VM Consolidation Calculator
How many physical hosts a set of virtual machines needs — from their vCPU and memory, the host's cores and RAM, the overcommit ratios you allow and the headroom you keep — with the binding constraint and the consolidation ratio.
Virtual machines rarely use all their allocated CPU at once, so hypervisors let you allocate more vCPUs than there are cores — four to one is common for general workloads.
How the vm consolidation calculator works
Virtual machines rarely use all their allocated CPU at once, so hypervisors let you allocate more vCPUs than there are cores — four to one is common for general workloads. Memory is less forgiving: overcommitting it means swapping, so most designs keep it at one to one. Each host also reserves some memory for itself, and the fleet needs a spare host for failover.
Whichever resource runs out first sets the host count; the other is slack, and the ratio tells you which one to buy more of.
Formula: hosts_cpu = ⌈vCPUs / (cores × overcommit × (1 − headroom))⌉; hosts_ram = ⌈RAM / ((host RAM − reserve) × ratio × (1 − headroom))⌉; hosts = max + spares
Worked examples
| Inputs | Hosts required | Note |
|---|---|---|
| 120 VMs on 32-core, 384 GB hosts | 6 | memory-bound |
| No overcommit | 20 | CPU-bound, many more hosts |
| Bigger hosts | 4 | half the count |
FAQFrequently asked questions
What is a CPU overcommit ratio?
How many virtual CPUs you allocate per physical core. Because VMs idle most of the time, 3:1 to 5:1 works for general servers; latency-sensitive or database VMs want 1:1 or 2:1.
Why not overcommit memory?
You can — hypervisors deduplicate identical pages and balloon idle guests — but when it goes wrong the host swaps and everything on it crawls. Most designs keep it at 1:1.
What is headroom for?
Bursts, maintenance and growth. A host planned to 100% has nowhere to put the VMs from a failed neighbour. Twenty per cent is a common reserve.
How many spare hosts?
One for a small cluster; enough to absorb a failure domain — a rack or a power feed — for a large one. The spare is capacity you pay for and hope never to use.
Which constraint should I fix?
The binding one. If memory binds, buy RAM-heavy hosts or trim VM allocations; if CPU binds, raise the overcommit ratio where the workload allows.
Where these figures come from
- IEC 80000-13 — Information science and technology (quantities and units) — the decimal (kB, MB) versus binary (KiB, MiB) prefixes used throughout
- RFC 4632 — Classless Inter-domain Routing (CIDR) — the address-plan arithmetic behind the subnet calculator
- RFC 1918 — Address Allocation for Private Internets — the private ranges the subnet calculator recognises
- NIST SP 800-63B — Digital Identity Guidelines, Authentication — length over composition rules; the basis of the password guidance here
- NIST SP 800-57 Part 1 — Recommendation for Key Management — key-strength comparisons used by the key-space calculator
- Australian Cyber Security Centre — Essential Eight — national guidance on passphrases and system hardening
Last checked: September 2026. Units follow the SI decimal convention (IEC 80000-13 defines the binary alternatives); network and security figures cite the defining standard.