Calwarden

VM Sizing Calculator

Estimate total vCPU, memory, and host count from application instance requirements and headroom.

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Inputs

vCPU
GB
%

Your values stay on this device.

Results

Recommended Hosts
Total vCPU Needed60
Total Memory Needed180
Limiting ResourceCPU

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Assumptions

  • No oversubscription beyond target utilization is modeled.
  • Storage, network, licensing, and affinity rules are not included.
  • Inputs remain local.

Sensitivity preview

Application Instances -10%: Recommended Hosts 3

Application Instances Current: Recommended Hosts 3

Application Instances +10%: Recommended Hosts 3

Formula: Total vCPU = instances * vCPU per instance * headroom factor. Total memory = instances * memory per instance * headroom factor. Hosts = ceil(max(total vCPU / usable host vCPU, total memory / usable host memory)).

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About this VM Sizing Calculator

VM sizing starts with workload resource demand, then adds headroom and maps the result to available hosts or nodes.

Who this calculator is for

  • Infrastructure engineers sizing VM clusters.
  • Application teams planning deployment capacity.
  • IT managers estimating host purchases.

When to use it

  • Estimate total vCPU and memory demand.
  • Calculate host count from target utilization.
  • Compare app scaling scenarios.

How to use this calculator

  1. Enter app instance count and per-instance CPU and memory.
  2. Set headroom and host capacity.
  3. Choose target host utilization.
  4. Review recommended host count and total resources.

Key assumptions

  • No oversubscription beyond target utilization is modeled.
  • Storage, network, licensing, and affinity rules are not included.
  • Inputs remain local.

Accuracy and testing

Review formula sources, default test cases, assumptions, unit handling, rounding rules, and changelog for this calculator.

View accuracy page

How this calculator works

Required vCPU and memory are multiplied by headroom. Hosts needed is the larger of CPU-based and memory-based host counts after target utilization.
Total vCPU = instances * vCPU per instance * headroom factor. Total memory = instances * memory per instance * headroom factor. Hosts = ceil(max(total vCPU / usable host vCPU, total memory / usable host memory)).

Inputs explained

Instances

Number of app or VM instances in the scenario.

Per-instance resources

vCPU and memory needed by each instance.

Host capacity

Usable vCPU and memory per physical host or cloud node.

Understanding your results

The recommended host count is rounded up. Check both CPU and memory drivers to see which resource constrains placement.

Example scenarios

App cluster

24 instances needing 2 vCPU and 6 GB each with 25% headroom require 60 vCPU and 180 GB before host mapping.

Key Features

  • Instant Calculation
  • Private & Secure
  • Mobile Optimized

Tips & Best Practices

  • Model production and nonproduction separately.
  • Reserve capacity for host failure if required.
  • Validate sizing with monitoring data after launch.

Frequently Asked Questions

Does this include CPU oversubscription?
Only indirectly through the target utilization you enter. It does not model scheduler-specific behavior.
Should I include high availability spare capacity?
Yes. Add it through headroom or reduce target utilization when N+1 capacity is required.
Is any infrastructure data stored?
No. The calculation runs locally in your browser and does not upload your inputs.