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EC2 Memory Utilization Calculator

Work out real memory utilisation after OS overhead, the GB per vCPU ratio, and the memory a target utilisation needs.

About EC2 Memory Utilization Calculator

Utilisation should be measured against usable memory, not the marketing figure, because the kernel, page cache and agents claim a fixed slice before your process starts. The GB-per-vCPU ratio then tells you which family you are on — roughly 2 for compute optimised, 4 for general purpose and 8 for memory optimised — so a workload sitting at 4 GB per vCPU with spare CPU is a candidate to move to an R-family shape with fewer, larger-memory cores. Memory is the one EC2 dimension CloudWatch cannot see by default, so it is where over-provisioning hides — and unlike CPU, running out of it kills the process rather than slowing it down.

Formula

EC2 Memory Utilization

usable RAM = instance memory − OS overhead; utilisation = used ÷ usable; memory needed = used ÷ target + overhead.

usable RAM
Memory left for the workload after the OS and agents
memory per vCPU
The ratio that identifies the instance family
target
Utilisation ceiling you want to leave room under

Worked example

With the default values below, EC2 Memory Utilization Calculator returns Memory Utilisation of Usable RAM of 31.1%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Instance memoryaccepts 0.5–24,576 GB
32 GB
vCPUs on the instanceaccepts 1–448 vCPU
8 vCPU
Memory used by the workloadaccepts 0–24,576 GB
9.5 GB
Operating system and agent overheadaccepts 0–128 GB
1.5 GB
Target memory utilisationaccepts 10–95 %
75 %

Results

Memory Utilisation of Usable RAM
31.1%
Memory Still Available
21.0 GB
Memory per vCPU
4.00 GB/vCPU
Memory Needed at the Target
15 GB
Memory Beyond the Target
17.0 GB
Matching Instance Family
General purpose shape — about 4 GB per vCPU

Step by step

  1. 1Values used
  2. 2EC2 Memory Utilization
  3. 3Memory Utilisation of Usable RAM= 31.1
  4. 4Memory Still Available= 21.0 GB
  5. 5Memory per vCPU= 4.00 GB/vCPU
  6. 6Memory Needed at the Target= 15 GB
  7. 7Memory Beyond the Target= 17.0 GB
  8. 8Matching Instance Family= General purpose shape — about 4 GB per vCPU

Assumptions and limits

  • The result is derived only from the 5 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 EC2 Memory Utilization calculated?

usable RAM = instance memory − OS overhead; utilisation = used ÷ usable; memory needed = used ÷ target + overhead. Utilisation should be measured against usable memory, not the marketing figure, because the kernel, page cache and agents claim a fixed slice before your process starts. The GB-per-vCPU ratio then tells you which family you are on — roughly 2 for compute optimised, 4 for general purpose and 8 for memory optimised — so a workload sitting at 4 GB per vCPU with spare CPU is a candidate to move to an R-family shape with fewer, larger-memory cores.

Why does EC2 Memory Utilization matter?

Memory is the one EC2 dimension CloudWatch cannot see by default, so it is where over-provisioning hides — and unlike CPU, running out of it kills the process rather than slowing it down.

What values do I need to enter?

This calculator takes 5 inputs: Instance memory, vCPUs on the instance, Memory used by the workload, Operating system and agent overhead, Target memory utilisation. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

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