EC2 CPU Utilization Calculator

Convert CloudWatch CPU averages into busy vCPUs, idle headroom and the monthly cost of the capacity you never use.

About EC2 CPU Utilization Calculator

CloudWatch reports CPU as a percentage of the whole instance, so multiplying by the vCPU count converts it into absolute compute. Costing the idle share is a first-order approximation because EC2 charges for the instance, not the cycles — the idle fraction is money spent on capacity that produced nothing. Sizing on the p99 rather than the mean is what keeps a resize from causing a latency regression. Average CPU under 20% with a modest peak is the classic right-sizing signal, and putting a dollar figure on the idle share is what gets a resize prioritised over a backlog item.

Formula

EC2 CPU Utilization

busy vCPU = vCPUs × average CPU%; idle cost = hourly rate × hours × (idle vCPU ÷ total vCPU); vCPU at target = ceil(busy vCPU ÷ target%).

average CPU%: CloudWatch CPUUtilization mean over the window
peak CPU%: p99 utilisation, the figure sizing must survive
target: Utilisation you are willing to run at steady state

Worked example

With the default values below, EC2 CPU Utilization Calculator returns Monthly Cost of Idle vCPU of $218.65. Change any input above and every figure here is recalculated in your browser.

Inputs used

vCPUs on the instanceaccepts 1–448 vCPU: 8 vCPU
Average CPU utilisationaccepts 0–100 %: 22 %
Peak CPU utilisationaccepts 0–100 %: 65 %
Target utilisation for sizingaccepts 5–100 %: 60 %
On-demand rate per hourminimum 0 USD/hour: 0.384 USD/hour
Hours running per monthaccepts 0–744 hours: 730 hours

Results

Monthly Cost of Idle vCPU: $218.65
vCPUs Actually Busy: 1.76
vCPUs Idle on Average: 6.24
vCPUs Needed at the Target: 3
Headroom Above Peak: 35.0%
Sizing Verdict: Reasonably sized against the target utilisation

Step by step

1Values used
2EC2 CPU Utilization
3Monthly Cost of Idle vCPU= 218.65
4vCPUs Actually Busy= 1.76
5vCPUs Idle on Average= 6.24
6vCPUs Needed at the Target= 3
7Headroom Above Peak= 35.0
8Sizing Verdict= Reasonably sized against the target utilisation

Assumptions and limits

The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges 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 CPU Utilization calculated?

busy vCPU = vCPUs × average CPU%; idle cost = hourly rate × hours × (idle vCPU ÷ total vCPU); vCPU at target = ceil(busy vCPU ÷ target%). CloudWatch reports CPU as a percentage of the whole instance, so multiplying by the vCPU count converts it into absolute compute. Costing the idle share is a first-order approximation because EC2 charges for the instance, not the cycles — the idle fraction is money spent on capacity that produced nothing. Sizing on the p99 rather than the mean is what keeps a resize from causing a latency regression.

Why does EC2 CPU Utilization matter?

Average CPU under 20% with a modest peak is the classic right-sizing signal, and putting a dollar figure on the idle share is what gets a resize prioritised over a backlog item.

What values do I need to enter?

This calculator takes 6 inputs: vCPUs on the instance, Average CPU utilisation, Peak CPU utilisation, Target utilisation for sizing, On-demand rate per hour, Hours running per month. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.