is the difference between RAID-1 and RAID-5 in vSAN?

RAID-1 (mirroring) creates full copies of data, so overhead is 2× for FTT=1. RAID-5 (erasure coding) uses parity, so overhead is about 1.33× for FTT=1. RAID-5 requires at least 4 hosts, while RAID-1 requires only 3. RAID-5 is more capacity-efficient but uses more CPU.

How do I convert raw capacity to usable capacity?

Usable capacity = raw capacity / (replication factor + 1) for RAID-1, or raw capacity / 1.33 for RAID-5, or raw capacity / 1.5 for RAID-6. Remember to subtract overhead and leave free space.

Is this calculator accurate for all vSAN versions?

The basic formulas apply to all vSAN versions, but newer versions (vSAN 7 and 8) have improved compression and deduplication that can reduce raw capacity needs. This calculator does not account for those features, so treat it as a conservative estimate.

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in usable | Usable capacity needed | GB | 10000 | 0
sel rf | Replication factor | FTT=1 (2 copies)=1, FTT=2 (3 copies)=2
sel ft | Failure tolerance method | RAID-1 (mirroring)=1, RAID-5 (erasure coding)=2, RAID-6 (erasure coding)=3
out raw | Raw storage required | GB | usable * (rf + 1) | 0
out hosts | Minimum hosts needed | hosts | ft == 1 ? 3 : (ft == 2 ? 4 : 6) | 0