VMware vSphere Memory Sizing Calculator
vSphere gives each VM a configured memory size and three controls: the reservation the host guarantees, a limit it can't use more than, and shares that decide who wins when VMs compete for memory. VMware advises against limits, since they can waste idle memory, so this calculator follows that: it sizes the VM's memory from the peak, as the cap, and the reservation from the average, with the limit left unlimited.
01 Usage data
Where do I get these numbers?
Measure from inside the guest: vSphere's own "active" memory underestimates what the OS needs, and "consumed" never shrinks. With node_exporter, run a range query over a representative period (a week is a good default):
node_memory_MemTotal_bytes{instance="vm1:9100"}
- node_memory_MemAvailable_bytes{instance="vm1:9100"}
Paste the Prometheus UI table output, the HTTP API JSON, or a Grafana CSV export.
Enter observed usage directly, in MiB (vSphere's "MB").
02 Workload profile
Advanced: margins and defaults
Values the result relies on that are our own defaults, or vendor defaults your setup may change. Each “?” says how one moves the result; the guide says where it comes from.
03 Sizing gauge
04 govc command
—05 How this was derived
How every number is calculated, with sources and assumptions (opens in a new tab) ↗
Result looks wrong? Report it
Memory, reservation and limit in short
A reservation is the guaranteed minimum allocation: a VM whose reservation can't be met doesn't power on, and a reservation can't be larger than the VM's configured memory. With no limit, the VM's configured memory is its effective limit. vSphere docs
VMware advises that a limit can waste idle memory, that you reserve the minimum acceptable amount, and that you leave at least 10% of the host unreserved. So the calculator sets no limit, sizes the VM's configured memory from the peak and the reservation from the average plus its margin, which is our reading of "minimum acceptable". The margins are MemoryLimit's own defaults and can be changed under Advanced.
Memory hot add lets you add memory to a powered-on VM, but lowering a VM's memory isn't covered by it, so we assume that needs the VM off. vSphere docs
Common input mistakes
- Pasting vSphere's "active" or "consumed" memory: those describe the host's view of the VM; measure inside the guest instead.
- Reserving the peak: VMware advises reserving the minimum acceptable amount, not the amount you would like to have available.
- A window that misses the busiest times: the VM's memory comes out too low.
Full derivation, warnings and every assumption: how the VMware vSphere calculator works. The shared formula and margin tables: the sizing model.
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